Thanks guys for the great info. At one time I swapped a head gasket on one engine , because I had a leak from the top of the motor. Found a slight mar in the head. Was able to correct it. That was when I brought the motors last year. Its a good idea to change them again. I found it strange that the carbs did not have a needle to adjust thr low end. They are air bleed, but most engines have a needle to adjust. I thought you might be able to swap carbs with a twin needle. I'll look into it. Since I'm using a pair of these, I would like a good idle. Do you guys think the Coolpower fuel is OK for these or should I switch?? Lou. you said 18% is what needed. What brand do you use?