Here's a long shot....I had similar issue with my old pal. Just getting it up and running. After much work and cleaning it still had a hard knock. Two suggestions: If your pan is off any way, take all your plugs out and roll the engine over by hand. It should not have any noticeable interference when rolling over. If there is, it could be something foreign like a screw or a something fell into the plug hole when a plug was removed. So if you feel interference, take the heads off and check for carnage. Hope it's something easy!!
