Few questions to help narrow down the possibilites: Did the S650G run before you made all these changes? Did you remove or rotate the distributor? Check/replace the fuel filter?
It shouldn't make a difference at low rpm, but is the vacuum advance connected and operating? What RPM does it idle at when you start it? Will it start if you choke it? Have you set the points gap? Without more info, it sounds like either the timing is way off or it's starved for fuel. When I replaced my points, the old points were so worn that the old timing was way off. (dwell on the old points wouldn't even register on the meter!) I had to tweak the timing by ear to get it close enough to get the rpm stable enough to set the timing with a light. I've also found that my fuel pump isn't exactly self priming, so if I run it out gas or drain the line, I have to fill the filter and line to get it to run. It's fine after that. Hope this helps.
