...some of the early GMS carbs (like mine) were miss-drilled for the inlet of fuel aprox.
40% restriction. This lead to a leaning out in flight, and deadstick. Most of them would
not respond to the main needle....you could open them up 7-8 turns and nothing would
happen due to the restriction. In other words, you could not get them richened up enough
flight. The problem was, they would run OK on the ground with the richness just enough to
deceive the operator into believing the engine was running OK. Then after a minute in the air
the inevitable leaning out, overheating, and dead stick.

Also the muffler nipple was restricted. This caused a loos of fuek pressure that exasperated
the problem. I went through my engine with a fine tooth comb before I ran it, because of
the problems others were having, and I isolated these two problems.

Enlarge this pic and see the restriction. Mine runs like a top. No doubt it would have run
poorly with these obvious problems.

FBD.