Been a while since I started this thread. Good time for a status update and some follow up question. Decided to take Carbking’s advice and double check the ignition system was all squared away. That was a quite a little journey in itself but long story short....
New set of plugs, new coil, dwell is now set at 32 degrees. Timing is set precisely to the flywheel ball. The ignition system is as squared away as I can get it. Engine pulls a steady 22 in of vacuum at idle. Set the idle down to 650ish.
Truck is idling as well as it ever has. Still have some valve noise I can’t quite explain but I think that actually got a bit better after I did the “running engine with diesel added to crankcase” flush.
I can’t say for certain that it’s still running rich. No black smoke (plenty of oil smoke still). I don’t get as much reaction out of the idle mixture screw as I think I should. But I don’t know what “normal” is for a B either.
That was a lot of words...onto the current symptom and my questions....
Under acceleration, and/or RPMs above some level (I’m gonna guess 1500-2000 and up), you get what I’d describe as a miss. I did some extended running and fiddling/tuning at 1000 RPM. The miss/shudder is not present there. I even validated that by pulling one plug wire at a time. Each cylinder reacted the same...run just a little rougher and RPM dropped by about 50. I should have replicated that test at a higher RPM but didn’t think of it.
Some theories below that I need advice to test. Also welcome any other theories.
1) Some sort of ignition related miss - Guess I could run the engine in the dark and look for arcing or sparks? BTW, the vac advance does function.
2) A fuel-related miss. I can’t discern a transitional stumble between the various carb circuits. The accelerator pump gives a good shot. The miss is present both during the acceleration and as you hold it at an elevated RPM. (Possible clue: I do have a leak at the throttle shaft. You won’t see it while running, but definitely will if you open the throttle not running)
3) A weak valve spring? Is there an easy way to test that theory or just have to go measuring them?
Last edited by JW51; 05/25/2021 4:48 PM.
