I have a 1995 yamaha timberwolf 250 that i recently rebuilt the top end in, it is running 145 psi compression. When running it sometimes will not return to idle after revving it up, it seems to be running very lean because if I richen the mixture screw it runs better but not perfect. When i turn the choke on it runs perfect

 

Air box and filter on? Some people try out a new rebuild without them and find that they run lean. I'd check the jets to see if any trash or dirt is in the carb. Having to choke one to make them run better is a sign that it's lean either in the carb jets,jets stopped up or fuel supply is weak,split intake boot,etc. If all that checks out it may take jumping up a main jet size or so.https://www.jetsrus.com/a_jet_kit_at..._1992-2000.htm

 

I had the air box on, i found that the foam ring that seals the air filter assembly to the air box was gone. Nothing is wrong with the filter and the carb is set to stock per clymer specs. It was also rebuilt and sent through the ultrasonic cleaner before it was re installed.

 

Since the foam piece is gone the machine is basically running unfiltered. Also it is great to hear from you "Old Polaris Tech", I am a young polaris tech, i just got my MSD Gold certifaction at age 19.

 

Yes the foam seal is critical! Plenty of work out there for good techs. Plus you'll be going back for more schools every so often to keep abreast with technology changes. Congrats..