Thanks for all the answers. No pinching of fuel or electrical lines that I can see. The problem arose pretty quickly. Had it running at home when I got it running for the season, but the cover was off. I thought about the choke being partially engaged but that seems Ok. Could the fuel mixture be too rich?1
IIRC, since about the early 60's, the motors I had had fixed high speed jets with only the low speed circuit adjustable. If it will pull max revs under load with the cover off the high speed jet is probably OK.
Possibly some debris has formed in the carb during the layover time, especially if you didn't drain the carb before storage. You might try adjusting the low speed jet, although it may have a plastic cap on it. 1-1/2 turns out was where I used to start then richen it up to the point it starts to roughen up a bit, then back just enough to smooth it out
There can be both air bleeds in addition to fuel passages that can be partially plugged with fine debris. You might try dropping the float bowl, turn on the gas, & tickle the float up & down to clear any possible debris in the float valve.
Could be the cover makes just enough restriction in the air flow to effect it with a small debris restriction. If it has any kind of air filter or sound baffle, check those out. If there is no exhaust leak, my guess is that you may be in for a carb cleaning episode. Maybe try a 50% mix of gas & carb cleaner through it before pulling the carb?
Let us know what you find.