so that's what it's for! thanks! that gives me incentive to keep working on it or find a replacement.

I got a new bellcrank cover. the old one was kind of cruddy. it still won't turn from the handle end. However, it does turn from the governor end. kind of like it's backwards but there is only one way to assemble it so that can't be it. In messing with it I discovered that with a wrench on both ends, the handle end turns if at the same time just a teeny pressure is applied on the governor end. it turns with what feels to me like just the right amount of resistance. maybe that's how it's supposed to work?? it doesn't make sense, but I don't understand the principle of how it's supposed to work and can find nothing on Google.

interestingly, the complete bellcrank assembly is not available.

Some of them take a fair amount of force to move the throttle.

you may be onto something about worn bushings 74A. upon further disassembly and closer inspection I found a badly worn shaft bearing which I suspect lets the part with the pins get offset just enough to jam. I ordered new ones (both ends of the shaft).

My bushings and the shaft lots of wear, creating a sort of notch that the shaft could fall into and stick. The bellcrank was sticky also. I had it apart from the dash to the governor control and changed everything I could find showing wear ridges. Works smoother than it ever has.

It's been a long and frustrating project but my D6C is back together and working perfectly (knock on wood). Many thanks to everyone on this and other threads who gave me invaluable tips without which the machine would have ended up as pricey yard art.
I ended up rebuilding the "bell crank" with all new parts and it's nice and smooth with just the right resistance.
Special thanks to PhilC for the tip that the governor won't operate until the motor is started. it's the only one of my machines with that "feature" and it may be in the operator book but I never saw it.
what a great club!