I have modified a standard prop stand to clamp onto the lower rear frame rail. Its been an absolute blessing, especially when dismounting when loaded up with camping gear.
After a few seconds on the side stand the engine dies, which isn't always convenient.
I am guessing its due to the carb float jamming against the float bowl although I haven't tested this yet.
Does anyone have any other possbile explanation?
Pretty much all AMAL carbs until the concentric was made in 1966 have problems with idling on the side stand. On your M20 when you put in on the side stand it will change the fuel level in the float bowl and make the engine run rich. If you tilt the bike over to the right side it will run lean. You can try adjusting the idle mixture screw with the bike on the side stand, but it may make it not idle so well when the bike is upright.
My M21 which has a brand new AMAL carb on it, even though it will idle perfectly on the center stand, will sometimes stop if I put it on the side stand.
The AMAL monobloc which replaced your 276 carb in 1955 is a little better but still can have problems on the side stand.
The concentric with the float bowl in the middle of the carb generally don't have this problem.