That 70 surpass would half to have some serious use to wear out the piston an sleeve, I've got a 91 that is 17 yrs old doesn't have alot of compression but boy it sure runs well.
Is there alot of play in the crankshaft if not you are more than likely having a fuel problem.
I've seen spray bars go bad from adjusting the low speed needle in to far an it ends up hitting the high speed needle. Your reg could also be bad. you can send it into hobby services and at least get an estimate. or just replace the carb with a non pumped carb