I think I am about ready to give up on this 4/6 and put it back in the barn or post it to craigslist. There was a time that I thought that I could figure out every problem - and when I couldn't there was always someone here on the forum who would offer some sage advice. But this one just has be bumfuzzled. No matter what I do I can't seem to keep it running for more than 10 to 20 seconds below 3,500 RPM. I had convinced myself that it was something obvious, but no matter what I check nothing has made itself obvious.
While I can't rule out carburation completely, the carbs have been checked, re-checked, cleaned and checked again. Crank seals have been replaced.
It starts easy, and runs like a banshee above 3,500.