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Hi All,

Are there any good engine guys out there who can help me diagnose an rough running engine???

Here is a video of the vacuum gauge with the engine running at various engine speeds:

youtu.be/1YYV77zbxog

The engine is a 1949 Packard Straight Eight and it was just rebuilt. I just adjusted the valves hot and replaced the spark plugs. All cylinders were compression tested and all are within 107 and 118 PSI during a hot compression test (spark plugs out and 5 compression strokes).

Can anyone PLEASE offer some suggestions/advice???

THANKS IN ADVANCE!!!

Bill

Sent you a PM. Is the vacuum to the advance plugged off since it is disconnected? I thought I heard a hiss noise like it might be open but maybe that is just something else the microphone is picking up.

A well-tuned engine in good condition should give a manifold vacuum of 18-20 inches at idle without much fluctuation so you're in the ball park. You might use the gauge to tweak the idle mixtures a bit - set for the leanest mixture consistent with maximum vacuum.

I don't understand why you timed to 11? BTDC, spec is more like 6?.

>I don't understand why you timed to 11? BTDC, spec is more like 6?.

I originally tried 6 BTDC however the engine had OK-ish manifold vacuum and OK performance. I advanced it to get the best engine idle vacuum. I will try a couple of high engine power passes (low RPM and full throttle in high gear) to see if it pings...if so I will retard ignition timing.

In regular cars (non-70 year old-ish cars) you adjust the low RPM ignition timing to get the most manifold vacuum and then adjust the high RPM low manifold vacuum ignition timing (full throttle mid RPM to high RPM passes) to get the timing such that there is no pinging.

With the timing that far advanced I would expect the engine to be quite "skippy", as it seems that it is.


The goal is not to get the maximum idle vacuum. The advance curves are calibrated to the combustion characteristics at many speeds and loads. At 11 or 15 degrees the car will not drive well, and will certainly not give maximum power as the cylinder pressure will peak well before the piston reaches top dead center. Even so, it might not ping, depending on the compression ratio. Also, the car will uncommonly hard to start when it is hot.


I would not be remotely concerned that the idle vacuum reading is a a bit low at this stage in the engine's life. It often takes quite a while for these engines to break in; until they do you have extra blow-by past the rings, and the extra friction will will take several points off the idle vacuum reading.

Just set the timing to 6 or 7 at a low idle and go for a long drive.

Agreed, Ross. I never did comprehend the use of manifold vacuum to establish ignition timing. At 11? BTDC I'd expect the engine to run very rough and be difficult to start when hot.

Thanks guys...greatly appreciated.

I will go back to 7 BTDC and see how the engine breaks in.

THANKS for your help!!!