04 RMZ250 Issue

[Chili]

Trying to help a friend with an issue that has cropped up on their RMZ.

Bike likely has roughly 50-60 hours on new valvetrain (Kibblewhite) and has been running very strong all season until recently.

Day the problem started the bike had been ridden roughly 1 hour and then transported to a different track to ride. During transport more fuel was purchased and the bike was refilled. Once at the track within a few mintues bike ran like crap. Symptoms were it seemed like the bike was bouncing off the rev limiter (cuttting out) but it was doing this at about 3/4 throttle and above, ran fine below 3/4 throttle.

Thinking it was fuel they dumped the tank and got fresh fuel, no change, bike running the same way. Since then the plug has been changed, carb cleaned repeatedly, jets swapped for new ones in case of small obstruction, valve clearances checked, CDI swapped out, kill switch disconnected and the stator changed. Bike still runs like crap under load above 3/4 throttle.

Any thoughts?

 

[Rich Rohrich]

Doug - Don't forget to look hard at the coil, the sparkplug boot, the coil wire and especially all the ignition connectors and grounds. Bad coils can do what you are describing, but simple ohm meter tests have never proven to be reliable in my experience. Swapping with a known good coil is the only good test IMO.

I would also verify fuel flow from the tank,and make sure the carb vent lines are all clear just to play safe.

One last thing to consider. The 2004 RMZs had a bad habit of having the cam gear slip on the intake side. This would put the intake events out of phase and make it run fairly odd. It might be worth pulling the cam cover if he can't find anything else.

 

[Chili]

Thanks Rich, It'll give them some more things to look at since they've reached a standstill.

 

[Chili]

As a follow up if anyone is interested, the problem was located. It seems an intake valve keeper wore away and allowed one of the valves to float. Not sure what caused the keeper to wear out but they were disappointed after getting 200+ hours out of the stock valve train ( I know it's a miracle!) they were hoping for an extended worry free life out of the Kibblewhite stuff.

 

[Rich Rohrich]

OEM keepers are used with the Kibblewhite valves and springs. It's all to common for mechanics to reassemble new valves with old keepers, and have problems down the road as a result. Keepers are cheap and should be replaced with the valves.

That said, it's much more likely that it wasn't assembled right in the first place if it punched out a keeper.

 

[Chili]

That's kind of what I was thinking Rich, as far as I know every component that went in was brand new as this kids Dad wanted one last trouble/worry free season before they upgraded to an 08 :bang:

 

[Rich Rohrich]

Lots of otherwise competent mechanics have trouble putting together a reliable cylinder head for a 13,000 + rpm engine. It's fairly specialized work to do it right.

 

[Chili]

Looked at the bag of old parts and the keepers are in the bag so he's pretty sure new keepers went in. You'd think with incorrect assembly it would have come apart before my very conservative estimate of 60 hours on this bike, no way to know for sure I guess.

 

[Rich Rohrich]

no way to know for sure I guess.

Post failure analysis is definitely as much art as science. For me it's one of the most interesting (and sometimes frustrating) aspects of engine work.