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Yamaha 250F Complications.

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Whats up guys, I'm currently riding what is now or soon will be considered a dinosaur in the dirt bike world. Currently, I'm riding a 2004 YZ250f and I have replaced the head, piston/rings, cams, carb rebuild kit, and valves. I ran about 6 tanks of fuel after getting the bike back from the shop and it ran perfectly. This past Sunday, I went out of town to the track and after the second practice session I noticed my bike was backfiring pretty bad when I would reduce speed or snap the throttle. A friend of mine noticed the Pilot screw was missing so a nice guy gave me a brand new pilot screw for a Honda carburetor and his mechanic stated they are universal. The mechanic then told us to tighten the pilot screw all the way down and then loosen the screw 1.5 turns, and adjust accordingly. This seem to fix the problem for a moment but then the bike began to back fire again and idle high for longer then normal after twisting the throttle until idling down. We then cleaned the carb and tried adjusting the pilot screw to 1/2 turn, 3/4 turn, full turn, etc. This didn't seem to fix the problem, and worse when we let the bike run for a moment the header became really hot (Turned bright Red), as if we set the pipe in a burner. There are no air leaks in the exhaust or intake boot system and new spark plug. The shop I had originally taken the bike to said the valves shouldn't have gone that fast and to try plumbers tape on the pilot screw, indicating maybe the bike is getting to much air. So....any thoughts are welcome. 

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 1.  when you bottom out the fuel screw, DO IT LIGHTLY or you might break off the tip of the screw

2.   read William's 'How to verify you pilot setting' at the top of the forum

3.  make sure there is no old or broken oring in the fuel screw passage

4.  make sure you have the fuel screw/spring/washer/ oring in that order


4.  if after all that, you still have an air leak somewhere

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