ATV set in garage for 2 years, would not start
-- Completely cleaned carb,
-- Pilot jet was clogged.

Now it runs, but it wo't idle without the choke and surges up to 3/4 throttle.
It runs great at 3/4 to full throttle. I am getting a backfire through the carb when I let off of full throttle. So, based on what I've read, it appears to be running lean. Something other than the carb?

-- I checked the jet needle clip (3rd groove) and moved it higher and lower.
-- Replaced pilot jet (one size larger and one size smaller).
-- Mixture screw backed off 3/4. Have tried other settings.
-- Replaced spark plug,
-- New fuel line, and checked/cleaned fuel shutoff valve.
-- Tried it with and without air filter

No change in surging.

-- Removed intake boot and checked reed valve assembly. OK
-- Compression test shows 90 psi. Too low?
-- I've sprayed carb cleaner all around the carb. and intake port -- no change in surging.
-- I tried to build a leak down tester (to test crank seals or other vacuum leak), but I haven't been able to get a good seal, yet.
-- Disconnected oil injector and tried mixed fuel

No change in surging.

I'm not sure the crank seals are the problem, anyway. Why would it run at low speeds with the choke, if I had a vacuum leak.

Ideas?

 

I had a problem similar to this with an older dirtbike I had once. I basically did all of the same things you did. Accept when I would clean out my pilot jet, it would run fine for a short time ( in the same day). After cleaning over and over, I finally took off the gas tank and poured out the gas. This was after sitting over the winter also. I found little amount of water and some trash. I cleaned all this out and it did much better. With you having to have the choke on for it to idle deffinately means you are not getting enough fuel to idle. I'm sure you know that by having the choke on changes the air to fuel ratio by allowing less air into the carburetor thus allowing it to run. I have worked in the car business for 11 years. But, I took a break for about 2 months and worked at a Yamaha shop. We had a 4wheeler that had this problem. The tech cleaned the carburetor and no more problems. Water would definately cause this problem I was told. I found out compression on a 2 stroke 250 is supossed to be around 115 psi or higher. I would assume that 90 is too low for a 350. But I am not sure. I would check with your local dealer for correct specs.