I hope you haven't heated it up too bad? I think you have a base gasket or INTAKE GASKET LEAK. Franco-Morinis ALWAYS run cool. They are like Minarellis.........KEWL Italian Stallions.
Drip carb cleaner or WD40 around ALL gasket connections,Rob. ANY change in RPMS indicates an inleakage which will lead to hard-starting and overheating. You need no more than a #64 jet. Re-read my post............your point gap is too wide.....especially if the points are used. You can go down to a .012" point gap.Set it at .014" for now.
Your SPARK PLUG is wrong as far as I know..........use the Champion #801 like I said. I've only run one of the Polini 60cc kits nearly 20K or maybe even more.LOL!
Have you used an induction timing light(battery powered from any separate battery) to check your timing? 23 degrees BTDC..........not over 26 degrees BTDC.
You may have the head too close to the piston, so check that you have at least 15 thousandths of tapered squish away from the domed piston crown. Look underneath the piston where the rod connects to the wrist pin and if it's blackened, the piston is carbonizing the FRESH fuel.
Franco-Morinis like at least a 40:1 gas to oil using a synthetic or semi-synthetic oil ratio.
Check for that deadly air inleakage...........it can even be a flyside crankseal inleaking on you.........in which case replace it with a Minarelli seal,which is stronger,better built. I can't post from work for 4 more 12 hr.shifts, but I'll hang weith you `cause I KNOW these engines are nearly bulletproof.