I have a 03 yz450 with a strange issue, I have owned the bike for 6+ years and never had a problem. I figgured it was time for a little maintence and totally ripped it apart this winter. I had a friend do the top end, sent the frame for powdercoating, all new bearings...you get the picture.
OK so I get everything put back together and broke it in a little before going to the first race of the year. I run in a local XC/harescramble series. The bike never missed a beat for about an hour and a half then started to get a pretty nasty bog when I would roll the throttle on. At that point I could run through it and the motor pulled hard after you got past the bog.
so I pulled in the pits and looked at all obvious things oil, coolant, carb boots ect...all was normal.
Now the bike will run until warm then constanty backfires misfires and cuts out when you apply the throttle. It starts and idles fine
Pulled the carb apart found a little dirt here and there but nothing blocked that I could see....no change
Tried a different carb from a good running 05 yz450...no change
Thought maybe hotstart was sticking open...new seal cap and cable didnt help
Thought maybe exhaust leak...since decel backfires are caused by that...new gaskets and packing for stock pipe and muffler..no change
Tried disconnecting TPS and Neutral safety....same
A friend has an 05 450 that we tried swapping parts (CDI and coil) from and it behaved the same.
up until this year I have barely had to adjust the idle on this bike so Im not sure what would have changed.
Heres a clue maybe. I tried adjusting the fuel mixture, set bike at fast idle and turned the mixture screw all the way in and it did not cause the idle to choke out. maybe even idled a little better. but the strange thing is another carb had no change for the situation.
I hope someone can point me in the right direction.
Edited by hglumac, June 02, 2013 - 12:42 PM.