To Run, or not to Run...?

'01 426-

Been riding for a year, never been rebuilt, ran great. In Jan put the 450 Cam in it, made a huge diff. For the past couple of months, the throttle has begun to get more and more finicky. When your on the gas, it runs, when your just holding the throttle, it doesnt know whether to keep running, or die, like its hesitating or trying to cut out. Very, Very rough, not even rideable.. This has been a problem that has slowly come on, getting progressively worse. I decided to go ahead and rebuild the top end. New piston, wrist pin, rings, and timing chain. Got everything back together, but has a couple of issues with timing using a new chain without any stretching. Found out I was 1 tooth off on the exhaust cam, adjusted, and it fired. Yook it to the track, and still had the throttle problems. It will run when you hold the throttle open and the engine is under load, but after the engine is up to speed, or it is maintaining speed, it doesnt know where to cut out, or keep running. Going back to the timing problem, I tore it down again, and checked the timing, now the Intake cam was way retarded, beyond what 450 Cam Mod PDF shows, so thinking I was once again off, I notched the intake cam back one and it looked to be perfect. Put it all back together, and tried to ride it, and now its even worse, completely unridable, it sputtered and was cutting out soo bad, I only got into 2 turns on the track.

Things I have checked-

Valve Clearance- All within Spec

Carb Jets- Main 160 Pilot 42 - AZ 1200ft

Re-Packed Muffler

New Plug

Air Filter Clean

No Exhaust dents or dings

Items to consider-

Cleaning Entire Carb

Checking Ehaust Manifold to Head Leakage

Timing (AGAIN!!!!)

I am at a complete loss. It never used to run this way. The only things I have adjust was the Pilot Jet and Main Jet when I went to the mountains. The bike has not undergone any serious crashes. We thought the performance problem might have been due to a worn piston, but now that it has been replaced, and the problem still exists, I am at a loss. It was a problem that grew progressively, and wasnt due to me changing anything out, or making any changes to the bike.... My fellow moto mechs/Buddies, seem to think its in the timing, but the problem existed prior to even starting the rebuild...

HELP!!! :applause:

It's difficult when you take over a bike that has been worked on by another guy. On my YZFs I replace the cog at the base of the timing chain when I change the chain because it wears and can cause the timing chain to jump a tooth. Is the cam chain tensioner operating properly? I always replace the tensioner when I change the timing chain. Also from habit I quality check the bolts that hold the cam shells down, they can stretch and distort if not torqued down to the correct pressure. (Can also make it very difficult to kickstart if they are too tight). It's concerning me WHY it needed a head gasket in the first place.

Okay, I'm in the exact situation as you are. Except I didn't rebuild my engine.

From what I know, the carburetor is a waste of time. I have tried many jetting combos, none of which made the slightest difference. I recently have had the carb apart twice and cleaned it thoroughly. Still runs like crap! Bought a zip-ty fuel screw thinking that might help. Nope. My floats were in specs, too.

Also tried a new plug, CDI box, clean air filter, and disconnecting the TPS.

On another thread here, a guy with a 450 had the same problem. He replaced his clutch plates and springs thinking it was slipping. That didn't help it.

Things left to try: Double checking all electrical connections, unhooking kill switch and timing check. I also have a new TPS on the way, so I'll tell you how that works. I'm kinda out of ideas, but I hope this helps. (P.S. maybe a new ignition coil might help.)

I thought about the iginition coil, since it has been somewhat of a progressive problem. I plan on putting back in the stock exhaust cam just to start back at ground zero. Its definately a missing issue, like a 4-cyl vehicle running on 3 cylinders, it runs, but sputters in doing so. I havent has any issues with this bike. One thing that I did notice after I rebuilt and the bike had sat, what after it started, it seemed like oil was burning off from the exhaust port, like maybe there was a leak at the header. I know that this can also cause issues, so that will be another point to check. If I could get a timing light, the only think that would check, for the most part is one, the flywheel is on correctly and sparking when it should, the second thing it might show is, if the flywheel is on correctly, then maybe the CDI is not functioning correctly, but at this point, its sparking fine, it idles perfect, it revs up fine, but when under a load, with sustained throttle, it misses, acts like its being choked, kinda like a vehicle you would buy from Sanford & Sons...LOL

does it do it at all times cold or hot?

have you checked the accelerator pump?if youve had your carb apart maybe you put the slide plate in upside down.i had a similar problem recently but only once the motor heated up and found it to be the coil.

Hot/Cold, doesnt matter, and as far as the carb, it hasnt come apart yet...

Have you had the slide out of the carb? There is a spring plate valve on the slide, and both these parts can be assembled incorrectly. The symptoms are very similar to what you describe: May or may not idle reasonably well, but runs very rich everywhere else but at or near full throttle.

Nope, I havent torn the carb apart. The bottom Bowl has been the only thing removed, in order to change Pilot Jet for the mountains.


The bike went to one of the best performance shops over a week ago. I called them Friday to get an update to see if it would be ready for this past Saturday's race and to see what was wrong with it. The guys there said between 3 of them they have about 5 hours into the bike, and really cant find a solid issue. He said they put the bike on the EGR machine, and it said the bike was running 6% rich. They screwed the fuel screw all the way in, and it had no effect. They said they had the carb apart 3 times. He said they put a 40 PJ in it and was able to lean it down to 2-3%. They said they checked the timing, TPS, CDI, coil, and everything is within spec and tests OK. The definately feel it is a carb issue. The only thing they can honestly find is that the needle in the slide is scored up and down pretty bad. They said they have seen worse needles, without this type of problem, so they ordered one. They said they think that might be the prob, but dont know for sure. He said if the needle isnt the problem, they will call me and let me know they cant figure out the problem... Go Figure...

Like Gray said sounds like the slide plate is in upside down. The hole needs to be at the bottom of the slide. If you took the float bowl off then this sub body off there is an oring between the main carb body and sub body. This may be damaged and is not a replacable item.

The Performance shop said they have had the carb apart 3x trying to find anything that would lead them to believe it is causing problems. If they put the slide in upside down, then there is some serious issues...LOL Especially at a MX Performance Shop...

i have had a problem like that on my old bike and it was because the air/fuel mixture had become way off from vibration, but this may not help in your case. Also it may be an electrical connection that may have a bad connection from dirt or rust but i would test the coil, sound like it might be the problem.

Check your carb; (do it yourself) i.e. remove it, clean all jets, recalibrate, and try again.

Long shot, but worth a look; My 426 had a cracked inlet manifold, which caused all sorts of problems. It took a REAL good look to find the problem. It was a while ago, but from memory, I think I had to remove the manifold from the bike to see the cracks. They were half under the manifold clamps. This was when the bike was only about a year old, but the rubber manifold had seriously deteriorated. As I said, worth a look. Initially, I would've probably guessed yours had a blocked pilot, but you say that the pilot's been swapped? Also, are you certain the slide's in the right way? :D

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