Technical Questions? Ask MX Tuner

Jan. 01, 2002 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.

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Carb Change
I would appreciate some information please, I have a 1999 CR125 an was recommended as a tuning tip to increase the top end performance to fit a PJ 38 carburettor of a 1996 CR250 I have a PWK of a KX, what is the difference between the two apart from the cresent and oval slide. Jetting was also recommended #172 main, #58 slow and 1468 p-2 needle can these be used on the PWK or are different (or equivalant) ones required.

yours faithfully Sam

The two carbs you refer to are similar but do have different characteristics. You should be able to make your CR run well with either one once it is jetted. Plan on having to fine-tune the jetting regardless of what you're told to start with.

MX Tuner

100 or 125?
Dear Mark, I believe this bike is a 1980 Yamaha DT100 Enduro. The number on the frame is 3A3-040671. The local Yamaha dealer told me that this should be the model from the number on the frame. However the tank decal indicated that it is a 125. I suspect that the tank has been switched out by the previous owner(s).

I bought the bike after it had been sitting up outside for a few months and found that it had rain water in the tank and carb. The previous owner said it ran fine before he let it sit. I drained the tank and took off the carb and drained all water from the bowl plug and reinstalled it and it started fine with fresh gas.

The bike was responding to throttle and idling fine for the first minute but started to rev up uncontrollably and would red line if I didn't shut it down. I'm thinking this is a carb adjustment problem. Can you help steer me in the right direction?

Thanks for your help.


Check the fuel tank for rust. Remove the petcock and check inside for rust or some other obstruction. I'd highly suggest running an in-line fuel filter between the tank and the carb. Check the carb itself for a plugged pilot circuit or a float that is stuck shut. For one reason or another, it is running on too lean of a fuel/air mixture. It is probably a fuel supply issue, a clogged jet or passage or possibly even an air leak. An air leak could be around the carb to cylinder rubber boot (look for cracks) or even a left side crank seal (under the flywheel).

You can look at the base of the cylinder for a casting indicating what the actual displacement is. Look at the side of the cylinder for either a "99ccm" or a "124ccm" for the size of the motor.

MX Tuner

Can I repair my CDI?
Hi, I have a Husqvarna TE350 from 1993 year. My CDI, it?s doesn?t work. Do you know if i can get the circuit to repair myself ?

Really thanks - Hernan.

I've never seen any diagrams from any manufacturer that shows the circuitry inside a CDI. The only other option besides a factory replacement (new or used) would possibly be an aftermarket ignition like a Wolf.

MX Tuner

YZ400F Won't Idle...
Dear MX Tuner,I have a 1999 YZ400F that has just developed a problem with not idling. It started with my last trail ride and basically it won't idle after I warm it up, it runs fine otherwise. Here are the things I have tried:

  1. I adjusted the idle speed up. This seemed to help, but then the bike acted like the throttle was sticking. It would idle high, then when I whacked the throttle, the idle would drop and the bike would stall. Going with the throttle sticking theory, I lubed the cables but this didn't cure the problem. I inspected the idle speed mechanism and it looked like it was working as it should. Adjusting the screw would move the throttle stop in or out as it should, so no problem there. The throttle cable does not stick either, I made sure everything works as it should.
  2. Thinking the air filter might be clogged (it looked fine, and I just serviced it, but I've read that if there is too much air filter oil in the filter, it makes for hard starting. It was an easy test and thought doing the easy stuff first would be the smart thing to do), I removed the air filter and started the bike in my garage and the bike still stalled.
  3. While revving the bike in my garage, I noticed the head pipe was glowing red, is this normal? I was working at night, perhaps I never noticed the head pipe glowing because I always run the bike during the day. Then this got me thinking, maybe the idle circuit is clogged and the bike is running lean at idle, so off came the carburetor. I pulled the float bowl off and used compressed air to blow through the jets and put it back together. I started up the bike, and it still stalls.
  4. After thinking about this some more, I'm convinced that there still must be some junk in the carburetor that I didn't get at. So I'm going to take the carburetor apart again and make sure that every passage is clean, paying particular attention to the idle circuit. I've got the manual and there is a good explanation on how the carburetor works and the different circuits, so I should be able to pin point where to pay careful attention. I'm also going to install an inline fuel filter so this won't be a problem again.

With this information, do you think I'm going about this correctly? The bike ran fine before, so I can't believe it's jetting issue, plus the problem only happens at idle. It's been cold and wet up here in Northern California, maybe in the 40's, but I wouldn't think that would have much to do with the idling problem.

Thanks for you time, hope this wasn't too lengthy, just wanted to give you all the facts!

L. Johnson.

Thanks for being thorough with what you've done so far. I'd much rather have too much info that not enough.

The head pipe will turn red after a short period of idling. That is normal and doesn't indicate a problem. Just don't let it idle for that long.

Sounds like you're on the right track. Check one other thing while you have the carb apart. Look for a cracked slide. Otherwise, I agree about it not being a jetting issue since it has run fine before this. If it was a problem due to the air temperature, a slight adjustment of the pilot screw would cure the idle issue.

MX Tuner

how do i set the valve timing on a 1979 honda x/r 250 dirtbirt, please explain this procedure to me

thank you - rob

Step one is go to your Honda dealer and buy the factory manual. There is way too much info in that book to not have one if you're into it that deep.

MX Tuner

Oil/Fuel Ratio
i have a 1980 rs175 (suzuki). And i was wondering what the ratio for mixing gas and oil is.

thank you vey much

Your Suzuki will do nicely run at the ratio of 40:1. Use a good quality two-stroke motorcycle oil. I'd stay away from outboard motor oils.

MX Tuner

Bogging CR
hi mark, well i need some help on a 1993 cr 250. i told my buddy to buy it and i was going to fix it, but i suck! i thought 700 was a good buy. anyway,this bike starts up and runs good about 1/4 throttle after that just boggs.we have put reeds(they were broken), resleeved (my buddies choice), piston ,rings,new crank bearings and all new gaskets and seals.well it runs exactly the same the first day we started.i tried jetting before the rebuild and recently tried a carb. off a bike of the same ran no different.i wish i could of tried the electrical but it was a"noway".this bike has seen a lot of use by the looks of the cylinder and bearings,but everything is new except for crank and rod.what im thinking now is cdi or stator.i was told a stator has a high and low speed coil,this one looks like one coil so i dont know.would the bike start with a bad cdi?can any of this stuff be tested ? i know nada about electrical components,so any help would be greatly appreciated. thanks

Resleeving is one of the worst ways of repairing a plated cylinder. You can get a damaged cylinder repaired for less than the cost of either a replacement cylinder or a resleeve. Resleeving alters the heat transfer characteristics of the motor.

I'd check the power valve to make sure it is operating correctly. On the left side of the cylinder (as you're sitting on the bike) there is a round cover (it has a 17mm bolt head). You can remove this cover and watch the end of the power valve move as you rev the motor. It should begin to rotate at about 4000 RPMs or so. Basically, just check to make sure it is moving. There is an "L" and an "H" mark that the slot in the end of the power valve will line up with. "L" is the low rpm position and it should rotate to line up with the "H" when it is revved.

MX Tuner

AVGAS vs. Race Fuels
I ride a '93 WR250. The motor is stock, but the bike does have a pipe, Boysen Rad Valve intake, and is jetted accordingly. My question is, what affect would running avgas, 100 octane low lead (100LL) have on my bike, or any 2-stroke for that matter. I know Race fuels like VP & Trick contain some lead, and Avgas is a very high quality fuel, the only major difference I know of is in its refinement it's made to resist vapor lock at high altitudes. Have you ever heard of anyone experimenting with it? I'm just concerned about how it mixes with the oil. Since I'm not running a high compression motor, anything over 100 octane would be a waste of money anyway. I'm just trying to find a cheaper alternative to race gas because this 91 stuff we have here in California ain't cuttin it and I can get 100LL easily and it's like half the price of race gas. Any information would be much appreciated.

thanks, Darin

Avgas won't give any performance gains over premium pump fuels. It's almost a trade off since it isn't formulated to run at anything similar to a two-stroke dirt bike environment. The octane rating is figured differently than pump fuel. An airplane engine is meant to be run at a much more consistent rpm and at a lower rpm range also. That being said, it won't harm anything in your motor and may be a decent source for a consistent higher quality fuel if you're having trouble getting good fuel from local stations.

MX Tuner

Pipe for Honda Rancher
I am looking for a complete upgrade exhaust for my 2002 Honda Rancher 350 ATV. I know about the Supertrapp IDS2 but prefer a pipe as well. Do you know if any one has or is coming out with a system??


I'd check with White Brothers and see what they have available. They have a wide variety of applications and are some of the better designed and built systems available. They have disc type exhausts similar to a Supertrapp.

MX Tuner

Suzuki 97' RM125
Hi I bought this bike about a year ago, and when I pulled the drain bolt out of the crankcase to change the gear oil, I noticed a small crack, over several oil changes it started to leak more and more, so I put JB WELD all around the crack, now I have decided to sell the bike and buy a yamaha 426, I plan to sell the bike for about $2350 is that a unreasonable price with the problem, and should I fix the problem before I sell it, and is it even that big of a deal?

Thanks Kyle

I have had limited success having the drain plug casting welded. It can be difficult to weld since the metal needs to be clean and oil-free. Another thing that may happen is a leak may develop at the seam between the cases due to the extreme heat causing expansion and contraction during the welding process. This is a risk you need to think about. If you don't repair the case, you should to disclose this to any prospective buyer.

MX Tuner

XR Stops...
I bought a used '94 XR250. After riding trails for 3-4 miles at moderate speeds, the engine will abruptly stop. It acts as though it is out of gas or losing spark. It will usually restart in 2-3 kicks. Occasionally it won't restart immediately but if I wait 5 minutes or so, it will usually restart in 1 kick. After restarting, it will usually only go 1/4 mile or less(sputtering some) before stalling again.

A mechanic friend of mine has:

  1. checked the vented gas cap for blockage
  2. replaced the stator
  3. checked the spark plug for spark
  4. completely cleaned out the carburetor

What should we look at next?

Kind Regards - Ed

The only other thing I can think of that might give that symptom is the carb vent hoses. If they get clogged, they can do exactly that. You need to determine whether it is losing spark or not. This obviously will take you in one entirely different direction.

MX Tuner

Crank Bearings
Hello, My son has a 199 Kawasaki kx100. the crankshaft bearings have gone out already, we are running torco oil at 40:1, and the bearings Don't look like they are getting much oil, I have heard of another 99 kx 100 that had the same problem. is this anything you can help us with, Terry

Torco is not a bad oil. As long as you're running at 40:1, you should be okay lubrication-wise. I think I'd take a close look at the air filter and the way it seals in the airbox. If it sucks any dirt, that'll ruin crank bearings (among other things) quicker than anything else. A dead giveaway if an improper sealing air filter is traces of dirt in the boot going to the carb. It ought to be 100% spotless, otherwise you have trouble getting ready to happen.

MX Tuner

Blowing Spooge
I recently noticed that my motor was spitting a little oil out of the exhaust pipe. It was time for a tune up and I changed the piston, rings and all seals on my 95' KX 250. Everything seemed to go together smoothly and it is running great. I have been taking it easy breaking in the new piston but the bike seems to still spurt some oil. Is this normal? The plug seems to be black and a little oily. Thanks for your time!

The spooge is due to a combination of things. One is the jetting being too rich. You need to lean the lower speed circuits. Start by leaning the pilot jet a size or two until you get some eye opening throttle response. This will cut down on some of the spooge. The other thing you can do is run a BR7ES spark plug. This plug is one step hotter and will help reduce/eliminate the spooge.

MX Tuner

KLX650 Mods
96 klx650r aftermarket. looking for parts and info on my bike . love the bike but want more low endthrottle response .any tricks or kits for the cvcarb? or replacment.its pretty fast but want more thanx.

Check with Thumper Racing or White Brothers. Both offer aftermarket mods that will do good things to your KLX.

MX Tuner

Jetting Away From Seizure
I am fixing a 1982 honda 205R ,went through the carb aand don't have a manual can you tell me what the carb needle adjustments are or starting point adjustments would ,I was told you can lean seize the motor if not adjusted right

Yes, running a motor jetted too lean can cause a seizure. But this would be during a sustained high rpm run. This would be caused by a main jet that is too lean. A pilot jet or needle won't cause any seizure related problems. You can experience a bog that is caused by a lean condition but you won't experience any damage. A good starting point is the center position for the needle clip.

MX Tuner

Tweaking My 125
i have a 2002 honda cr 125, it has an fmf fatty pipe,fmf shorty silencer and a 53 tooth sproket. With these modifications is my bike still inferier to the yamaha and if so what else could i do to tweak my 125 in to a horspower god? and do you think the cr 125 engine is crappy or does it have serious potential?

Your CR does have the potential to run with most anything out there but it may take a few more mods to get there. You need to run an aftermarket reed cage. My choice would be a Delta V-Force. This reed cage just plain works and works great. The next step will be a good porting that can fine tune the power output to your liking. Don't go with one of the big aftermarket companies since you'll get a "cookie cutter" port job. Expensive and not tailor fitted to your needs. I highly recommend Eric Gorr. He is undoubtedly one of the best in the business. You'll get a better job at a cheaper price. One other option is a 2mm overbore. This will put it in the 133 cc area. Still legal for AMA amateur competition and a big gain in overall power. Eric can help with that also. Go to for more info. Tell him Tuner sent ya.

MX Tuner

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