This is what is happening:
Bike is warmed and runs amazing off the line and for the next two laps, but then half way through the third lap it starts to backfire, then sputter and dies. No flicking of the throttle can keep it running for long. This has happened to me twice and both times were around 2-2.5 laps. The first time it happened I check the spark plug and it was wet and looked fouled. Since it was running so well at the low end I went to a smaller main jet. The second time it backfired/sputtered/died the plug looked OK (see below). Both races had brand new plugs. Once in the pits, after I looked at the plug, bike starts right up and runs amazing again...arg!
My questions:
1. What typically causes backfiring?
2. Any idea's on what is going on?
3. Steps you go though to diagnose and issue like this?
4. I'm open to any tips you want to share for setting up 95 to race.
Pertinent info to help with diagnosis:
Bike is stock except for new carb.
Race day was hot and sunny.
I race at high rpms.
I set timing the day before race with timing light.
Gas - Non-Ethanol 92
PreMix - Castor927 at 32 to 1
Plugs - NGK B8HS 5510, both races had new plugs.
Brand new 28 mm Mikuni running 35 pilot, needle 5EJ13 clip on center, 180 main on race that plug fouled and 160 main on that race where plug looked OK to me.
Inline fuel filter. Gas tank was treated for rust but I do have a small trace of rust still in it. I figure the filter will capture anything.
I rebuilt both top and bottom ends this winter.
Thanks for any pointers,
Jay
This is the plug after the second race.
