Following this thread with interest. My M37 rebuild has had similar problems. The truck sat for a lot of years befor I got it. Every time I found one problem, another would pop up. At this time , the truck starts well, idles well, has lots of power, right up to 35 mph. Then it just will not go any faster. Even down hill. Tried a different carb, different dist, elec fuel pump putting out fine and 20 psi vac.
My question is, could it be the governor? I read the manual on adjustment where it says to turn the adjusting screw but that thing is just brass and it is tight. I don't want to strip the slot head and there was no mention of loosening the adjusting nut/collar.
Any thoughts?