JW there are a few things that are very likely to wreck your power piston. Ethanol-laced gas, a warped air horn/float bowl (or anything that causes a lack of vacuum), the wrong strength of spring (really common on those that gone through the rebuilder mill a few times because the same parts do not get matched with the same carburetor) to begin with. The best way to diagnose the problem is to remove the air horn and and make certain the piston moves freely in the bore. If it does, then make certain your carb isn't warped to the point vacuum is being lost. You can see what I posted a while back about straightening:
https://tinyurl.com/8pydzfd6
