I have a 1977 F250, 460. The thing shudders at idle. Runs like a champ otherwise. Aside from the shake, it has a very clean idle. It's not misfiring at all, smooth under load, plenty of power, no hesitation. Here's what's been done.
1. New distributor and cap
2. New plugs and wires
3. New carb
4. New harmonic balancer
5. Timing set up and checked twice
6. Motor mounts are good.
7. Ruled out vacuum leaks
Took it the local guy (known as the local classic Ford expert) who checked all my work, redid some of it, and did the balancer for me. All we can think of is to pull the engine and send it to a machine shop to get it re-balanced. Before I go that route, I am hoping for some fresh thoughts.
I don't know about the camshaft.
Timing is 12 degrees per the sticker
I didn't personally do the compression test but shop told me it was OK.
I have a steady 19 on the vacuum, I can't find any small leaks.
It's difficult to understand exactly what shudder means. If you could send a video with audio it would help. If you didn't do anything to it to upset the balance I doubt that balance is the problem. Since it probably hasn't been doing this since 1977 think back about what may have changed that could be associated with the shudder. If nothing, then check the ignition module. I've seen them do some strange things. Just swap it with a known good one and see if anything changes.
Replaced the ignition module today, shot carb cleaner everywhere I could find. No luck. The shake is not rythmic (near as I can tell), so balance is doubtful. That leaves ignition and fuel. It has a new everything. This is driving me crazy.
Could be! Truth be told, the old Motorcraft carb was so far gone there was probably no way to tell how the thing would run. So I did all that work- brand new Holley classic carb, new distributor, wires, plugs, coil. It's a mystery.
Could the vacuum advance canister(if you are using vacuum advance) be hooked to manifold vacuum instead of ported?
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