OK, now we believe it kind of "develops" the issue. We believe it is not the trans/clutch. That's good. Probably not the timing chain either. A rough idle, then runs OK, then develops a problem is what I read. This is a little odd.
You did not jump out and remove gas cap as advised. Shame. Must try to follow suggestions and advice. That would have eliminated another cause.
Elimination and testing is the proper way for most things. Inexpensive items, and easy to install items, are OK to replace without test. Points, plugs, wire, condenser should be replaced regardless of who, when and how in the past. Maybe even the coil, dizzy cap and rotor. Need a good solid electrical baseline. A tune up doesn't hurt anything.. Then Time the motor. Points MUST be adjust correctly. Then test drive. If problem persists, we probably move to fuel delivery and possible vacuum leaks.
Focusing on CARB, choke, fuel pump, ALL filters (including any internal carb filters and AIR FILTER), floating clog in tank or lines......Gas cap not venting. All vacuum hoses, fittings, holes, plugs and lines. Including vacuum advance (vacuum and mechanical components).
1. What carb do you have?
2. What upgrades does motor have?
You can do a fuel pump flow test if you like. Not hard to do.
(option) Not to make Jerry mad, but a new one is cheap and easy to install instead of test. Depends on the person, their tools. their experience and there desire to save a few bucks and start a fire instead.
