86 200SX Fourtrax, Idles High and then slows down, backfires and dies

[Sha35297]

Hello, I have an 86' 200SX. Customer brought it in with rusty dry gas tank and said it ran good, but would not idle. I cleaned the carb, completely cleaned and coated the inside of the tank with fuel tank kit. Came out great. Fresh fuel and Cleaned carb, adjusted valves. The bike will run great all day long as long as you don't let it idle. It will idle very high, then slow to a low idle for a few seconds and then backfire and die. I replaced carb with a brand new one, still does the same thing. Sealed off the air box with no improvement. Sprayed the intake manifold with carb cleaner while running and it did rev up the rpms. So I removed it and coated it good with some Flex Seal (just to test if it would help) After reinstalling the it, It still does the same thing. Any Ideas of what else could cause this problem if it is not the Intake manifold? I'm running out of options. I also replaced the O-Ring on the Intake.

[davefrombc]

It  sounds like a very lean  run at  idle.  If you have thoroughly  checked the manifold  for leaks I would suspect a carb setting.Did you  change the  setting on the needle in the throttle slide?. If  it is in the top  slot try moving it down one notch  and see if idle improves. My 300FW runs best with that needle two  down  from top.

[Sha35297]

Thanks for your response. Yes I have moved the slide needle to every position. It has some effect, but nothing that fixes the problem. I’ve tried larger and smaller jets with no change in idle performance. I believe I’m going to replace the intake manifold and see from there. 

[Russell Holt]

Plugged pilot jet.

[MarkinAR]

Check the oring between the intake manifold boot and head.  I had a Bayou 220 that had the same issue and out of curiosity I tried a new oring and that was it.  Looked fine, but must have had a flat spot.