troubled.......you have 2-problems, weak sparkies and miss-adjusted carb. You do NOT have a timing issue!!! The 1/2-hr timeframe for boggin' down is typical weaksister ignition switch. ($15, cheap)
2nd issue, carb is RICH. There are 2-adjustments for the M/S carb. Adjust the down-pointing mainjet to 2-turns and LEAVITT!!! Adjust the side-pointing idlemix for FASTEST idle, NOT SMOOTHEST idle. Understand the difference??? Adjust the behind the carb idle speed for extreamly SLOW 400-rpms. (ennythang under 500-rpms is good) Do that idlemix adjust 3-times. The last time I adjusted my eazy start 6-volt 8N, my idlemix was 1/8-turn and idle was 435-rpms on new proofmeter. Remember the side pointing idlemix is BASSACKWARDS; out fer lean, IN fer ENRICH.
BTW, recommend AutoLite 437's gapped 0.025"........Dell, yer self-appointed sparkie-meister