>>28598845
I'm now having similar problems with my 500. Honestly, I've had cement mixers and lawn mowers with smoother and easier running engines. As >>28598857 said, check timing first as it's quite simple to do. I'm not sure if this carries over to your 600 engine, but another thing to check is (assuming you car has one) the carb spacer. The 500's and 126's have a plastic spacer between the carb and the head, and over time they become brittle and split when people reassemble them. If your engine has that spacer, check that it's not sucking in air from anywhere, be it from the gaskets or from damage.