The engine should feel smooth when the piston is at the bottom of the stroke. At the top, it should grab.

If you feel roughness, and the piston is at the bottom, then you might consider squirting a little oil in, or pulling the backplate to make sure there aren't any filings or bits of crud in there. It's not common, but does happen once in a while. A quick squirt of oil, or fuel if you are going to run the engine can wash stuff out and give some extra lube.