Packard Forum Index » Pre-War (1899-1942)

Tune-up data is kind of hard to find for this year & model.
I did find this info in the SHOP MANUAL but it is confusing to me. Perhaps one of you tune-up specialists can enlighten me.
Here under distributor is what it states:
VACUUM ADVANCE:
Start 0 deg at 7"
Intermediate 1 deg at 8 5/8"
3 deg at 12"
4 deg at 13 1/2"
Full Advance 5.5 at 16"

What does this mean?? At idle I get approx 14 to 15" of Hg at around 500rpm. Normal?
Anyway I thought when you rev the engine the vacuum would go up appreciably. But when reving the engine it maybe goes up to 17" but not an appreciable amount. Is that normal for this engine? I think in more modern engines they are talking about vacuum in the 30" Hg range.
On most engine tuneups they usually tell you to disconnect the vacuum before timing. All I found thus far, was to set the timing 4 notches before TDC. That is not much advance is it?
I could find no data that states to disconnect the vacuum advance nor at which speed to check the timing. All confusing.
Any advice or help at all would be appreciated. Thanks.

Your vacuum reading is modestly low, is it steady or also fluctuating? If steady it may just be a reflection of improperly adjusted idle mixtures or ignition timing. Old Motors Manuals have a good section on diagnosing vacuum readings. Large rapid fluctuations can indicate valve seating issues.

Your engine if in good tune and internal condition should give a steady manifold vacuum reading at idle of 18 - 20". Under heavy load as in accelerating it will drop dramatically, and upon recovery as in coasting it will rise well above 20".

The figures you provide give the number of degrees of timing change that those various amounts of vacuum provided to the vacuum diaphragm should produce. The vacuum units can be adjusted if necessary to match those values by springs and shims bearing internally against the rubber diaphragm. Best done on a distributor test machine such as those made by Sun.

You check the timing at idle speed and as I noted to you in your inquiry on the Packard Club forum, the idle speed must be below the speed at which the centrifugal spark advance initiates, in other words less than 500 rpm. Nice idle speeds for those motors are in the range 425 rpm.

The vacuum line to the carburetor is ported into the carb bore above the closed throttle plate position. Therefore there is no vacuum present in the line or diaphragm at idle and hence no reason to disconnect it to set the timing. If the vacuum line entered the carburetor bore below the throttle plate, then the opposite would be the case.

Appreciate your message & knowledge. You say my vacuum @ 500rpm is low. What could cause this? Could timing cause it? I still have not checked the timing because not sure what criteria to use. If I check the timing @ 500rpm what mark on the balancer should it be on? One source I looked at, states 4 marks before TDC. Isn't that kind of retarded when most more modern engines are more advanced before TDC?

I notice that when I increase the idling to around 800 or 1000, the tack pointer really jumps around. Is that normal? I believe it is because the high speed part of the carb is trying to cut in, but it sure isn't doing it very evenly. What could be the cause of this or is this normal? The tach is steady when I reduce down to 500rpm.
Lee

As I said in my last response, two reasons your vacuum is low could be improperly adjusted carburetor idle mixture, or timing. And there are other reasons as well (for example, a vacuum leak) but first adjust your idle mixture to maximum vacuum consistent with the leanest mixture, then check the idle speed and if OK (less than 500 rpm) put a timing light on it and set the timing - forget "marks" and "notches", the front damper is marked degrees. Set to 5-6 degrees BTDC. Readjust your idle speed if necessary, recheck the timing again, and now see what your vacuum is.

It will make timing with a light much easier if you put a small dab of white paint on the pointer and degree line on the damper.

Forget timing specs of modern engines, not applicable.

No, the tach should not start "jumping around" as you increase to 800-1000 rpm.


PS - timing marks in degrees, from 1941 Owner's Manual.

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Agree with O_D, that the specs for the vacuum advance are those one would use with a distributor testing machine where the amount of vacuum is adjustable. I don't think these readings can be duplicated with the distributor installed and connected. However, with an adjustable vacuum source, one could disconnect the vacuum line and connect the adjustable source to check. This job would most likely be done on a machine when setting up the distributor after servicing. Anyway, this is the way I did it.