Bought used 2004 Sportman 400. Owner claimed ran great before it sat for long time. Drained fuel and replaced with new plus fuel treatment. Didn't help after running about 15 miles of fuel through it.
Starts and idles fine. Runs mediocre when cold at low speed, not at higher speed and rpm (blubbers and craps out). After 5 minutes of running barely OK, it gets warm and barely runs. If put choke on, it quits. It runs much better when airbox cover off... but still not great.

Cleaned air filter, replaced cracked carb rubber boot going to airbox and airbox rubber seal. Thoroughly cleaned carb and all jets. Seemed clean to start and all jets and passages seemed clear. Put back together and leaked much gas into motor, leaked out exhaust pipe. Perhaps float stuck so I removed carb again and re-assembled. Put in new plug after drying out for the night and still runs poorly. Removed fuel pump and diaphram and check valves all seemed OK. Working with mechanic much smarter than me and we're both puzzled. Anything else we should check?

Appreciate any help you can give.

 

Check the oil for gas. If it dumped a lot of gas in the oil it will run poorly and there is a lack of lubrication with the oil being diluted. Also if you transport on a trailer make sure you turn the gas off. Bouncing around on a trailer will fill the crank case.

 

i would definatly check the oil. if the cylinder filled up then it will leak past the rings into the crankcase.
you could have an electrical issue too. try cleaning all grounds and check wires for shorts or bare spots

 

Thanks a ton guys. I'll check that