Sounds like the bearings may be worn, when's the last time they were replaced? It's probably not hurting the disc valve, since the pull of the engine will pull the crankshaft to it's furthest forward position, but it's still an issue that seems to be robbing your engine of power. Bearings would be my first guess... of course if you've spun a bearing you could have damage to the case as well so when you replace the bearings check the case for wear.