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'64 C60 w/ 292/420/2 speed and original Rochester.

When I first rebuilt the truck I drove it how I always drive the larger trucks and would use engine load to assist in slowing down, rarely applying the brakes until <10mph. At the time I was on regular 292 Delco plugs and a points dizzy. Everything worked beautifully. After a year I upgraded to an OEM HEI unit that I sourced off a 250(?). Put heavier advance springs in, rewired the ignition, matched with HEI wires, and installed larger gapped plugs (Autolite 3923 copper gapped at .044). Truck still runs like a dream and pulls strong under load.

I cannot however, utilize engine load for braking anymore. Doing so causes engine popping/miss, almost like backfire but not exactly. Typically symptoms of over fueling as the same things happen if accelerating with the choke partially on. I've torn down the carb numerous times, checked float levels etc, and all is within spec. Pulling and inspecting plugs seems to indicate good combustion. If she's running rich the plugs aren't indicating. Cold starts (sub 32F) require heavy choke but once warmed up she idles great with no choke and doesn't idle rich based on both smell and exhaust color. For the last two years i've scratched my head on this and cant seem to diagnose it. Again, under any throttle the engine purrs like a kitten, but as throttle is released while in gear and rpm start to fall, it occurs.

Am I missing something small? Does the carb need tuned differently than standard? Maybe its with the plug gap or advance springs? Im at a loss and appreciate any input. For now I just drive and use the brakes, but id like to get back to engine braking to save on pad and drum wear since brake parts are hard to come by.

Put the points back in and see what happens.

Why did you change the vac advance springs? They might be too stiff now.

When you let off the throttle, your vacuum will change significantly, especially if you are using ported vacuum for the distributor.

I would echo Otto's question as to why you changed the springs.

I changed the mechanical advance springs. There are no vac advance springs. The 292 has a different power curve than the 250 and stays retarded for longer before advancing as rpm's climb. Thus, stiffer springs. I tried like the devil to locate 292 advance weights or specs at least to modify the 250 weights but struck out, thus opting for slightly stiffer springs. The only vac advance present is just the disc on the side of the HEI. Same vac disc from the original dizzy just swapped over to the HEI. This vac advance ( to my understanding) only applies advance at idle. Other than that, I never touched anything vac wise.

The dizzy I was on was slowly failing, thus the swap to the HEI. Couldn't put it back in if I wanted right now.

Another point, I have the timing set to +10 deg on the dot as per factory spec.

Are you using ported vacuum or full intake vacuum on the advance? When coasting, you should have little or no advance from the vacuum canister if the port is above the throttle plate. Someone has BADLY misinformed you about how a vacuum advance works. It adds about 8-10 degrees of total spark advance (crankshaft degrees) at light throttle cruise, and drops that advance off during open throttle pulling, and usually when coasting with the throttle plate(s) closed. "Ported vacuum" comes from a fitting slightly above the position of the throttle plate at idle, so it's possible your idle speed is set a bit too high if you're getting vacuum at the distributor while idling.
Jerry

Alright, went out and did a sanity check. Vacuum advice is hooked to the standard vacuum port just below the base of the bowl, so manifold vacuum.

Disconnected and plugged the vac line and checked initial timing. Its set at 5 degrees, manual calls for 4 degrees. Idle is set between 450-550 rpm. With vac hooked up, timing is 6 degrees at idle. When throttle is applied total advance goes up. Everything for all intents seems perfectly fine.

As a correction to my earlier statement, the vac advance canister on here is the one that came with the HEI, thus for the 250. I tried to swap but its a different style mount than a points dizzy canister thus I left it. Is the advance canister engine specific?

Playing around under the hood, whenever throttle is released this occurs slightly, Its merely amplified 10x when under engine load.

I had some backfiring from my 283 prior to a complete new dual exhaust, now no backfiring on engine braking. Point being, do you have exhaust leaks near the engine, on deceleration it will suck oxygen into the exhaust leak and backfire.

I'd suggest doing a test drive with the vacuum advance disconnected and plugged to see if the exhaust note changes when coasting. Running full manifold vacuum to the advance is generally not an ideal situation unless you do some tinkering with the total travel of the centrifugal advance weights and/or the spring rate as well. Drop me a PM and I'll respond with a phone number. I have a few ideas that might be better addressed in a 2 way conversation instead of with a keyboard.
Jerry

No exhaust leaks present that I can tell. No carbon tracking at the head/manifold and I periodically check for exhaust and vacuum leak here since I know the manifolds like to loosen or warp with time. Its all brand new stainless single out exhaust to a Massey Ferguson muffler and downpipe right under the cab. Same setup as prior to the HEI.

Just had a nice long talk with Jerry who was beyond helpful. Im thinking the current advance setup still needs fine tuned so I am going to focus on that and see if dialing it in lessens this phenomena. I've got a nice 2.5 hour drive on Friday with the truck to pickup a water tank in Gary and plan to tinker with the advance and total timing during the drive to see how the truck responds. It may take a minute with trial and error but ill report back with what I find/learn in the process.