2002 YZ426 Backfiring Issue

A little background:

This bike ran perfect for friend of mine, then (suddenly? maybe after a few months of sitting?) starting backfiring so bad (accel and decel), bike was un-usable. Based on advice, he had carb rebuilt (twice) by local mechanics and new gas - no improvement. Then he put in all brand new stock jets from Yamaha - no improvement. Decided to have valves checked - clearances were in spec. but had the mechanic install a Hot Cam so it would be easier to start for the new owner (me). [As a side, I notice that the decompression lever is still installed and connected but it's not being used and bike starts on the first kick]. After cam install, same problem - on advice from another mechanic we disconnected the TPS connection and backfiring subsided. But this "fix" was not consistent. Sometimes it still backfired on both accel (violently) and decel.

Bought a new TPS unit and installed yesterday. At first seemed to run OK around yard (easy, low rpm stuff). Took out on road and at high RPM started backfiring. Backfiring got progressively worse and at all throttle settings. I made several incremental timing adjustments to the TPS unit but no improvement.

My next step is to drain gas again, and put in new plug. Based on my limited experiences with engines, It sounds and acts like a timing issue. I'm suspicious of the CDI unit but I read that they do not go bad very often and I don't want to start throwing money at it till I determine the root cause.

All troubleshooting advice and experience is appreciated. Thanks in advance.

Adjust the fuel screw.

Also check the common threads Sticky in the YZ forum, there are a bunch of threads and people with this exact same problem. Popping and Backfiring is normal with these bikes but if it is excessive 90% of the time it is pilot circuit related. Here is a good post from Eddie_Sisneros, Try this before you go into anything CDI or timing related. Good luck.


p.s - You posted this twice:bonk:

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