Packard Forum Index » V-8 (1955-56)

Whether the vacuum line to the distributor advance needs to be disconnected and plugged depends upon where the port for the vacuum line is located in the carburetor.

If the port is below the throttle plates, then the port (and line) are exposed to vacuum all times when the engine is running - requiring disconnection and plugging of the.

If the port is above the throttle plate, then the port is not exposed to vacuum when the engine is running until the throttle is opened - no disconnection of the line is needed.

Now to complicate things: There are some applications where the vacuum port is a slot and part is below the throttle plates and part above. This results in the portion of the port below the plates to be exposed to a partial vacuum, with full vacuum achieved when the throttle plates are opened. This design may or may not require disconnection and plugging of the vacuum line.

I know, I know, this is way more than you wanted to know, but I couldn't help myself.

When in doubt, try it out.

JW -

Good explanation - seems to cover it all. The evolution of distributor timing control can be very confusing.

I believe the port for vac advance on the Packard V8s is the slotted type, but I never inspected it closely enough to see where the throttle plate rested, in its closed (idle)position, with respect to that port. My guess is that the slot is cut to be first exposed only when you set the choke and the fast idle cam kicks in - for higher RPMs.

Timing advance is specifically needed to accelerate the engine RPM - a demand situation which is "driven" by the driver's foot, through the throttle blade angle.

With the various carburetor types and brands used on Packard V8s, (and even the possibility that someone has adapted an aftermarket carb), it's a good idea to verify how things are set up for any particular engine. It's simple enough to check whether vacuum advance is a factor in initial timing (at idle). Pull the rubber line off at the diaphragm fitting and put your finger on the end while the engine is idling. If you feel suction, there is vacuum present and disconnecting the line will affect initial timing. If you don't, it won't.

But this is assuming the line is connected correctly to start with. There is often more than one place to tap into manifold or carb vacuum. The shop manual might show the routing of the line, if it gets that detailed.

Following the manual specs for initial timing gives a good starting point for reference, and may well be the best setup in the end. If that nasty Engine Rattle persists with 5? BTDC initial timing, good points set to spec, and all else in good working order, there are other possible causes besides "too much" timing advance that could be the problem (carbon build-up, lean mixture, bad gas, etc.). Good idea to check for these conditions, because simply retarding the timing may get rid of the noise, but result in other performance problems, like poor mileage, poor throttle response, loss of power or hesitation (to name a few).

Be sure to test the vacuum advance control unit for a vacuum leak. An engine can be tuned around an overlooked vacuum leak, but the leak and lack of timing advance will cause plenty of driveability issues down the road.

A MityVac handheld vacuum pump is invaluable in this regard, but can handle several other automotive tasks - pays for itself in short order.

Mity good idea there, Brian,

This thread really has a lot of good info on a topic that isn't discussed much for some reason. About the only thing not yet addressed is centrifugal advance inside the distributor. Specs on that would be a nice addition. And a reminder that poorly functioning centrifugal advance/retard can really louse things up, (as of course, a wobbly shaft can).

When you get down to it, the electrical system from the distributor to the plugs is one of the most accessible and important maintenance areas for any vintage engine with point-type ignition. Few special tools are needed and dramatic improvements in economy and performance are possible with little time invested. Plus, it can be fun!

Anybody here remember the old "Magic Window" on GM distributors? A nifty little design when it worked, which was most of the time. It was actually designed to help set points "by ear" with no gauge needed.

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I didn't know tyhat they had a name for that little window. It is truly the best system I ever worked on to set dwell. I did a tune up on 65 Olds this summer which has that distributor and it is a real pleasure to work especially with the distributor at the back of the engine. The "Octane" adjuster" on my Packard distributor is also a great idea.

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That sounds cool, more info please! Is it a dial setting, or what?

Attached is a photo of what Packard Autolite distributor with the fuel compensator attached. With this device the ditributor can be turned 12 degrees advance or retard. I set my timing to factory setting of 6 degrees BTDC and mark a spot on the engine block at where the dial indicates "0" and can then move the distributor either way the 12 degrees with the ability to return it to the factory setting.

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Simple and effective little piece of gadgetry. That device dates back at least as far as 1935 on the Delco 662W distributors used in the Eights and Super Eights.