I have a 74 TNT that I picked up about a month ago. Really good shape and ran good when I bought it. When I got home I was going to go for a ride and found out I was running on one cylinder and went nowhere with a load on. Decided to give it a new belt, plugs. After that it backfired bad out the carb, exhaust and up the pulse line(?) from the crankcase. Discovered poor spark on the mag side. Installed a new coil. Cleaned points. No more backfire but now won't stay running after it starts warming up(30 sec to a minute, after cold start, fires but won't run) Have set the Tilly carb to spec many times. Any suggestions would be very apreciated!
When you cleaned your points you may have changed the gap. When things get warm they expand and may be out of spec now. When it is cold and running is the choke on or off, may be starving for fuel clean the carbs and reset idle screw and float level
I had 1 of those. My wireing harness hit the exhaust pipe and melted, the electrical system did not like that. Check it for previous damage. I went through coils and I think it may be because the coils are mounted on the cooling shroud and not enough air gets behind them to cool them. When the coils are cold it will run and when they heat up they stop working. The book I have says your point gap is 0.35 - 0.45 mm or 0.014 - 0.018 inch, The points start to open BTDC 2.8 - 3.3 mm or 0.110 - 0.130 inch. The spark plug is BOSCH W260T1 the gap is 0.5 mm or 0.020 inch. You did not ask for this but yamacatowner could be right.