Hesitating after top end rebuild??

[Pozi]

I recently put a new top end in my 2001 WR426 and now I am having problems with it running. It seems to hesitate when the throttle is applied. Doesn't really matter whether or not it is 1/4, 1/2 or 3/4 throttle.

Any ideas what might be the culprit to this problem?? ? Thanks

[Indy_WR450]

Check your cam timing. I bet it is off.?

[jchantzWR400F]

I doubt the SOB would even start with the timing off. Is it a hesitation and then dying problem? Or, is it a starting and continuing to run problem. Have you tried running and then pulling the hot start to check for a rich condition, or pulling the choke to check for a lean condition? Did you mess with the carb at all during your rebuild?

[Pozi]

[QUOTE=jchantzWR400F]I doubt the SOB would even start with the timing off. Is it a hesitation and then dying problem? Or, is it a starting and continuing to run problem. Have you tried running and then pulling the hot start to check for a rich condition, or pulling the choke to check for a lean condition? Did you mess with the carb at all during your rebuild?

The hesitation only occurs when throttle is applied. While idling (after warmed) it is fine.

For being what it is it starts ok. It has always started ok even before the rebuild. I have not checked for a lean or rich condition by doing what you stated. I will do this. As far as the carb goes....it was not touched in any way during the engine work. Only removed and reinstalled.

[Hamish]

just for the record, the 426 will run and does have clearance with the exhaust 2 teeth retarded, but it doesnt like to idle and the low down power is non existant.. Does it do it when in gear, or just when you are twisting the throttle in nuetral? If in nuetral only, sounds like you are in need of the BK mod.

Also check for air leaks around the intake manifold. To do this quirt pertrol around the manifold while the bike is running. If it has an air leak (which will cause a lean condition) it'll stall (or run rough) when the petrol hits the place where it's leaking as the petrol gets sucked into the manifold making it over rich.

You also need to consider how stuffed the engine was before the rebuild. If it was completely worn out, it's quiet possible that tuning changes will be required to get it to run properly now that the engine is more efficient.

[jchantzWR400F]

I agree with Hamish. My personal experience tells me that if you retard the timing further than what is stock, the bike simply won't start. This was however my 400 and not your 426. If the timing were advanced by 1 tooth on exhaust, you would have yz timed your bike and it would start and run fine-advanced by more than 1 tooth, would be surprised if it would run or start at all. Just my thoughts.

If you have this condition when the bike is in gear, then I would double check timing. Make sure you are positively at TDC and check in and out cam position.

[Pozi]

just for the record, the 426 will run and does have clearance with the exhaust 2 teeth retarded, but it doesnt like to idle and the low down power is non existant.. Does it do it when in gear, or just when you are twisting the throttle in nuetral? If in nuetral only, sounds like you are in need of the BK mod.

Also check for air leaks around the intake manifold. To do this quirt pertrol around the manifold while the bike is running. If it has an air leak (which will cause a lean condition) it'll stall (or run rough) when the petrol hits the place where it's leaking as the petrol gets sucked into the manifold making it over rich.

You also need to consider how stuffed the engine was before the rebuild. If it was completely worn out, it's quiet possible that tuning changes will be required to get it to run properly now that the engine is more efficient.

The bike mainly hesitates when in gear and not while idling. I did the BK mod some years ago already so that is not the case (in neutral). Also I YZ timed the bike the same time I did the BK mod. And when I put everything back together I re-YZ timed it. I will double check my timing efforts this weekend.

Thanks for all the help.