Too rich or too lean could make it gassy. A lean misfire, the cyl is too lean to fire but the unfired (even tho lean) mixture which is still potent enough then goes out the tailpipe & you smell it. Yes an LM1 would be state of the art for tuning but something is deteriorating/going bad unrelated to your current jetting cuz it was running OK before now it ain't. lets find/fix it. I never discount a basic compression/leakdown tests then I'd start with the plugs/plug wires for a possible misfire culprit (& rotor phasing/radial gap may be borderline) & float level/saturated for a rich condition. Holler how it turns out. EDIT reread & saw the hesitation, I'm thinking lean misfire, vac leak maybe