I think u mean 155 main cuz 142 is stock mine is 170 and i need to lower it cuz its too rich. I'll give u a crash course in jetting - The carbs job is to mix air and fuel at the right ratio to feed the engine. The main jet is basically a screw with a hole in the middle that lets gas get in. So when u changed ur filter and too off the airbox lid, u increased the amount of air getting to the carb so u need a bigger jet to equal out the ratio. When i say mines too rich that means too much gas, too lean means too much air. U can diagnose this by taking out ur spark plug and checking the color of the end that goes in the engine. If its all black - Rich white - Lean You want it to be light brown. I f u want any more info on that i'll be glad to share anything I know or I'll look for some good sites for u to check out.