[Speterson6973]

I have a 1986 Fourtrax 250, it starts and runs great, idles nicely until it gets warmed up. After I have run for a few minutes it won't idle down, and the engine speeds up considerably. I have adjusted the air screw and idle stop disassembled and cleaned it but it still won't idle. The throttle return spring is fine, the butterfly and piston are closing properly. I'm out of ideas. Any thoughts other than replacing the carb? A friend has a carb I can have cheap but it is a little different. There is no mechanical linkage for the vacuum piston, I'm guessing it is vacuum operated and the throttle cable attaching screw is smaller on the carb. There is also a plunger on the side that I'm not sure what purpose it serves and it would be inaccessible when intalled.

 

[rustynut2]

Creeping idle is a sign of lean condition. Most likely a dirty carb/low speed jet. Suggest you remove and clean carb. Remove all jets and blow passages out.

 

[SamUK]

These 250's run the same carb as the 250 ATC so you could also search for a second hand unit from one of those. DONT buy Chinese, they DONT work, we hear stories on a weekly basis of folks fitting Chinese carbs.
Your best bet is to stick with your original and buy a rebuild kit and some carb cleaner, I found the 250 carbs need to be Surgically clean in order to run properly, I had a right old nightmare with mine. Be patient, you WILL get it running great

 

[retro]

I got nothing different for you than you already got... just a longer version. Theres some solid advice you got here!

Definitely start with the OEM carb! The cheap china made knockoffs are useless garbage. Get a copy of the factory service manual HERE, print it out and use it as a step-by-step guide for complete disassembly. Clean the carb and all metal parts of it thoroughly in a solvent soak then blow every passage out clean with compressed air. Follow the FSM step-by-step and reassemble the OEM carb to the exact specs listed there.

Don't buy a china made carb kit they are junk, buy OEM parts only. When you get done adjust the cables like the service manual recommends before starting it up, then all that remains is to fine tune the idle mixture & speed.

Clean out your fuel tank real good and put a new fuel filter on it before you put the carb back on the motor. Clean or replace the air filter too. Check the carb boot for leaks when you are done.

Welcome to the forums!