Packard Forum Index » V-8 (1955-56)

55 400 idle variations
With engine warm and the vacuum advance disconnected and the port plugged, idle is unsteady. A vacuum gage connected to the rear port of the carburetor reads max on the gauge. Visually comparing the distributor position to a 56 400 and firing order are the same. The 56 idle is smooth and vacuum is normal. I am at a loss as to what could be causing this unsteady idle, and high vacuum. Any suggestions as always would be deeply appreciated.
Happy

Best guess is that your timing is vastly advanced. You will need to check it with a timing light.

Thanks for the feedback. High vacuum due to my using the wrong port. I plan to remove the distributor. I wish there were one of the old Sun machines near here but, they are almost nonexistent. Timing mark unsteady. moves +/- 5degres.

Happy

I wonder did you have it connected to the vacuum pump instead of the manifold?

I can grab some pics off my car.

Pics

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Yes, but now connected to proper manifold port. Thanks for the pics. You may recall during your visit that I have a running 56 400 that I have been using for reference while restoring the 55. The wiring order is correct, distributor orientation is the same. I have verified TDC and rotor positions are correct. With all this the RPM and timing mark on damper are unsteady. It is a puzzle.

Happy
For what it's Worth: when the engine was dissembled for rebuild (with less than 38,000 miles) number 7 piston had hole (burnt?) in the top compression ring grove. possible crossfire ?

Detonation due to low octane fuel could cause piston failure. Another possible cause is a vacuum leak that caused that cylinder to run too lean, allowing it to detonate. Ring gaps not staggered correctly could be another cause.

Defective points, worn distributor bushings or other worn parts in the distributor advance mechanism could cause the timing to "float". Too much friction at the distributor cam/rubbing block could also cause it. A weak spring in the point set might cause it, or a spring that was way too stiff might.
A throttle plate that is loose on the shaft or a loose throttle shaft can also cause erratic idle, and the erratic idle might affect the timing as shown with a timing light.

There was also a change between 55 and 56 to space out the plug wires to prevent cross firing between cyls 5 and 7.

Here is the STB

https://packardinfo.com/xoops/html/downloads/SC/SC-VOL30NO3.pdf