I have the mods you see below and still have bone stock jetting. the only time the carb was touched was turning the screws out to 3 turns. The plugs are a nice shade of tan, maybe a hair on the light side, but no excessive heat, and runs very strong, (dont know about cold weather yet) every now and then I do get a backfire on deceleration but not much and not often. Im at 1000 ft. Is it time to rejet? or leave it alone?

 

When you start seeing black specs on the white porcelain insulator.

 

I've been considering exhaust or the muffler mod but a little afraid of rejetting. We did my brothers p400 with a K&N and dynojet kit and its never been right since. tried about every combo I can think of and still worse than when we started. now its back to stock and still isnt right. Mine is running the best its ever ran and would really hate to loose it.

 

If all you are going to do is the muffler mod I would bet money you wont have to rejet. Even if you did you would only have to go up one main jet size with each carb.