Ok this one has me stumped so far. I have a D-400 that started well until a few months ago. Now when cold starting, it will run at varying slow speed for about 15 - 30 seconds, then rev up to normal speed by itself. This happens with or without priming while running in this slow condition. Once it's up to speed it runs fine. It does not occur on hot start.

The exhaust ports are clean. Tried new points, flywheel, and checked the coil gap and spark advance weight. Nothing seeems to work.