I have a 51 chevy with a 235 and a powerglide. The engine has a newer HEI distributor and Idles perfectly. It also does well when accelerating, but when I let up on the pedal to just cruise at whatever speed it stumbles horribly and backfires like crazy. I just stopped this morning and picked up a new carb because I had planned on changing it anyway. But Im thinking timing? Any help would be appreciated.
Have you set the timing? With a light? That would be the first thing I would check. Then on to the fuel /carb thing.
Timing was about dead on with the light. Turns out it runs great with the vacuum advanced disconnected and plugged. Why would that be?
I have an HEI on a older 235 and the instructions advised me to disable the vacuum advance system. Mine runs fine without it. I have over 12,000 miles on it since I installed the HEI. Normbc9
You mean it accelerates fine when the pedal is down and idles ok? But driving at a steady cruising aspeed it backfires out the carb abd stumbles? Sounds like fuel delivery issues from the off idle and or main jet circuit. From personal experience; You need to check the total mechanical advance if using a 70's-80's stock HEI on a 235.Distributors from EGR engine might have 45 degrees built in.......that's why the mention above of not using the vacuum advance.235 should like about 36 total degrees at 3000 rpm or so.Vacuum advance adding about 12-15 degrees more at cruise speeds helps response and fuel mileage.
The reason I ask is that years ago the older type HEI distributors that had the module inside of it,what use to happen to them is the advance would pull the pole piece to advance the the ignition,and by doing that it would flex the wires that go to the module and over time the wire would break at the pole piece and cause the same symptom and even quit running at all. Its been a while since I've worked on any HEI but I thought it might help you.
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