I have had one of that era that had a bad powerpack and made it act like it was only running on 1 cylinder despite showing good spark on a spark test. But those ignition problems are typically either p[resent all the time or they show up after it warms up which is the opposite of what you have going on. Sounds to me like it is a fuel issue. I'd start at step 1 and work up from there just to rule everything out and get a good baseline. I'd check compression first and then see if you are getting spark that will jump a 3/8" gap. I'd then replace the fuel lines from the barb on the motor pan side to the fuel pump and pump to the carb with clear fuel line. I always add a clear inline fuel filter somewhere in that run as well. Then pull the carb and at least inspect it and blow out the low speed circuit and high speed jet with carb cleaner and compressed air. I also always replace that gray fuel line that comes on the premade primer bulb lines with regular black automotive fuel line that doesn't have a liner in it. Finally I would bench test the ignition components. CDI Electronics has the test procedures and specs on their site somewhere. I have a factory service manual too and if I can find it I will post it here.