sounds like it,s out of time about a hair. hook up a timing light to it. if it has points then they could have slipped a little. it also could be the vacuum advance too. make sure it pulls in when you start it and also make sure the hose is hooked up right. it,s not suppose to be hooked to the intake, it,s suppose to be hooked on the ported side of the carburetor( above the throttle plate on the carburetor) so it can advance itself when you give it gas.