360 running on 4 cylinders
vacuum gauge. It's bouncing so wildly,
In order for that to happen;
Firstly the gauge is not properly damped.... but if it is telling the truth, then;
Secondly, you have some cylinders that are pulling vacuum, and some others that are not only not pulling, but are actually sending a pressure spike up into the intake............... after the engine reaches operating temp.
To me this points to sticking intake valves or intake valves not closing, which I might interpret as lifters pumped up, or pushrods too long, but I can't think of anything else.
If you opt to do a LD test. you MUST do it HOT, when the engine is in the failed mode. If you wait until it has cooled off, you will miss it.
But, IMO, the gauge is already telling you about the problem, and a LD test is a lotta work. There is an easier way; just pull the plugs out of all the cylinders that you know are faulty. Plumb your gauge, and start it up. If I'm right, the pistons in those cylinders are now gonna pump the pressure out the plugholes, and the vacuum gauge should settle down. Altho, with 4 missing cylinders, I would expect the actual reading to be quite low.
With those cylinders now not upsetting the Idle fuel mixture, the richness should also disappear.
It's gonna be loud.
Yes I'm guessing, but this is IMO, a logical interpretation of the symptoms you have detailed, that began at cruising speed, lightly loaded. Why those 4 just happen to all be on the same plenum is a mystery to me. or
Thirdly that one-side plenum could be receiving multiple pressure pulses from some other source, like say a floorjet into a cracked or broken cross-over passage. But I think that's really stretching it.
You only need one bad cylinder pumping into the dual-plane intake, to mess it up for the other 3 om the same plenum, so I might be tempted to run it on all seven others, with only the #6 plug removed, and see what the vacuum gauge says.
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