1999 cat 250 2x4 keeps fouling plugs
 

My stepfather had a new carb put on. Still hard to crank and runned rough. Whoever done his carb work then put larger jets in. Now it will foul a plug about every 30 minutes. Where do i start?

get stock size jets. they mayve went way too large on them

Fundamental internal combustion knowledge base. You can not go to a larger jet unless you improve your exhaust. If your trying to cure an off idle lean condition then low speed jets up one size at a time with stock exhaust untill the lean sputter goes away. If the main was enlarged then the exhaust needs to be opened up, this may create more noise than results. The jet may be to large. If you won some good screw drivers (craftsman, MAC, Snapon) you can swap out jets in about 10 to 20 minutes. Do this on a clean work bench out of the weather.