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The problem: My '48 Victoria convertible developed a rough idle in the summer of 2017 and I had an email conversation with Dave Czirr that fall, and we were unable to pinpoint the problem. I wasn't able to tinker with the car last summer, but now I am back on the trail in search of a solution. At that time I did an idle screw adjustment with a vacuum guage and was able to achieve only about a 16.5 reading. Manual says it should be in therange of 18-21. I attempted to pull the plug wires one at a time in an attempt to narrow down the problem and discovered that all of them had an effect on the idle performance except for #s 7&8. I am totally perplexed why those last two cylinders are apparently contributing nothing to the engine's performance.

Some additional information and things I tried at Dave's suggestions
*the timing is dead on
*compression test was fairly consistent #1-130, #2-123, #3=118, #4-130, #5-118, $6-110, #7-120, #8-128 (car was purchased from an elderly couple in Kansas, who claimed to have a valve job done before I bought it)
*tried blocking off the vacuum lines from both the electromatic clutch and the vacuum booster portion of the fuel pump and re-tested, but got same 16.5 reading.
*replaced all of the spark plugs
*tried squirting some oil along the interface of the intake manifold and the block adjacent to #7&8 to see if there was any improvement, thinking that the intake gasket might be leaking. There was no improvement, but I did re-torque all of the manifold connection for good measure

Dave and I ran out of things to try, so I am hoping someone here might be able to steer me in the right direction. I do have a video that illustrates the poor performance uploaded to my Dropbox, but am not sure how to make that available here?

I wonder about the vacuum advance unit could be bad?

Are plugs 7 & 8 actually firing?

Thank you for your responses. I will check the vacuum advance and try to verify that the 7&8 plugs are actually firing.

Dave: I assume that you wanted me to remove each of the 7 k 8 cylinder plugs one at a time and verify that they were each firing on the business end when the engine was running. I did just that and both were sparking at the electrode. Engine sounds like a Model T when running with a plug removed.

I wasn't quite sure how to see if the vacuum advance was functioning properly?

Well, if the plugs are firing and both cylinders have normal compression as you indicate, I don't understand how there could be no discernible difference in engine idle when you short out those plugs. Are you still certain that shorting out 7 and 8 makes no difference in idle?

1, 2, 7, & 8 fire off the same throat of the carburetor, are 1 & 2 firing normally?

Make sure the firing order is correct...
A way to determine a vacuum leak is to squirt brake cleaner at the suspected areas and listen to see if there is a change in the sound of the engine. The cleaner is flammable so be careful.
At the distributor remove the vacuum line and plug. Then see if there is a change in the readings.

You can also remove the distributor cap and attach a vacuum pump to a line attached to the advance, pull in some vacuum and see if the plate the points ride on moves and stays moved. If the plate moves back there is a hole in the diaphragm.

Maybe a leak in the carb to manifold gasket.

Just some more ideas....

Thanks, Dave, for hanging in there with me on this one. The number 1 and 2 cylinders appear to be firing normally, as the engine stumbles when either of these is shorted out. However when I short out either number 7 or 8, no discernable change in the idle. I was pulling the plug wires off the plugs before, rather than shorting them out, and assume that the effect is identical? It's almost as though something is blocking the gas/air mixture from reaching the 7 & 8 cylinders, and the vacuum from those two cylinders from reaching the intake manifold; does that make sense? If so, what step could I take to check or remedy that?

Thank you 50 2382 for your directions on how to check the vacuum advance. I'll also check all the plug wires to confirm that the firing order is correct!

Good afternoon all...I am wondering if the plastic distributor body is cracked or distorted such that there is no power going to 7 and 8, or if the two plug wires are not conducting...Ernie in Arizona

Okay let's see. Good compression, low vacuum.

It's been a while, but doesn't the windshield wiper tap off of the rear most intake runner which feeds cylinders 7&8?

So try blocking off the windshield wiper feed.