If your idle is at 650 rpm, the idle mixture screw won't have as much effect as it would at 450 or 500 rpm. At 650, you're beginning to move away from the idle circuit and into the off-idle circuit. The idle mixture screw has less effect on that. "Normal" for a Rochester B is not always easy to quantify, however. Sort of like "normal" for Dr. Frankenstein. Try lowering the idle back to around 450 and see what happens. It is important to remember the idle circuit still feeds fuel in as you move into the off-idle zone. The Rochester engineers thought this would provide a smoother transition from idle speed to driving speed and it was at least as good as some of the other thoughts they had. Under some circumstances it will work.
Could be an ignition related miss. This might be from a worn-out distributor (allowing the shaft to wobble which causes odd dwell variations and possibly other problems). A couple of years ago Jerry (Hotrod Lincoln) was working on modifications to the original distributor to fix this...and he posted something about this just today. And I will tell you it could be a phantom problem from a seriously worn camshaft retainer. There should be a gap of only .001 to .005" between that retainer and your cam gear. That is to keep your camshaft as stable as possible. Apx 50 years ago I had one that absolutely would not stop misfiring and it turned out the retainer had worn so much over time (maybe .020" or more) it was allowing the cam to travel in/out as the engine ran (which caused the timing to advance and retard enough to make hiccups). After replacing every ignition component completely and checking the distributor on the machine three or four times, a fellow mentioned that had happened to him. It was not fun to replace (nor really cheap), but it stopped the problem, and it isn't anything you can visually observe.
A fuel related miss is always a possibility. Running too rich will always cause misfires and if the carburetor is constantly running rich the misfires will be at all speeds. The leak at the throttle shaft probably will make you run a bit leaner...as air will be sucked in while the engine is running, however. Try wiggling it up/down/back/forth and see how much wobble there is. The thing is with the Rochester B there are a few important things that will cause vacuum loss inside the carburetor and that isn't good. There are also parts of the air horn that could have been knocked off by clumsy or inebriated "rebuilders" and this can/will cause problems if you have a vacuum loss. If you have a vacuum loss, it can/will allow your power piston to go all wonky. This will invariably make your carburetor run rich because it will keep the power valve open---possibly constantly. What happens is there isn't enough vacuum to overcome the power piston spring (which by design forces the power piston down...which is the very last place you want it to be) and when you're accelerating you cause this to happen by virtue of vacuum drop also. And while it is a long shot, there's a vent hole for the power piston (going back into the throat of the carburetor) that must be open. If it isn't open, vacuum can suck fuel up into the passage where it will head right back into your intake manifold. Isn't the Rochester B a wonderful design? Look at the last image below. That vent is shown by the yellow arrow. The red arrow is the power piston, the blue arrow the vacuum passage that operates it and the green arrow...well the green arrow is a silent arrow, just like the letter h in John (which was omitted from my name to avoid further confusion).
A weak valve spring? Maybe, but sort of a longer shot in my mind. I've seen sticking valves be a more frequent problem than a weak valve spring. Particularly in #5 and #6 cylinders. Since 1965 I've had exactly 3 valve springs in 235s break, also. But you know that instantly.
I'd focus on the Rochester and try to study it well. Is the air horn warped (check without gasket as it sits on the float bowl)? Are there pieces missing? 6 and 8 shown below should be there. If not and you have a vacuum leak these will lead to problems. 6 is the vacuum passage that operates your power piston. 8 feeds your idle circuit. Check also the ports in your throttle body. They're shown below. One image below shows the Rochester B all straight and nice again. Yours probably won't look like that, but hopefully it will.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
