Technical Questions? Ask - MX Tuner

Oct. 01, 2001 By Mark Klein

Mark Klein is ready to answer your technical questions! Mark has extensive experience race tuning off-road motorcycles for local and national races. If you check back through our 1998/99 archives you will see a series called "Dirtbikes 101." Mark brings his expertise to fielding your maintenance and tuning questions.

Zingless YZ
I have a 1986 yz490. The other day I was riding and the thing died on me. I pulled the plug on the prong was bent completely down. I put a new plug in except now i can't get it to run right. The mid range is just fine., but off the line the thing wants to die, it bogs down. As soon as it hits a certain rpm it takes off. I cleaned the carb thouroghly adjusted the air screw to the right setting but I still can't get my bottom end back. Do I need to re-jet it with a smaller jet maybe? Help.

The first thing I'd check is to make sure the top end is okay mechanically. The fact the spark plug electrode wasn't it's original shape would concern me. I'd pull the head and make sure a piece didn't break off and gouge the cylinder. You're describing typical low compression symptoms. Only after I made sure the top end was okay would I start rejetting.

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Broken Clutch
hi. i have a 89' XR200 and last weekend i was trail riding with some friends and i came off and broke the clutch lever, the bike was still running so i turned it off. I managed to get the clutch fixed up so it would work but it still wouldn't roll when engaged, only in neutral, and it wouldn't kick over. It was justed jammed. I haven't seen it since cos we just left it there. What do you think? Is it seized?, or what could it be? Thanks

Was the actuating arm at the other end of the cable moving when the lever was pulled in? Did the lever pull feel normal? The only way the internal release mechanism could get damaged is if the cable got pulled too far during the crash. This would be fairly unusual. Make sure the cable is pulling the actuating arm fully. It may be that it just isn't pulling the cable far enough.

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Drip, Drip, Drip...
hi. i have a 2000 rm 125 that leaks gas out of the overflow tubes when i ride at high RPM's. can you help me?

How can you see the gas flowing out the overflow tubes if you're moving that fast? Regardless of when it happens, if gas is flowing out the overflow tubes, the float needle isn't working properly. It may be misadjusted or possibly have a piece of dirt preventing it from stopping the flow of fuel correctly. First place to start is a thorough carb cleaning.

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Fouling in Slovenija
Hello, I have a suzuki rm 250 99. Jets are 155 and 55, airscrew 1 1/2 turns out, I burn 2,5 % TOTAL racing 2t, new piston, neddle in middle position and fairly black exhaust. I dont drive it very fast but i dont have any problems with it when rev ving. Spark plug NGK B8EG or BR8EG can last 2 to 3 days before it completely dies.Faults occured with old piston to. It doesnt burn any oil from gearbox and the cooling water doesnt leak.I tried mainjet 175 but it doesnt rev.Spark plug gap is 0.024inch or 0,6 mm. Second: If I rev it high during driving and when i loose the throttle, when the rev s are lowering at certains i would say middle revs the engine or maybe clutch produces louder rotating noise than it should. I checked clutch and it has 0.7 mm play, i spotted no play in any other mechanical parts, could it be the gearbox? I drive my bike in Slovenija pretty much the same climate as Austia

Thanks Denis

Your spark plug fouling problem is the pilot jet being too rich. You'll probably end up around a 50 pilot depending on the weather conditions at the time. Switch to a BR8ES plug. These are much more foul resistant and cheaper than the "G" plugs. I would put the stock main jet in it until you have a definite reason to switch. While you have the tools out, I would suggest repacking the silencer. Make certain the core holes are clear and unobstructed before wrapping the new packing around the core. This can substantially affect the low speed jetting also.

It's hard to guess at what the noise might be since I can't listen to it.

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Power to the XR200!
I have an '88 Honda xr200 which I am in the process of doing some engine work after a recent valve swallowing. I was wondering if you had any relatively inexpensive tips/ tricks that I could try to give it a bit more power (since I am in the process anyhow!)

Thanx for your help!!


I'd be careful trying to get much more power out of your 200. You can make it unreliable very easily. Plus, squeezing more power out of a four stroke is generally much more costly than a comparable two stroke. The first place to start would be an aftermarket exhaust. The next would possibly be a big bore kit. Now were already up to about $600. How fast do you want to go?

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Fouling XR80
My son has a 2000 XR80 that seems to have a plug fouling problem. We purchased the bike last December and everything was great with it. I had not done anything maintenance wise on it, other then gas and oil changes, since it was purchased. The other day I thought it was time and gave the bike a once over. Everything was just fine until I got to the spark plug. It appears to be oil fouled. It is not wet oil but what appears to be burnt oil. In the past I would have thought this might have something to do with the fuel mixture, but with it being a 4-stroke I am a little confused. After replacing it, it seem to foul again after a few rides.

Sounds like the air filter may be clogged. This is a key part of regular maintenance. This needs to be done very frequently. Depending on how dusty it is where he rides, it might be necessary to do it after every ride. This is more critical on a two stroke but still very important on a four stroke. If this doesn't cure the problem, I'd suspect something with the carburetor (float level too high, etc).

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Crunching RM
I have just got an suzuki rm 125. I think it is a 1995/6 model. The problem is, you can only put it in 2'nd gear (or neutral). Also when I try to put the bike in second (the only avalible gear) gear to pull away, it crunches a bit. It's not the clutch though.

Someone said it is the gear selector barrel - Help!

Thanks, Alain.

It is definitely something preventing the shifter from moving properly. This might be anything from a broken piece of clutch plate stuck in the end of the shift drum to gear teeth caught between the gears or shift forks. The only way to determine the cause is to tear into it to inspect it. Start by removing the clutch and visually inspecting the shifter mechanism. If nothing appears to be holding the linkage from moving normally, then the cases will most likely have to be split.

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1970 Suzuki TS-90
I have a 1970 Suzuki TS-90 on/off road bike that I have restored. It seems to foul the plug right away when started from cold. I thought it may have a float shut off problem since there was gas/oil in the crankcase when I checked after it sat a day or two. I have ruled that out. It has been bored over about 2 sizes and I haven't changed the jets, I have played with the needle in the carburator. It also seems to have poor throttle response at the bottom end. I have no idea what jets and what needle settings this thing is supposed to have and I haven't been able to find a shop that has manuals that old. This bike has a rotary valve which is not unusual for a bike that old. Any thoughts or ideas. I can usually get it running after I put a second plug in it and after riding for a while the plug will come out dark tan - a little rich?

That's what it sounds like. Now, what is causing it? Chances are nobody rejetted it. I think I'd look to a carbon build up in the exhaust, more specifically the baffle. The entire pipe may have an accumulation that can affect performance but the plug fouling may be from a plugged baffle. Generally speaking, rotary valves are trouble free. Either they work or they don't. It can get out of time but then you'd have more dramatic symptoms.

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Valve Ajustment - YZ250F
2001 YZ250F, I seem to be having valve trouble?. The bike is a bear to start cold and is better once warmed, BUT, after adjusting intake valves to #155 shims from # 175 shims it fired right up BUT! after a couple races we had to bump start it! We seem to be chasing our tails any ideas? Thanks - Gino

Out of all the YZF valves I've adjusted, I've never had to move more than one shim size. You went four. I think I'd recheck and make 100% certain you're getting your measurements right. The valve clearance isn't as big as the 400/426. This would give approximately the difference you came up with. Make sure you're going by 250F specs. I've seen a few of the 250s that are more sensitive to the start procedure. Most people (who are having starting issues) are flooding it right away. Try starting it without giving the throttle any twists. The basic mixture may be rich enough that you may not have to give it any squirts. I've seen this on the big bore Yamahas also.

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I Need Complete Technical Data...
I need complete technical data on a 1983 Suzuki RM 125 , I need to know the size on a stock bore , the stroke size , fuel mixture , plug size and gap , transmission oil and oil type , oil capacity , radiator mixture and antifreeze type ,sprocket size (front@rear) , chain size , engine size in cc's , power output , and any special specifics that are on this bike like ... powervalve, powerband , whats the peak on this bike? Is it high,med,or low RPM , whats the torque rating? Whats the trans ratio? My head is mildly ported and polished in the intake and the exhaust. I received the bike in a non-running condition and it has a mixture of parts on it and I am not sure what belongs and dont belong on the bike. I'm in the stages of completely rebuilding this bike and I'm not into expensive but cheaply made performance parts like handlebars and sprockets. I seen the bike go before it ate a piston due to an improperly pressed in sleeve. The sleeve seems to be ovaled because at the bottom of the cylinder on both sides where the clips go on the wristpins I see two patches where the ring of piston never even wore off the crosshatches! the cylinder has a weird wear on it . uneven in a few places plus a slight scalloping near the top of the ports. When these bikes came out I was 13 so I have little knowledge of this bike. I have been waiting on a manual but I really can't wait to do up this bike because I want to ride and they cant give me a date on me receiving this manual. I'm looking for an original 1983 Suzuki RM 125 manual. I don't trust CLYMERS manuals because I'm an ASE Certified Mechanic and I've run into bad luck with some of their info. They try to incorporate too many types of bikes in the same repair and info gets squirelly. Plus what you need info on isn't there,like the inportant stuff. If you also know of a reliable web site or address to someone who can get parts for a 1987 YZ 125 or my RM let me know also. Please help I'm kind of impatient like I said. I hate to see this bike sit in pieces any longer.

Sorry, I don't have lists of OEM specs on all dirtbikes. You're either going to have to wait for the manual or try to talk someone at your local dealer into digging for the info. Clymer manuals can be a source of decent info if you can sort through the omitted parts for individual bikes. The RMs of that era were all very similar so I'd think a Clymer manual would be acceptable. If you're ASE certified (I'm an ASE Master myself), you should be able to tell the good info from the bad.

The only way to tell if the bore is faulty is to accurately measure it with a bore gauge. The scalloping at the port edges may be the way the ports were chamfered when the sleeve was installed. The lack of the crosshatch wearing along the sides of the cylinder is relatively normal since the piston rocks front to back and not side to side.

Concerning websites for parts, you might try doing a search using "Suzuki parts" or "Yamaha parts".

MX Tuner

I Was Hoping I Could Get Some Jetting Expertise.
I have a 2000 kx 250. I have an FMF fatty pipe and a Boyesen Rad Valve for aftermarket performance. I ride mostly in the desert in Phoenix, but I occassionally go up to Flagstaff, AZ which is at 7,000 ft. I can't seem to get the jetting dialed in up there. Stock jetting is a 160 with the clip at the 3rd groove. The last time I was up there I tried a 154 with the clip at the top groove of the needle to lean it out. It's running decent, but I think bottom end could be better and the plug is still pretty wet when I check it. Could you tell me what would be a good jetting setup at 7,000 ft and also at desert level in Phoenix. Last time out in the desert I ran a 158 and put the clip back in the 3rd groove. The plug was still wet. I'm just concerned of running too lean and doing engine damage. Should I change the pilot jet? I bought one down from stock and haven't tried it yet. Would that give me better bottom end. Any help would be appreciated.

Thanks, James

You're trying to get jetting that works at both altitude extremes by only changing some of the jetting. You need to get the jetting correct (all carb circuits) at the lower altitude. Then go to the higher elevation and correct the jetting there (all the circuits). You really can't expect it to run well by only changing some of the jets. First jet that needs to be changed is the pilot jet since it is the lowest speed circuit. Next adjust the needle position. Then change the main jet.

We run into the same problem running the Vegas to Reno desert race. We start out at Las Vegas altitude and go all the way up to about 6500 feet in Reno by the end of the day. It can be a jetting problem, to say the least. At least you have time on your hands to alter the jetting. As the race goes higher up, you can definitely hear the bikes running richer and richer.

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Mile High KX 500 Jetting
I have a 2001 kx 500 and ride most of the time between 4500 and 5500 feet elevation I have a problem with the bottom end running too rich.I am using a 165 in the main and a 52 in the pilot. do you have any sugestions for this bike? Thanks in advance.


If the bottom end is rich, lean the pilot circuit. One other thing that can help significantly is running a hotter spark plug. The stock spark plug is capable of running in the desert at continual high rpms. If you're running in the mountains, chances are you can get by with running at least one heat range hotter than stock. Turning the idle speed down lower will also allow you to run a leaner pilot jet and get much crisper throttle response. I don't have specific jetting numbers but it seems a 52 pilot is a bit rich for that altitude.

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Round Allen Bolts
hello there my name is Aaron i own a 1989 ktm 125. The bike runs fine. On the left bottom and the right bottom of the engine there are drain plugs for me to drain the crankcase oil, the bolts that i need to unscrew are hex or allen wrench bolts, i cant unscrew because they are stripped or because i have the wrong size wrench but they are like a circle instead of angles, how long can i keep oil in there without it messing up the bike???

Normally, you should change the transmission oil every few rides. Fresh oil is cheap insurance for gears and clutch plates. You might try getting a standard size allen wrench and tapping it in place. This may fit tighter and allow you to break the drain plugs loose. Before you try to tap the allen wrench into the plug, tap on it with a hammer. Give it a few good whacks, but just don't break the cases. This tapping can take some of the tension off the threads and help make it easier to loosen.

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Hard Starting 500
I have an 84 CR500 that is incredibly hard to start. By the end of the day I am barely able to start the bike due to a soar foot. ( I do have proper boots ). The top end has been recently rebuilt. Do you have any suggestion that would make this bike easier to start.

I also have and 81 KDX that spews black sludge out the silencer no matter what oil is used. I have been using a 30:1 mixture. Any suggestions

Thanks - Denton

Even with all things being in good condition, your 500 may be a handful to start. I've seen some CR 500s wear the kickstart pivot so the lever points down just enough that you can't get a good healthy kick. One other trick is to lay the bike over until gas pours out the carb over flow hoses. This will give it a richer mixture for starting. The mid 80's CR 500s were brutal to say the least. You might try adding a base gasket or two to lower the compression a bit. This can help make it easier to start also.

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ATC 250
Hello I own a 1984 atc 250r I just reworked the top end 20 over with a wiseco piston The bike will start up first kick runs strong pulls front wheel off the ground first three gears Will run excellent till what it seems like it gets warmed up then it starts bogging and sometimes will shut down. Sometimes you can work the throttle and keep it going (barely) I had one guy tell me it might be the stator coils could this be possible it runs so good when it is cold and sits there and idles like a baby

Thankyou - Larry

It could be an ignition issue. Electrical components can function normally when cold and break down when hot. I think I'd make sure it isn't something fuel mixture related first, though. If it is too rich, it may do something like you describe. Make sure all the air passages in the carb are clear. Check the obvious. Is the air filter clean? Check the exhaust also. Is the pipe dented or the baffle restricted?

Be careful checking electrical components. The manual will give you symptoms to check between certain wires for each component. Use a grain of salt when checking. If a component gives a value of say, 200 ohms and you get a reading of 220 ohms, don't automaticaly think that is your problem. It may not be exactly wha the manual says it should be. And on the other hand, if the part does measure exactly within specs, that doesn't' mean it is a good working part. I know this doesn't help much. The best way to diagnose electrical components is to swap them with a known good part.

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Burning Out Bulbs
hello i own a 1990 rmx 250 my question lies in the electrical system. when i bought the bike the head light was missing and i bought a after market one. acerbis.but every time i hook it up there are three wires there one gray and two black witha white stripe it blows out the bulb.

i hook it up using the blk/wht as a positive and the gray as a neg and have taped of the remaining blk/wht wire as i see no place fror it to go. i have the factory service manual but it doesnt go into wiring to that extent when i first start the bike the light is super brite then blows out and i'm tired of replacing bulbs.what gives??????

thanks in advance. Frank

It sounds like the regulator may be missing. The headlight has wires for a high and low beam. Look at the back of the socket and pick which two you want to hook up. You can buy an aftermarket voltage regulator for about $15 through Tucker Rocky or Parts Unlimited. It will have a bolt that mounts it to a good grounded point (the frame with the paint cleaned off) and one wire that connects to the hot wire going to the headlight.

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Out of Gear
Mark, I own a 2001 RM 250. I race in a local harescramble series and GNCC. The bike has been jumping out of gear for a month or so, especially at a higher rpm. Then it locked up between 2nd and 3rd gear. We split the cases and found a slightly bent gearshift shaft and one of the two screws that hold the gearshift cam stopper were loose. All other gears and forks have some wear but all the spec's are in line but close to the limit. Am I missing something ? I don't want to replace parts for the sake of not knowing. We use NAPA oil 10/40 or 20/50 in hotter temps. Change atleast once a week.

Is that enough?

Crank bearings look good but should they be replaced since the cases are split? I am replacing the transmission bearings. Thanks alot for any help you can give,


If the detent cam wasn't working 100% correctly, that could be the whole problem. If the shift fork or shaft were bent, that could also give the same symptoms. The thing to look for on the gears are the corners of the dogs (the teeth on the sides of the gears that mesh with the corresponding gear to the side of it). If these get rounded, it can pop out of gear as soon as a strong load is put on it. The more the corners get rounded, the less of a load it takes to pop the gears out of place. If the shift fork ends have any wear on them that can be felt at all, they should be replaced. If the fork shaft is bent, it too needs to be replaced.

Your choice of oil shouldn't make it pop out of gear. It might contribute to a notchy shifting transmission but it shouldn't be causing any problems since it falls within what Suzuki recommends. You might consider trying Type F automatic transmission fluid for a smoother shift and better clutch action. As long as you change whatever you use once a week, you will be fine.

The crank bearings should be replaced (along with the crank seals) since they have had the stress of being installed and uninstalled placed on them. They would work fine for a while but will wear prematurely due to the side loads put on them during assembly, disassembly and reassembly.

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My sons 1989 YZ 250 is really loud. Can you tell me how to re-pack the muffler?

Thanks for your help.


You have no idea how nice it is to have someone ask a question like that. I wish most of the 50cc riders parents would ask that.

Once you remove the silencer, there will be some fasteners at each end of the silencer. These will be either screws or rivets, normally 3 around the sides. If they are screws, you're battle is half over. If you have rivets, these need to be drilled or ground off. Tap the remainder of the rivets into the silencer body. If you have rivets, you'll need to re-rivet the thing back together. Rivet guns aren't expensive and should come with a small assortment of rivets.

Getting the core out of the body can be a bit difficult depending on how long it has been since it was last repacked. Try holding the pipe going into the end and tapping the mounting bracket on a sturdy object. I use my vise to tap the mounting bracket on. Once you get the inside core out of the body, remove the old packing from the core or the inside of the shell. Use some type of solvent (aerosol brake cleaner works well) and clean out the oil residue out of the shell. It doesn't have to be spotless, you just want to remove the stickiest mess since you'll have to slide the new packing and core in there and the old oil won't make it any easier.

Once you have the core in hand with no packing on it, you need to clear any obstructed holes. The core is a sheet of metal that has small holes in virtually the entire core. These holes can get restricted, or even completely plugged, with a carbon residue. This needs to be cleaned out of every hole. Yes, every hole. One at a time, if necessary. Find something that you can poke through the holes to clear them. If these holes aren't cleaned out thoroughly, the silencer loses a percentage of it's efficiency. This can also prevent it from running as well as it could, as this affects low rpm jetting.

Once you get all the holes clear, cut a section of the new packing the same length as the core. Go ahead and get the correct type of packing from a bike shop. Household insulation can work but won't last nearly as long. The real stuff isn't expensive so it just isn't worth the savings to be a cheapskate. Get the stuff that comes in sheets. I usually get either the MSR or Tucker Rocky stuff that comes in green sheets. The "bag of string" stuff can send you to the mental ward trying to get it to go in to the silencer body. You need to wrap the core completely with the packing as many times as the sheet allows. You want it firm but not too tightly. It should cover the entire core from one end to the other right up to the end cap. You can wrap the packing with thin wire or masking tape to help hold it in place as you slide it back into the body. Make sure you don't get the one side upside down in relation to the other, or the mounting bracket will be on the wrong side. The bolt holes may dictate which way it goes together. As you slide the core into the body, you'll need to line up the other end as it goes into the outlet. You can usually feel it by sticking a finger in the outlet end of the silencer and guiding it into place. When you just get the end cap about to slide into the body, smear a thin layer of clear silicone sealer around the surface that slides into the body. This will help prevent any spooge from leaking out the seam.

Reinstall the bolts or re-rivet the pieces together and you're done. Another layer of clear silicone sealer around the silencer where it goes into the pipe will help prevent spooge from leaking at that joint.

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I have a 1994 kx 80 and the motor will not rev up. I had the top end rebuilt do to severe cylinder scoring. I had the jug resleeved and put in a new piston. I bought the bike with this problem. I thought maybe the top end rebuild would fix the rev problem. When I hold the throttle open, it sounds like it has a rev limiter.

when I ride it there is not top end (sounds like it has a rev limiter). The motor starts and idles great. Carb. seems to be in excellent condition. I tried changing the timing and it doesn't make any difference. Could it have the wrong electronics? The electronics seem to be the proper ones. Everything looks like it fits properly. The color coding is the same throughout. Any ideas or suggestions to try?

Thanks, Rob

A couple things come to mind. If the piston was installed backwards, it might give these symptoms. If the reeds aren't sealing against the cage properly, it could give some unusual running problems. I have heard of a CDI box that isn't advancing the timing correctly but I'd think that changing the timing would at least change the symptom. Is the exhaust restricted?

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I have a 1993 Kdx 200, and am having problems with 2 things: When it sits in the garage, fuel leaks out heavily, even with the petcock turned "off". It also bogs when the throttle is opened all the way. I know the silencer is in rough shape, a couple bolts came out of the side of the silencer, but I cleaned the entire carb. Any suggestions for a dirtbike half-wit?

The petcock can leak fuel even if it is turned off. The carb float needle may not shut off the flow of fuel through the carb if this is the case.

If the power valve isn't opening correctly, it can give a high end bog. You should be able to remove the power valve linkage cover on the right side of the cylinder and watch the linkage move when you rev it up. If it won't move, the valve is stuck and needs to be removed and cleaned. There are small aluminum teeth that operate the valves and these teeth can shear off over time. This is relatively common on higher mileage KX/KDX models.

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Stuck Impeller
I have a 1997 Kawasaki KX 125, and I blew a water pump seal a week ago. I have fixed it before, but this time i had trouble with getting the impeller off. The first time I fixed it, i wedged something in there to stop the impeller from turning, so that I could loosen the bolt holding the impeller (which is aluminum), and it worked. But I tried to do it again, and I ended up cracking one of the impeller "fins" while trying to wedge something in it to stop it in oder to turn the bolt. I have already ordered the new impeller, but I was wondering what is the best way to get out the impeller with out damaging it or anything inside? do they people at the shop use an impact wrench? If you know of a way to get it out with out harming anything, I would greatly appreciate it if you could tell me.

thank you, David

You should be able to hold the impeller shaft from the other side of the cover, where it meshes with the primary drive gear. Any time you replace a water pump seal, it is generally a very good idea to replace the bearing, the inside seal and sometimes the shaft. This requires removing the entire right side cover.

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I just bought a 2002 KX 125, and the same day I got it, it started leaking oil out of the tailpipe. I havent had the time to take it back to the dealership yet, but I was just wandering if that was something small I could fix at home, or if that was a major problem. Thanks for your time.


Hey Rob, I ride out at Bremen fairly often. Nice track. The oil leaking out the silencer is the low speed jetting a bit on the rich side. That, and if it isn't ridden aggressively can make it spooge worse. The higher rpms the bike is run at, the less it will spooge. But I'd start out with going to a pilot jet one size leaner than stock.

I'd recommend a synthetic or a semi-synthetic two stroke oil. Stay away from castor based oils (Maxima 927, etc) and stick with something that burns cleaner. Yamalube 2R, Honda HP2 or Golden Spectro are a few good choices that are readily available.

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Throttle Stop
I own a 2001 Yamaha WR 250F and have been told that the bike does not get full throttle. I have been told that by removing the small allen screw on the carburetor and cutting off the end that this will allow full throttle. Is this true ? How much do you cut off the allen screw ?



I remove the screw entirely and plug the hole with a dab of silicone sealer. When the WR 400 first came out, people were saying that you could run the throttle open too far and the bike wouldn't run right if the stop was removed entirely. I've done it both ways and can't tell a difference. My WR 450 has no throttle stop in it at all.

Your 250 is a great bike. I've built a couple with a 2mm overbore. Makes it about a 263cc motor and gives it a huge boost in power everywhere, especially top end. The only downfall is that it raises the compression to 13:2 to 1 which requires a steady diet of race fuel. A White Bros E-Series exhaust works great with a stock or modified motor. It is a bit louder than the stock pipe (even with the baffle removed) but helps immensely.

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Current scenario: 2001 CR250, Las Vegas (2000-2500 ft., 95-105deg.) 400 main jet, 6BEH1-76 needle w/clip in 2nd from top pos., 27.5 pilot jet. Boyesen Rad Valve, FMF SST and Shorty, Boyesen power valve cover. NGK B8EG. Red Line 2 stroke oil mixed at 40:1 with 76 "race" gas. Mostly desert riding, some track.

Q. I feel I'm pretty close to correct jetting, however, I still foul occasionally. Usually when riding slow techy areas or down out of the mountains at partial throttle. Throttle response off idle and partial throttle is excellent. I still get "some" spooge at the back of the pipe after a long ride. I am considering using Engine Ice coolant due to their claimed drop in engine temps. Do you have any experience with this product? Does using race gas (76 or VP) make for a cooler running engine? Given this, I am planning on going another size smaller on the pilot jet and maybe opening the slide cutaway by two mm. Do cooler engine temps. encourage plug fouling? Should I use a hotter plug or keep jetting leaner till I get indications of performance drop? Which coolant brands are best for aluminum?

Great column. Thanks for the info.


The 400 main jet sounds a bit rich. But your symptoms do indicate the pilot might also be a tad rich. Usually, your off-idle throttle response will suffer if the pilot is too rich. But then again, with all your mods, especially the reeds, you'll need to go leaner.

On the subject of coolant. Engine Ice is okay if you really are running too warm operating temps. This is where good marketing comes in for them. Yes, most of the factories run some type of fancy coolant but they have motors that are as hopped up as they come. A highly modified motor usually needs some type of added cooling help. But most all stock compression motors don't need that unless they are run at very low speeds (tight woods, etc). This goes along with the hotter plug issue. The hotter plug helps keep the combustion temps up closer where they need to be to adequately burn the fuel/air/oil mixture. If it doesn't get hot enough, spooge occurs. The last thing most people need is cooler engine temps. One of the mags recently printed something along the lines of "you can't have an engine that runs too cool". Nonsense. You want an engine to run a consistent temperature. Not too hot but not too cool either. In your case, I think I'd concentrate on getting the jetting sorted out to control the spooge issue rather than use a hotter plug. The hotter plug would help in the mountains but the first time you get into a long sand wash wide open for a minute or two, combustion temps will go up higher than ideal.

Normal coolant is ethylene glycol. Amsoil sells a high performance coolant for something in the neighborhood of $25 a gallon. This is propylene glycol and is very good coolant. You can get the exact same stuff at Wal-Mart for about $6 a gallon under the brand name Prestone Low-Tox. Yup, the same stuff that is safe for pets is the hopped up stuff good for your race bike.

MX Tuner

Project Bike
I am currently looking for a CR500 motor for my project bike. It is a road racing project and I wanted to know what motor would be best for this appication. I have heard that the pre 90's motors are stronger and that the 85-88's have a wide ratio transmission. Do you have any info on the motors as far as which one would be best?

Thanks, Chuck D

The mid '80s CR 500s were brutally strong. As time went on, Honda made the bike more user friendly by making it easier to ride fast. This meant taming down the power delivery and altering the gearing. But I wouldn't eliminate using a newer year motor because of that. The older ones were harder to start and harder to tune because of the state of tune designed into the motor. A newer motor can be ported to suit your needs better than an older one can. In fact, I'd think a newer motor could end up a better end result than trying to turn a mid 80's motor into your project. I don't have any gear ratio info. I know the Honda service manual is very thorough in covering quite a few years, including specs for each model. Internal gearing could easily be changed since the cases are virtually the same for all the later (90's) years.

MX Tuner

CR 500 (89)
I play on hills and trails with a little track play. The 500 is a beast.

A year or so ago my friends (who race) talked me into letting them dial in my bike. They switched from red line oil to Dumonde Tech Oil. Rejetted for the synthetic oil put a procircuit pipe on it and dialed it in. It was so fast and had a top end that was pipey like a 125. It flew up the hills. The problem: It fouls plugs every ride. It's too hard to stay on the pipe. It won't idle, dies on all down hill trails. I had a fatty pipe and had put a weighted flywheel on it so I could enjoy my rides. I'm 47 years old and just want to ride for fun. the oil mix is 400cc of dumonde tech to 5 gals. of gas. It ran great for the first month but has been frustrating with each ride. I'm thinking of putting a gnarly pipe on it, rejet it use redline oil again (it never fouled a plug) . Does it sound like simple jetting problem or could there be something else going on????? Please help me enjoy my bike again yet still have the power to climb and fly around the hills Thanks.


I have no experience with Dumond Tech oil. Then again, I have no experience with the Red Line oil either. Honda HP2 is one of the best (and readily available) oils out there. It sounds like they jetted the pilot circuit richer. This would account for the plug fouling. If they took the flywheel weight off it, put it back on. I found a 15 ounce weight helped the CR 500 in every way imaginable. I wouldn't think the PC pipe would take away from the bottom end power. I'd switch to the FMF last if everything else didn't return it to it's previous personality. The jetting can totally alter the power output of your bike. Definitely don't overlook that.

MX Tuner

Next Bike...
My son is 13 years old and has been riding a little suzuki 80 for about two or three years. I'm thinking about getting him a four stroke for dependability. which is the better bike, dependability,performance and fun for riding hills and playing on trails (no racing)? I am looking at the XR 100 and the TTR 125L. Thank you for your help. Bobby

Either bike is a proven product. The Yamaha would be a little more capable bike for someone who is big enough for the bike. The additional 25ccs helps along with the frame and suspension that is geared for a little larger person. I know of quite a number of adults that ride TTR 125's and ride them hard. They are holding up unusually well considering the abuse they are put through. Don't get me wrong. The XR is a tried and true proven product also but I'd think the TTR would be a better bike to ride as his experience grows.

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Failing 50
hi, i have a little 50cc dirtbike that i'm trying to fix. For some reason it's not running very well. It takes a long time to start, and when you slow down the engine stalls.

i'm trying to learn how to fix dirtbikes and i thought this way would be the best,by starting on small dirtbikes to learn, and i was just seeing if you had any good ideas on how to learn about fixing some. And if you had any idea what is making it do this, please tell me how to fix it. thanks!

A shop manual is the best place to start. It can give you a huge amount of useful info. Most have a good trouble shooting section that can help immeasurably.

MX Tuner

Oil In Coolant/Bent Radiator
I own a 1999 YZ I have 2 problems

  1. After riding it for a month I notice a very little drop in the radiator fluid. If I look carefully at it I can see little black spots that look like oil. I flushed that radiator once but it didn't seem to help. I am not sure what way to go. It only loses a very small amount of fluid. I don't mind adding a drop of coolant every once in a while if its not hurting any thing. I change the oil a lot and never see coolant in it. What do you think I should do?

  2. I had a little fall and bent the left radiator a tiny bit. I went to get a new one and they wanted $207. Is there a place to get them repaired?? Thanks for ever bit of help you can give me!!

Dave from New York

Keep a close eye on the condition of the transmission oil. If it starts to take on a milkshake type look, you probably have coolant getting in through a faulty water pump seal. Otherwise, a head gasket (o-rings) could give the same symptom you have. Even if the bike runs fine, you might still have a small leak at the inner o-ring that seals the head and cylinder. If the trans oil looks okay, you might want to pull the head to inspect it.

If you find evidence of a leaky o-ring, I'd suggest resurfacing the head. This is easy to do with a flat surface (finished marble, glass, etc) with a sheet of emery paper taped to it. Spray the emery paper with WD40 and slide the head along in a figure eight, applying even pressure. You'll be able to see where the aluminum is being removed. Continue until the wear marks are even across the entire head sealing surface. Install new o-rings and you should be good to go.

MX Tuner

1994 Kawasaki KX125
Hi, I just purchased a 94 model KX125 and it came without the owners manual.

I've never owned a dirt bike (or road bike for that matter) and I'm unsure as to what the fuel/oil ratio would be for a dirt bike or this one in particular if different models have specific ratios.

Thanks alot,


1991 KX-80
question: what 2-cycle oils can I use ? does it matter? how about pennzoil premium out board multi purpose oil? what does the tc-w3 rating mean? should the mixture be 40:1


I'm going to kill two birds with one stone here since the same answer applies to both questions.

You could use the outboard motor oil but I'd suggest an oil designed for the rpm range of your bike. The twc-3 rating is a rating system for outboards concerning oils. The twc-3 rating is about the most common. Golden Spectro, Honda HP2 and Yamalube 2R are all very good oils and are available at a large number of dealers everywhere. The 40:1 mixture ratio will be fine for most oils you find.

MX Tuner

Cold Starting a WR
I have a yamaha WR400F and i just have one question about it I can't seem to get it started from a cold position i can push start it and run it for a while and then stop it and try to start it again and it will start right up but it won't seem to start cold i tried choking it and even used the hot start instead but it just won't start what should I do???

I'd start out with pulling the carb to do three things. One, check the float level. Two, clean the accelerator pump linkage. If it hasn't been done in a while, the linkage can, and will get dirt in it that will prevent it from moving. This also prevents you getting the priming squirt when you twist the throttle as part of the normal starting drill. Three is setting the fuel screw at 2 turns out. This should be in the ballpark for decent cold starting. This adjustment can be done with the carb on the bike but it is much easier with the carb off.

MX Tuner

Jetting for FMF
Hello My name is Alex and I recently purchased CR500R. Last week my father bought me a new FMF pipe and silencer. I was wondering if you had any jetting specs. The FMF recommendations were not to useful. It has a stock carb and reed valve.

Any tips would be helpful.

Thanks Alex

No, I don't have any specific numbers. Specific jetting specs vary due to anything from local altitude to humidity to barometric pressure to outside air temperature. The best thing to do is jet your bike for your local conditions. Usually you can go down one size on the pilot jet for a pipe and silencer. You may be able to go leaner with the main jet also but I can't see anyone needing much more top end power from a CR 500. Eric Gorr has a very good jetting guide to help you rejet your bike. Go to www.eric- for more info.

MX Tuner

Shocking Seals! $$$
hi mark how are you good i hope. our family lives in australia and we own kawasakis ones a kdx 200. 97 model the other is a kx 80. 90 model we resantly had some bad luck when we whent to rebuild the rear shocks on both bikes.As our kawasaki dealers down here tell us the rear shocks seals only come as units on the shock shafts the price of these units down here is $689.00 the other is a lot more. As you can see this is a lot of money as we only need the seals we have been to every seal bearing & hydrolic shop and no one can match a seal im told they are hydrolic seals. will normal seals last . do you know anyone selling these seals.

I've heard horror stories about parts availability down under. See if you can locate a Race Tech distributor somewhere. Race Tech sells all the parts you need individually. Even if you find the parts, you still need to find someone capable of installing them. It isn't rocket science but it is very specialized work. Don't expect to be able to "match up" any seals. They are very specialized seals. The dealer can only get from Kawasaki what parts they carry. Kawasaki isn't the only one that sells shock parts like that.

If you can't find anyone locally, e-mail me directly at and we'll get a solution one way or the other.

MX Tuner

Shiftless When Hot
I own a ' 97 YAMAHA TTR 250. When is becoming hot it is almost impossible to shift gears. Is it a problem of lubrication or what? Kindly waiting for your guidance.

Either lubrication or the clutch adjustment. Make sure the oil is relatively fresh and at the correct level. The clutch cable should have just a little free play in it at the lever. If it has too much, or if the cable is stretched, it can give the symptoms you have.

MX Tuner

Carbon Build Up
My brother just got a 2001 yz 426. The bike is fast as hell but if he doesn't rev it the bike will die. Then when you try to start it again it will not start unless you put a new plug in it. We also noticed that the plug had a massive amount of carbon on it. The plug is NGK CR8E and we run 100 octane pump gas. My question is what causes this and what can we do about it? Thanks


Sounds like the fuel screw is adjusted too rich. Try turning the idle speed up just a little. The 426 is sensitive to jetting changes. If someone has been fiddling with things that are best left untouched, then you could have problems such as yours. You might want to check all the carb jets and settings (needle position, throttle position sensor, fuel screw setting, float level, etc) to make sure they are all stock. Rarely does a 426 require any significant changes in jetting.

MX Tuner

Flying Goo
Hi, I am new to this wonderful sport. I talked my dad into buying me a 2001 YZ 125. When I first got it I was running Yamalube at 32:1 and a BR9EG plug and wasn't fouling plugs for the first couple rides. Then all the sudden I was fouling plugs and I noticed a goo coming out of the silencer. I then changed the mixture to 40:1. I talked to my local dealer and they said that I wasn't revving it high enough and suggested that I didn't rejet it so when I got better it wouldn't be to lean. So I went and got a bunch of spark plugs and worked around the problem but I am still fouling plugs. I have gotten much better and I only foul plugs when "putting around." So my question is should I try cleaning the silencer? Or try a different type of plug? Or if you have any other suggestion? Thanks


The silencer may be saturated but you'll still need to lean the low speed circuit to cure the plug fouling. I'd use a BR9ES instead of the more expensive G series plugs. They are advertised as more foul-resistant but I've found exactly the opposite. Check some of the on-line jetting guides. There is a good one at

MX Tuner

Whirring CR 80
Mark, My son rides a 1995 CR80. The bike is making a "whirring" vibration noise when decellerating from high RPMs. This is the only time you will hear the noise (not while accellerating, at idle, etc). I've been told that the main bearing are going. If this is indeed the problem, is it a good idea to replace the crank at this time along with bearings/seals? Any inputs in this matter would be greatly appreciated.


If this is the case, I'd definitely look very closely at the condition of the lower end bearing. If there is any up and down play in the rod, the crank should be rebuilt. You can check the condition of the main bearings by removing the left side cover and trying to move the flywheel/rotor up and down. You may have to take a screwdriver to pry gently against the side case or the stator to make any movement apparent. You're not trying to force it to move so don't force anything. Any play will be fairly apparent.

MX Tuner

Gearing Down
MX Tuner, I just purchased a new 2000 KX125 and would like to gear it down. The front sprocket has 12 teeth and I was told that going to a smaller front sprocket (11 teeth) could cause too much bend in the chain. I'm limited in increasing the size of the rear sprocket by the chain guide. Is it true that going to a smaller 11 tooth sprocket on the front is too small?

Thanks, Russ

I don't know of anyone who makes an 11 tooth front sprocket. You can put a larger rear sprocket on. You shouldn't need to go more than one or two teeth either way on the rear to accomplish most anything reasonable.

MX Tuner

Power to the XR200! #2
Hey, I dont really have a problem, but more of a question. Currently I have an 1986 XR200 with a SuperTrapp. I've been using it about everyday, and Im starting to want more power and acceleration - lots of times riding up the mountain the bike struggles etc, and Im also quite fed up with drum brakes....but I dont want a heavier bike..... And the suspension is way too soft for jumps - it bottoms out all the time and my back starts to complain after a while.... Do you have any suggestions as to what I should get? Or should I mod my current bike? If so, what should I do?

Thank You - Andrew

Your XR is a good bike for what it was intended. Don't waste any money trying to modify it. What your choices are depends a lot on your budget. There are dozens of great bikes available. Take into consideration where the good bike shops are in your area and patronize them. Shy away from the guys who are only out to make the sale. Ask other riders to find out who the shops are who cater to riders instead of just the buyer.

Think about a KTM 200, a KDX 200 or 220, a WR 250, an XR 250 for starters. Generally you get what you pay for in new bikes. In other words, the more race oriented bikes will cost a little more but will be more worthy off-road.

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