What I have: 1919 Speedster, 6v battery only, Texas T dizzy, no starter, Rajo head, Stromberg carb.
Took the Speedster out for its first "real" drive in 35 years. Got a few miles away, and all seemed to be going well. Just driving along a back-road in high gear, going maybe 35 mph. I noticed a few backfires, then shortly after it started missing; sounded like it was running on three cylinders. It didn't quit, but obviously wasn't right. A little concerned, I pulled in a driveway and turned around for home. A few hundred yards down the road, in high gear again, it cleared-up! Ran fine the rest of the way home; no misses, no backfires.
Points are gapped at .018" plugs are at .038" per rec from Texas T. I'm sure the carb needs some further adjusting also, but like I said, other than the few minutes during the drive, it ran quite well.
I never did like mysteries much.
Good news for most of the drive anyway, it should be a real fun car. Your experience sounds a lot like some joy I've been having with a PV 544 Volvo I got for my wife. Runs great some of the time, like junk some times too. Has caused me to think unkind thoughts about SU carbs. The are coming off, getting a thorough cleaning and inspection today. I switched my 26 coupe over to an OF yesterday, a bit of adjustment and it is real happy. The NH that was on it liked to stick the float, not sure why yet. It has a Viton (sp?) tip needle and it all looks fine at first blush. Float level correct etc. Oh well, don't need it right away so won't worry about it until a few other things get finished.
I hope you sort out why it was unhappy for a while and come up with an easy fix. Have you checked the float level and made sure that it moves easily? Then again, I heard somewhere that more than 90% of all carburetor problems are electrical.
Thanks for the encouragement, Walt. Up till now, just on short neighborhood runs, I've had no problems. Stan Howe recently cleaned-up the RF for me, so I'm sure it's in good working order, just needs some tweaking I think. Will take another test run to the gas station today. Gas in the tank has been there since December, which also may be a contributing factor.
Made a carb adjustment and drove to gas her up. All went well. No backfire, no misses.
It could've been just a bit of trash in the fuel line.
I would suggest possibly a drop of water in the fuel and it passed through. If a tank is emptied with high humidity the moisture in the air will condense in the tank, maby just a bubble but that is enough to cause your issue. Hopefully it has passed through.
Possibly a sticky valve after 35 years. Oil the heck out of the overhead and run a little 2 cycle mix. Those overheads have no oil supply and you have to do it or the valves will not close when they should. I have had the same thing happen to me with a Fronty that sat for decades.
Thanks, all. Ran it again this evening, and no additional problems. I've been keeping the top oiled pretty well. How much 2 cycle oil in a full tank?