Its a long shot, a young gun used to road race one around here in the mid eighties, and at one meeting the bike started no worries, but would not run anything over the pilot jet. Turned out the young bloke had re-packed the muffler, and in his desire to seal EVERYTHING up he used great lashings of silicone on the muffler/pipe joint, which ended up with a massive blob of silicone partially blocking the muffler. When muffler was taken off, bike was faultless, so you may be on the right track with the muffler. i assume the top end, reeds, carb etc are all ok?