I also believe if there is a vacuum leak it makes timing fluctuate along with chain stretch and wear. Etc. A vacuum leak can be detected by spraying carburetor cleaner around intake manifold/base of carburetor. Our rides are old but if we get close by moving and tweaking.....become as one man and machine as one adjust until it feels right. That is what I did to zone in. Feel the force young jedi. Let it flow through you. Yes