My 115 starts good , but after a few minutes it will start popping( like a backfire ) through the exhaust. When it does this, the rpm's drop and the motor almost stalls. It doesn't do it quite as bad when you increase the rpm's and it pretty much goes away altogether once you get above 3000 rpm. The worst is @ idle. Carbs were rebuilt last summer. It has fresh fuel and filter. Any suggestions? Thanks in advance.