Vederstein, I don't like to criticize, but PM research are at fault: The flywheel needs a counterbalance weight to minimise the imbalance of the piston, rod, crosshead, con-rod and big-end masses. I simply hate manufacturers that sell kits that have not been properly engineered with a counterbalance cast-in to the flywheel. They make a lovely, well made model look "amateurish" when finished and running. But it is nothing you have done, following their instructions. Just their poor omission. You can add a balance weight, (lead and clay/plasticine for experimenting?) with a bit of trial and error, and reduce the imbalance oscillation considerably. Then the engine will be able to run much slower as well.
(Old man's rant finished!)
But a smooth runner all the same, which is proof of good machining and assembly.
Well done.
K2