IMO 850's way too big for what he's got and the rpm range it needs to run in even if it was set up perfectly. Is the M1 a single plane? Is the air door adjusted properly on that carb (Thunder AVS?)? If there's not enough airspeed when the throttle is cracked and the secondary air door is too lose it will stumble bad.
If a normal head gasket was run and estimating the chamber size around 81ccs(probably larger than that...) I'm coming up with around 9.1:1 assuming the deck height was blueprinted to 9.98. Normally they are much taller than that. So if the deck had a clean up cut it's still probably closer to 8.5:1 than 9. If the heads were not milled a bit and nailhead valves used, the chamber size is probably larger than 81 (they ussually start around 84-85) which would lower it even more. So it's all a mismatch. It will be hard ti tune with the parts you have, and if you really want it to wake up lower down the cam will need, and the carb may need to be replaced.
