This is important information that you did not disclose previously.. This isn’t stumbling under load, it is stumbling and backfiring off idle. This is usually a problem of lack of idle fuel or a intake manifold vacuum leak. It is hard to distinguish these possibilities, but try to rule out the manifold leak and then test whether disconnecting the idle fuel cut off makes any difference. Turning up the idle speed screw to make it run off the main nozzle often helps it run better, but this is bandaid.Couldn't agree more! The idle is ok at 7 BTDC but it stumbles and backfires as soon as you take off. Fuel pump looks relatively new, I am going to replace the fuel filter next. I'll check the advance and see if the timing changes at higher RPMs.
At 7 BTDC it will idle ok, but backfire/sputter as soon as it is revved. Pretty much not driveable. At 20 BTDC it seems to run fine with no backfire or sputter. The only issue is the idle is high and it may not be good to run long term with that much advance.I also tried planing the intake/manifold with good results. You can also pull one plug wire at time if there is is no change in idle that could also be an issue. May have several problems. Run some sea foam in gas tanks. Good. Does it back fire while at idle and Rev up the engine and than you get backfire ?
I believe I have a retarding distributor and its vacuum is hooked to the VSV. I’m not measuring any vacuum at that port.i was thinking along the same line as Doug above. What distributor do you have and where is it connected to carb? I recently picked up a 69 that was running poorly, stumbling backfiring stalling, and it turned out the PO had the vacuum advance distributor connected to manifold vacuum instead of ported vacuum...
In the video I am parked in neutral and there is some hesitation. I’ll try it again and see if I can get the idle down. I brought the timing back down, and with the VSV disconnected, I am not seeing the carb backfire like I was. That vacuum loss through the VSV appears to be at least part of my problem.Fuel should not be dribbling out of the primary nozzle at idle. Can you adjust the idle speed down to 625 RPM? The fact that the engine stalls when you disconnect the solenoid indicates that the idle circuit is providing fuel.
Does the engine stumble when you rev it in neutral and parked?