My 2001 Yamaha SX700R developed a problem last winter that I was unable to solve. After idling 30-60 seconds the engine won’t run clean when given quick wide open throttle (drag race). The PTO spark plug will be wet if checked immediately. The sled is stock, has even compression (134 psi), the main jets are 1 size leaner, idle mixture screws are turned in 1/8 – 3/16 of a turn which seems to help a little. I’ve checked and cleaned all jets, checked float height, needle and seats. The needle clips are in stock position, and the reeds look to be fine. Sled runs great all other times. Any help will be appreciated.
Sounds to me like it is getting too much fuel at idle. You would need more air, not less air, from what youâ€™re calling the â€œidle mixture screwsâ€. I assume this to mean the air screws on the bottom of the carbs. If youâ€™ve turned them in, this will make the problem worse (less air means a richer mixture). Try going the other direction with them.
Also, check all of the choke plungers as you might have a stuck choke that is enriching the mixture â€“ especially on the PTO carb.