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Well back in Aug of 2018, I started this post about my jerky 29 coupe. Guess what, here it is Sept 2019 and I have the same problem. Same situation, ran it is 4th of July parade, let it set until now (sept) and the same problem started again. The problem was that when I would start to go, press on the gas it would jerk and backfire in the carb.
So I did some thinking about the situation as to what it was doing--- would "miss" when pressing on the gas starting out from a stop. Well so many people recommended Marvel Mystery Oil so I got carried away. A few days ago I took the plugs out, dumped in a bunch of MMO in each cylinder, and let it set for a few days. Today I got in it, started it up(and it did start) and I had a concern that the neighbors would call the fire department. So I backed outside and let it run for awhile blowing smoke all over the neighborhood. Then I got in it and drove it around the block a few times in 1st and 2nd. At first it still had the problem. Then after a few rounds it all cleared up-smoke and the jerk (not me).
After parking it after a few miles, I turned it off and let it set for while. They I tried to start it. I had spark up, gas lever up and just a touch of the starter, it was up and running! Wow! This start was better that it had done for years. I also pulled one of the spark plugs and looked at it. Before the MMO, when I took them out they were covered with black junk. This time the electro look nice and brown and no black stuff.
Anyway to sum up all these comments- it does look like I had a stuck valve. Putting the MMO in the cylinders and running it after that did clear up the problem of a jerky start where it would jerk and backfire in the carb.
Hope this helps someone.