ask - MX Tuner

Technical Questions?

Oct. 01, 2000 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.


Persistant Water Leak
Make: Suzuki
Model:RM 250

I bought my RM three months ago. When I first got it it had a water leak and my crankcase got water in it. So we replaced the mechanical seal on the water pump side of the clutch cover. But even now my oil still gets water in it. I let it drain overnight then put a fresh 800cc's of Schaffer's Micro Moly SAE 40 in. The radiator seems to stay at the same level, it might go down a mm I am not sure but I haven't had to add any water in ten hours. Where else could water be getting in? I am getting tired of changing the oil after every ride. Also I am wonder what Carb settings you recommend? I live in Kansas at around 1000 feet above sea level I don't exactly know what the pilot and jets are set at but I am pretty sure they are the stock 50 pilot, 320 main and the needle in the second position. The BR8ES plug looks all black and wet when I get done riding which definitely suggest way to rich but the bike seem to run alright on Amoco Gold 92 with 24:1 Maxima oil. It is a little boggie down low but has great throttle response in the mid and high rpms. I just rebuilt the top end also. Thanks for any help you can give me.

Topeka, KS

It may take a few oil changes to clear the transmission of all the water residue. Try using Type F automatic transmission fluid and changing it after about 10 minutes riding time. Do this a few times in a row to clean the water out.

I'd switch to a BR7ES plug and try a 45 pilot and lean the needle one clip position. This should get you going in the right direction. You may be able to go leaner than that with the pilot jet. You want to go as lean as you can with the pilot jet. It'll idle down slowly and it'll feel like it's running out of gas if you get too lean. You won't do any engine damage experimenting and will be rewarded with the best possible throttle response.

MX Tuner

High Idle
I have a 1987 KX 125 that I have just rebuilt. The bike starts and runs great with a lot of power, but it has a high idle that takes awhile to come down. After running it at any rpm the bike stays running at a higher rpm then finally idles down after about 20 seconds. I have checked the carb for leaks and made sure the pilot jet is clear. I have also checked and made sure that the throttle cable is free. The bike has a new top end with a Pro x piston, both power valves have been replaced and move freely. Could I have a problem with the reeds, the top set is tight to the cage, but the bottom has a gap though it falls within KX specs. Or do I have a case leak. I ride with the previous owner and he said the bike always did this. When riding in to a turn and braking hard the bike stays revving , giving you a dead spot when getting back into the throttle. I run Maxima 927 at 40:1. Any help would be appreciated.

First of all, lose the Maxima oil unless you plan to rebuild your top end once a month. Just because Ricky Carmichael uses it doesn't mean it's best for you. I don't recall which version carb your KX has. The idle speed may be incorporated into the choke knob. Where the choke knob comes up from the carb body, look to see if there is a small arrow cast into the carb. It should say "idle" right under it. Turning the choke knob will adjust the idle speed. An intake leak would be much more erratic than to control the idle that predictably. It could account for the slow idling down, but not the inability to lower the idle speed.

MX Tuner

Stiffer Shock Spring?
I recently bought a 2000 KX250. The bike was previously owned by a local Pro SX racer. Enzo Racing revalved the forks/shock but left the springs stock. The previous owner was 35 lbs lighter than me. I weigh 180 lbs without gear and I'm a beginner to MX.

The problem I'm having is that I bottom the bike frequently and on smaller jumps than I think it should. The compression clicker on the shock is set at 6 clicks out and the race sag is set at 98mm.

I think I need a stiffer shock spring - but I'm getting conflicting information on which spring I should use - 5.0, 5.2 or 5.4. I don't want to swap my current problem for a too stiff spring.

Do you have any suggestions? I'm currently thinking that I should start with a 5.0 and move up as needed. I'm wondering how much difference there is between the stock progressive-rate spring and a 5.0?

Thanks, Mark

I find it hard to believe Enzo would revalve the rear without recommending a straight rate spring. That is definitely the first thing to change. A 5.0 is what you want. You won't have to go up from there and the 5.0 won't be too stiff. It'll make a BIG difference. The valving may prove to be too soft for your weight. If the spring doesn't do what you want, you'll need to have the valving updated.

MX Tuner

Short Shifting an XR200
The bike is a Honda XR200R and is a year 2000. I was wondering why it won't power shift between 3rd and 4th gear.

Hope you can help, thanks.

Probably because it wasn't meant to. You might try a better oil. This can help shifting. Try Mobil 1 synthetic 15w50. This very good oil might help. Riding your XR 200 the way you imply will result in a short lived Honda. Remember what the bike was designed for.... NOT racing.

MX Tuner

Mods/Plastic - Killer CR 500
I just bought a -97 CR 500, and it's a killer bike!! But it looks old! Does the plastic from the -96 CR 250 or CR 125 fit the CR 500? (rear fender, sidepanels, airbox, seat, tank, and rads.) Or do I have to make some adjustment to the frame?

And what about bolt-on tuning parts, who makes them? (pipe, silencer, etc.)

And do you know any tech and tuning tips for the CR 500?

None of the plastic from a 125 or 250 will fit except the front fender. You should be able to buy all the plastic for your bike from aftermarket sources like UFO or Acerbis. Any number of companies make graphics that will help with the looks.

There are many things you can do to your 500 to make it either more powerful, easier to ride, or both. A flywheel weight helps the 500 hook up substantially. We put 15-ounce weights on them. Steahly makes a bolt on weight for a little over $100. A Delta V-Force reed cage is a great improvement to the 500. But until you've done the most you can with the stock bike, I wouldn't do anything. Leaning out the low speed jetting will make a big difference in your 500. That and some suspension upgrades will make a nice change.

MX Tuner

Jetting - 92 KX
I have a 92' KX 250 with tag bars, a FMF gold series pipe, and power core 2 silencer. I am a beginner in this great sport and need some help on the jetting side of things. As of right now, I have the stock reed cage with Boyesen reeds on the bike. I also have a Boyesen rad valve as back-up. Since I bought this bike used, I didn't get a manual with it and therefore I do not know the proper carb settings and jet numbers. (The local shop boys don't know them either). Can you please let me know the optimal carb settings for the stock reed cage and the Boyesen rad valve? this is the only Website I could find info for my bike on.

It starts fine, it gets into the powerband fine, but there is a little junk sound in the upper rpm range. The jet needle is in the middle groove and the airscrew on the outside of the carb is 2 turns out. I run hp2 at 32:1 with 94-octane pump gas. Is this normal? if not please help!!!

Thank You

Any Kawasaki dealer should be able to give you stock jetting specs. Boyesen should be able to give you ballpark jetting for a bike with their products. Boyesen does have a website at which can be helpful. I'm a little unsure of what you describe as a "junk" sound.

MX Tuner

Bucking XR200
I just recently got a Honda XR-200 and it starts and runs but when you try to ride it at about half throttle it seems to fall flat and seems to be starving for gas. If you pull the choke out half way it is a little better but still does not run with more than half throttle. If you twist more than half if bucks and the engine sputters and wants to cut out. It does not seem to have full power. I do not know a lot about the bike but I have pretty good mechanical skills. Can you suggest any thing to cure the problem???

Have you checked the basics? The air filter needs to be cleaned and re-oiled periodically. Has it been cleaned recently?

MX Tuner

Bottoming KX 60
I recently bought a 1998 KX 60 for my son he had been riding a PW 80 for about 4 months and was ready for a bike with better suspension and more power. He is 7 years old and weighs 65 lbs. The front shocks are air adjustable and seem to be too soft for him. It is hard to find anybody who really knows how to adjust them right for him. He rides pretty good and is bottoming out on the jumps at the local track A couple of the better riders at the track told me the bike needed to be adjusted for him. I am not able to find a good manual on that bike that is any help. Could you please give me some advice on the right way to make the correct adjustment

thank you

First thing you need to do is buy the Kawasaki manual. Get the one Kawasaki publishes for the bike. It is very useful. You won't regret it. I'd add air to the forks as a last resort only. You can add oil to the forks or you can change to a thicker oil. Adding oil will increase bottoming resistance in the second half of the travel. Changing to a thicker oil will make it stiffer everywhere (even over small bumps). First thing I'd do is have the forks serviced. This way you know the internals are clean, the oil is in good condition, and that the right amount is in the forks. Your new service manual will give a height measurement for the oil level. Set it in the middle of the range they provide. Then, if it still bottoms excessively, you can remove the fork caps without removing the forks and add a few ccs to each leg. This will raise the oil level to fine-tune the bottoming resistance. I'd add 5ccs to each leg at a time until it gets acceptable. You can buy an infant medicine syringe at any pharmacy for $2 that works well for this.

If this doesn't achieve what you need, then I'd recommend going to a thicker oil. This can make the forks harsh over small bumps but will help with bottoming resistance.

MX Tuner

Bottoming KX 100
I have a 2000 KX100 and my dad and I wanna stiffen the suspension but we really don't know how. The manual says what to do but it's too confusing! We were wondering if you have any easy steps to take. We don't wanna take it in because the place were we got it is just so busy right now!! Would it be too hard for us to do it? Also, I have some good size jumps in our pasture and because my suspension is loose would it be bad for the bike to be landing that hard? Could this cause the chain to jump off the sprockets? cuz that happened and I was wondering if jumping with the loose suspension would cauz it to do that.


How much do you weigh? I'm guessing since you just bought a new bike, it *is* big enough for you. You can get stiffer springs but you can try to adjust with what you have first. First thing is to adjust the rear spring according to the manual. Once you get this done, you can adjust the compression adjusters to help control bottoming. Turning them *in* will stiffen the damping. You need to become familiar with the manual and the adjusters to take full advantage of the adjustability of the suspension. You can also add fork oil to a higher level. This will help in the bottoming resistance.

As long as the chain is kept properly adjusted, it shouldn't come off regardless of the suspension. If you continually bottom the suspension, you can eventually encounter frame breakage problems.

MX Tuner

Stalling XR600R
Me and my buddy both bought 96 XR's in late 98. We both did the same mods. These included; putting three extra filtered 1-inch holes in the air box and removing the top cover, and placing a filter on it. Upping the jets, and removing the stock silencer, I put in the White Bros. replacement and he chose the thumper one. We have both had the same stalling problems. At idle it will die sometimes. We have both been religious about plug changes and valve clearance checks. My buddy even tried a hotter plug. The worst part is when you cut up traffic at a stoplight and it stalls before the light turns green. Please help.

Which jets did you replace? My guess would be the main jets. The main jets don't affect carburetion until over 1/2 throttle. Sounds like your pilot jet may be too small. You might be able to adjust this out with the low speed screw. I'll be very general. if the mixture screw is in front of the carb slide, it is a fuel screw. It controls a fuel passage. Unscrewing this screw will result in a richer low speed mixture. If the screw is behind the carb slide, it is an airscrew, unscrewing this screw will result in a leaner low speed mixture. I'd try richening the mixture and see if this helps. You may be able to get the stalling to an acceptable level without replacing jets. There's only one way to find out.

MX Tuner

Three for All
1989 CR500. A friend of mine has one of these monsters. Here's the problem. It was just rebuilt on the top end, including crank seals, piston, rings, and new crank bearings. The bike runs very crappy at low rpms. Then, when it comes into the mid-range, it runs like a raped ape. I'm thinking that the pilot and maybe the needle should be leaned out a little to maybe make the transition a little smoother. Right now, it's like a 125 with the hit of a 500. Oh, it does have an FMF fatty pipe on it.

second, I have an old ATC90. I just did a top-end rebuild on it, including new points & plug. Valves are set to specs. I set the points at .015. it runs very radical at low rpms, then, after you play with the throttle, it cleans out, but only at high rpms. I have cleaned the carb out with carb cleaner, but, I'm thinking that I didn't get all of the garbage out of it. Any suggestions on these two bikes?

I also have a friend who has a 98 or 99 KTM MXC 200. His son raced it the other day & we had to bleed the rear brake about three times that day between motos. There aren't any leaks. I noticed that the rear caliper, after he finished a moto, was super hot, as the burn mark on my hand can attest to. Is this normal for the rear caliper to get this hot? All we had was some DOT3 fluid to replenish the system. Thanx, The old "Stumpjumper."

Three for one, huh? Okay, I'm feeling generous.

The CR comes jetted too rich on the lower speed circuits. Keep the idle speed turned down as far as you can (off preferably) and lean out the pilot circuit. There is nothing finer than a crisp running CR 500. and, as always, be sure the silencer is in good condition (fresh packing and the core holes are clear and unobstructed).

The ATC 90. First of all, set the points to .012". Next, buy some Yamaha carb cleaner. This comes in a 1 liter black bottle. This stuff is great for soaking carbs in. Now strip every piece off the carb you can (except the choke flap) and soak it overnight. Then spray it out with aerosol carb cleaner. You need to verify all the circuits are unobstructed. You can do this with the aerosol carb cleaner. Be sure to wear safety glasses of some type. You *know* you're going to get squirted sometime during all this.

The KTM. The KTMS have sensitive rear brakes. Some say they are too grabby. Aggressive riders can easily overheat the rear brakes. First, make sure there is some free play in the master cylinder pushrod. It doesn't need much but it has to have *some*. Then bleed it with a good quality fluid like Motul 600. That should prevent the fluid from boiling during normal use.

MX Tuner

Sagging TC
I have a husky TC 610 '98 model . I recently replaced the fork seals and cleaned a lot of thick gunk out of the valves and shims .I tried to get it all as clean as possible inside, and then put 5 weight oil in the rebound fork and 7 in the compression fork... I'm having trouble now with the forks feeling very spongy and sagging too far. I think the sagging could be fixed with some new springs but I was wondering if there's anything I can do myself to fix the compression problem, short of a revalve, that is. Any info about these forks would be appreciated really...

Chris Edwards

Sounds like you have too low an oil level in the forks. Those should run a pretty high level. I'd run them at about 85mm.The oil level is measured with the fork fully compressed with the spring out of it. If the oil level is too low, it'll give the exact symptoms you describe. If something changed from before to now, a revalve won't address that issue.

MX Tuner

DT Straights
I just purchased a 1995 Yamaha DT200R 2 stroke street/trail bike with about 16,000 kilometers on its odometer. The bike works fine but it is very quiet. Is there any company out there that sells aftermarket exhaust systems for this bike??.. I searched the Internet with no luck at all.

I don't want to replace the expansion chamber, but the spark arrestor/silencer part is what I'd like to change. are there any two-stroke YZ parts that I could replace it with? Or perhaps could I put a straight pipe made of copper on instead of the second part of the exhaust? Also...would this make the bike's engine run hotter, possibly causing damage??

Thank you for your time...

The only aftermarket silencer I can find is a universal spark arrestor silencer from FMF. It comes with assorted mounting hardware to fit a variety of applications. I'm not too familiar with the DT 200 so I can't comment on what, if any, YZ parts would fit. I'd be surprised if any of them did. Don't attempt to run the bike without a silencer or with the straight pipe on it. It won't hurt the motor but will drive your neighbors (and yourself) crazy due to the noise. You may need to alter the jetting when you get the different silencer on your bike.

MX Tuner

Drop on Top
I have a 1996 YZ 250 that is cutting out on the top end when I open it up. Took it to a local shop (bad idea) got it back and it now screams on the top end but bogs out on the bottom end real bad, and now it fouls plugs consistently. I personally have worked with the normal adjustments to carb. /air/raised the needle/dropped the needle. According to the shop: Compression is great, reed valves are good, they said the jetting was O.K., but they adjusted it a little more for our elevation (possibly why it fouls now). They said they think it could be bad electrical, or the power valve, but they also said they have to tear down the top end to check the power valve. On a 96 YZ 250? Hmmm. Anyhow, any assistance you may have will be great.

Best regards,
Jason B.

It never ceases to amaze me the quality of mechanics the dealers hire and allow to act as mechanics. You'd get better jetting help from Kroger. You do NOT have to tear down the top end. The pipe needs to come off. Then removing the left and right covers on the cylinder will give you access to the power valve. There is one long bolt (4mm Allen head, I believe) that holds the two halves of the valve together. Remove the linkage and the long bolt and the valve will come out in halves. There are two small locating pins that help align the valve halves. Make sure these do not fall into the combustion chamber when you remove the valve. I truly doubt if the valve is causing any of your problems.

First thing to do is return the jetting to stock settings. Now find someone who can jet your bike to your individual requirements. You may have to recruit one of the local racers. Ask around. As always, be sure the silencer is in good condition (fresh packing and all the core holes are clear and unobstructed).

One other possibility is an electrical problem. Begin by checking the ground points of all the electrical components (stator and coil mounting tabs can corrode).

MX Tuner

What!!! No filter???
A friend of mine has a 100cc year 88 Yamaha and I want to buy it but he never runs it with an air filter and he wants about 500 or 600 dollars for it. Is it worth buying?


Absolutely not. The motor internal components require a clean air supply to live on. I've seen a crankshaft get ruined in about 15 minutes running time because it was run with a big air leak between the air filter and the carburetor. If you buy this bike, you can plan on having to spend another $600 on getting the motor rebuilt soon.

MX Tuner

What Ratio - What Oil?
I just got a used (1996) Yamaha WR250. I haven't had a motorcycle in about 10 years, but used to ride all the time. Since it's been so long, I am a newbie at the gas ratio that I need. I would like to know what the best fuel to oil ratio is (how much pre-mix oil for a 5-gallon gas can, if there is a certain brand I need.) I have searched everywhere to find it, but it seems to be pretty elusive. Dealers seem to like to tell me 3 or 4 different ratios, and the Yamaha site gives no info on it. I was hoping you know what is best.


I'd use 40:1 for your WR 250. Yamalube 2R is one of the best choices that is readily available. Honda HP is another good oil. Motul and Spectro also make very good oils. Stay away from castor based oils (Maxima Super M, Maxima 927, etc.). They are excellent lubricating oils but require the motor be torn apart and cleaned at very close intervals. These oils are good for professional racers who have their motors torn apart every few races.

40:1 would be one pint (16 oz) to 5 gallons. Most oils come in pint bottles so you don't have to worry about measuring. The reason you get so many different stories is people's experiences and misconceptions versus manufacturer recommendations. Yamaha recommends 32:1 for your WR. Since you're not racing and running at extended high rpm conditions, I recommended 40:1. This will burn a little cleaner than 32:1 and still give more than adequate lubrication for your motor. Many riders think burning less oil in the fuel makes a bike more resistant to fouling spark plugs. This is NOT true. 98% of the time a bike fouls spark plugs is because it is getting too much fuel in the fuel/air mixture. This is determined by a few things but mostly carb jetting. Most bikes come jetted too rich from the manufacturer and will benefit from having them rejetted properly.

MX Tuner

What Bike?
I am 13 and weigh about 122 pounds. I am looking for a good dirt bike. I live in Europe so some American brands are hard to find. I want to start racing cause I have been riding my friends bikes throughout the past year and just wanna know what would be a good starter - but still fairly competitive

There are many variables to consider. Do you want to race in the woods or motocross? A small four stroke like a Honda XR or a Yamaha TTR would be a good choice for a woods bike and still be somewhat competitive at beginner levels. If you're wanting to race motocross, you almost have to have a two stroke to be competitive. The Suzuki RM 80 has a good motor from the standpoint it isn't difficult to ride. The Yamaha YZ 80 is among the faster bikes but it is more difficult for a beginner due to the way the power is delivered. It is more of an expert rider's bike.

Your height is another big determining factor. You don't really want a bike that you can touch the ground on both sides flat-footed. That would be too small for you. But of course, you don't want something that you can't reach the ground on at all. Are you a fast learner? Be honest. You don't want a bike you'll grow out of (experience-wise) within two months but you also don't want something that will intimidate you right away either. There are plenty of good choices out there. Whatever you buy, be sure and have it inspected by a competent mechanic before you buy it. This means even if you're buying it *from* a mechanic, you need to have someone else look it over thoroughly.

Good luck.

MX Tuner

YZ Timing on a WR
I wish to inquire about converting a 2000 model Wr400 to YZ timing. I'm from Australia, and I've heard a few reports of guys who have done this have experienced fouled plugs and not the performance boost they were expecting. Can you offer me any assistance on this and tell me your point of view. I recently fitted a White Bros. E-Series exhaust, White Bros. air filter, and removed the snorkel from the air box. I've read that many guys over your way have completely removed the top of the air box. Is or does this create any major performance increase? Having said that I've replaced the standard exhaust (Doesn't come with a removable baffle over here) and air filter, I also increased the size of the main jet, up by two sizes from standard to 168. Unfortunately, I didn't get the performance increase I had hoped for. I was told to run eight disks in the pipe, but unfortunately, it was a case of all noise and not the go to match. I stepped it back to five and this seemed to help. The altitude where I'm located is below 8000 feet. Do you have any preferences on how many disks I should be using?

Although the bike is registered, the bike is mainly used for hard trail riding with a bit of motocross as well. So it gets some fairly hard work. Once again after reading some of the other articles, it seems that most guys are running a lot higher jet size (main) then myself. My next move will be to replace the standard header pipe with a White Bros. unit. They tell me that this will definitely increase the midrange to low end power. Do you agree with this? I will then also be looking to increase the jetting to see if this will help, considering it should be getting more air with the other filter and snorkel removed.

Thank you for your time. Any help you can offer would be greatly appreciated, as it is a bit hard to come by in my area.

Stu Berry

I have a '98 WR 400 with one or two mods. I have the 450 kit, the White Bros. Promeg E-series exhaust, and the YZ cam timing. I tried some recommended jetting (trying to save me the research time) and ended up back at stock except for a YZ needle. Main and pilot are both stock. This is running VP race fuel (c-14 to be specific). All the recommended jetting was way too rich. I'm at about 1000 feet above sea level.

Yes, I would (and I did) remove the entire top of the air box where it unscrews. Retarding the cam timing to YZ specs won't give the anticipated results unless you run a less restricted exhaust. Your E-series will do nicely. If they didn't get the gains they were expecting, and they have an aftermarket exhaust of some type, the cam timing wasn't done correctly.

I'm running 10 discs with the spark arrestor end cap in place right now. This was what I found to work best with the stock 400 bore. When I went to the 450 kit, I left the discs the same. It runs so well, I've had no reason to change discs yet. It is brutally strong well into the rev range. I'm running the tapered head pipe, as you should also.

I can't give you any jetting recommendations except start off stock and jet to your individual needs. I can't see you having to go richer than stock, especially at any altitude much over sea level. And remember, the fuel mixture screw is very sensitive. A 1/8 turn is a substantial change.

MX Tuner

Fork Seals
I have a 1990 RMX250. I replaced the front fork seals, which blew the first time I took it our on pretty heavy trails. Is this a trait of this model and if so, what can be done to improve the situation? I remember a bloke telling me to get used to replacing the seals, so now I am scared


Your RMX is no more prone to blowing fork seals than any other bike with upside down forks. You might want to inspect the lower legs for nicks that may have been the reason for the first set of seals to start leaking. You also need to make sure the lower fork tubes are kept relatively clean between rides. The main reason for seal leakage is dirt getting jammed up into the seals. If you stop riding with mud splashed on the fork sliders and it dries, the next time the forks compress, some of that dirt can work its way up into the seals. Otherwise, make sure the oil level was correct when the seals were changed. The incorrect oil level (by a large margin) can contribute to seals leaking prematurely.

MX Tuner

Stiff Forks
First of all top web site!!!

My problem is with a 1999 wr400f. The problem is that the front forks are to stiff in the first part of its travel - common complaint I know. I have tried backing the adjusters off a few clicks but still the same. I have thought about fitting a set of gold valves to the forks but have been advised to reshim the original valves, as they are of better quality true or false? However, I am not even sure if I am looking in the right area. Springs or oil - maybe all 3? I weigh 83 kilos and on a one too ten riding scale ten being very fast, I am about number 4.! Any advice would be great thank you.

Yours Truly - Capt. Arm Pump.

Let's see... 83 kilos... if I have my calculator jetted correctly, I come up with 182.6 lbs. The springs that come with a US version WR should be close for your weight. I don't know what springs come with your version. They may be lighter springs. Otherwise, the quick fix would be to disassemble your forks and clean the internals thoroughly. Refill with 2.5 or 3 wt oil at about 95mm. This should make a significant improvement. Damping is definitely the culprit here.

Gold Valves are a little on the controversial side. I've heard a lot of competing suspension companies that really talk bad about the Gold Valves but I've installed a lot of sets and have had VERY good results with them. I don't follow Race Tech's recommended valving, but I don't vary too far from it. I've gotten the best results with woods bikes and Gold Valves. If you're going to go into your forks, there are a couple other mods which can be done to insure more consistent damping. The midvalves can be returned to a conventional check valve design. The compression valving needs to be altered to compensate for this. Your Forks also have what is called a cylinder valve (or a quad valve). This is the part the magazines so unknowingly refer to as the midvalve. This cylinder valve is located at the top of the cartridge and can bleed off oil. Race Tech makes a repair for this for about an additional $30 or so. These mods can be done at home but only if you're somewhat familiar with the fork internals. You can cause some expensive damage if you don't do things *just* right. You might want to check with a Race Tech authorized service center. They are located throughout the world.

Concerning you being a "4". I can't count the times I've talked to less advanced riders that think they aren't fast enough to take advantage of a suspension revalve. Nothing could be further from the truth. In fact, that can make a huge difference in rider confidence. Don't hesitate to get your suspension done right.

MX Tuner

High Speed Miss
I have a '89 KX250. It has been a great bike with a great motor. It will beat or hang with newer 250's in a drag race. However, it has a high speed miss under full throttle. It doesn't seem to hurt the performance, but I'd like to finally identify and fix the problem. It is pretty much unnoticeable to the person riding. When I listen to someone else ride it, the miss is obvious. It has been there for years, through several top end rebuilds, new reeds, new pipes, and a variety of main jets and needle settings; compression is good. I use a BR8, which never fouls (the factory recommended BR9 will foul). I use Honda HP2 at 32:1 and the engine has always been very clean when taken apart. I run it slightly rich (the plug is on the black side of tan), but it never stumbles when accelerating. Maybe it is a bit too rich or maybe an ignition problem at high rpm. Any ideas?


Some of the KXs had problems with air leaks in the bottom end. A lot of the KX's benefited from having the case sealing edges resurfaced on a flat surface. When you eventually do the bottom end, be sure to plan on doing this while you have it apart. Normally, the air leak miss sounds more like an intermittent popping noise at full throttle.

You may be too rich on the main jet. That can give a high rpm miss. Doing a plug chop can give a better indication of the main jet mixture. Find a long section where you can run full throttle and carry a plug wrench with you. After an extended full throttle run (1/4 mile), pull the plug and inspect the porcelain. Ideally, it'll be light colored with a black ring down where the porcelain meets the metal part of the plug.

Check all the electrical connections and the condition of all the wires. This means the plug wire and cap. Check the wires where they come from the stator through the case. Is the rubber grommet still there and in good condition? Check the ground points for the entire system (this means the coil mounting tabs also). Are there signs of moisture (dampness, rust, etc) under the mag cover? Disconnect the kill switch from the wiring harness of the bike and try it. Kill switches have created some hard to find problems.

MX Tuner

DS = Doesn't Start?
I have a 1979 Suzuki DS 80. When I go out riding, the bike gets hot, and if I either turn the bike off or if I stall it, it won't start up again. When I try to jumpstart, it only works sometimes. I can eventually get it started after about half an hour. I would really appreciate if you could write back and tell me what I can do to fix it. Thanks a lot.


Sounds like it is too rich. You may have some things contributing to the hard starting, though. First thing I'd do is make sure the air filter is cleaned and re-oiled. IF the silencer baffle is restricted, that can also add to the problem you're having. Remove the baffle and clean it thoroughly. This can be done with some strong carb cleaner (a tub that you can soak the baffle in- you'll end up using 46 cans of aerosol cleaner) or with a torch. Heat the carbon up as hot as you can and it will flake off much easier.

If the baffle is fairly dirty, you might want to remove the entire exhaust pipe and inspect its inlet for carbon buildup. This buildup can significantly restrict the engine performance.

MX Tuner

XR250 engine stops unexpectedly.
I have been working for some time to fix my brother?s 1995 Honda XR250. It was purchased used a few months ago. Typically, with choke on, it starts in one kick. (Very nice.) The problem we are having is that after running or riding it for about 4 to 7 minutes the engine dies suddenly. It happens rapidly as if gas or spark was terminated abruptly. If you pressed the kill switch, that is how sudden. It can happen when being ridden or when sitting idle. Once stopped, you can attempt to kick it and restart but you only end up tired and thirsty to no avail. Waiting about 10 minutes for the engine to cool and you are back to starting in one kick.

Attempts to remedy:

  1. Removed plug, placed back in plug wire, touched to case and saw spark. So, we assume it has ample spark.
  2. Disassembled carburetor all the way down, checked for rust or dirt particles, cleaned and flushed everything out and reassembled (did this more than once.)
  3. Replaced spark plug just for the hell of it.
  4. Bike had no fuel filter when we got it, added fuel filter, removed the tank, flushed tank out to be sure there was no rust particles. There was none seen.

All attempts thus far have been in vain. After completing any of them, the engine will still die after a few minutes or ordinary riding. On occasion, you can get it to run for a short time longer but the abrupt lack of what we believe to be spark or gas is eminent.

If you can help with troubleshooting advice or next steps, please do. We?d rather be riding than wrenching.


Even though you have a visible spark, you still may have an electrical problem. In fact, an electrical component can fail exactly as you describe. A resistance check may show a bad component. But, then again, unless you check it in a failed state, it may check okay even if that is the problem. You might want to go over all the ground points (coil mounting tabs, etc) and ensure they are corrosion free.

You need to try to narrow down what you're losing (obviously, huh?). When you try to restart it and pull the plug, is it wet with fuel? Does it seem to stop running during an extended high speed run? This might indicate a low float level problem. Have you checked the carb vent hoses for blockage? This can give some unusual cutting-off symptoms also. The gas cap has to be vented also. If the vent hose is pinched/blocked, it may starve it for fuel.

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