Vac advance debate!!

A good distributor reference is in this month's Mopar Action, December 2020, pages 80-87.

Upgrade to a Mopar Performance Electronic Distributor Kit P3690426 Electronic Conversion Kit, Small Block. OEM is best if you can find one of these kits.

For the record, the vacuum advance is required for a street driven car to keep the fuel burning efficiently and not washing oil off the cylinder walls. Richard Ehrenberg has preached this for years and in a practical sense with pump gas required a carefully tuned vacuum advance to not detonate (ping) and cause engine damage.

Tim Ziegler said:

Just cuz we are in to the distrib, I couldn't get a 350 Olds to run and with a dwell meter I saw dwell variation where the needle wouldn't stay still. The cam lobes weren't true anymore. A new 49 dollar distributor and that car came on like gangbusters. Probably not your issue, just threw that out there

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The distributor on my car is the original one so that may be the case. I'll have to look into it. Thank you!

I have seen the vacuum advance both ways with ported on some cars and manifold on others at idle. Some like the extra timing at idle as it helps the car run a bit cooler but most I have seen seemed to use ported from the factory. And you have to remember that ported vacuum is actually manifold vacum thats just above the throttle blades so it has no vacuum at idle. I saw some Fords in the early to mid 70's when they were putting alot of emission controls on cars that used ported vcuum at idle but had a vacuum switch on them so if the car got to hot at idle the switch would move up and send manifold vacuum to the dist at idle to speed up the eng and fan and advance the timing which all help cool the eng more. I agree vacuum advance is good for a street car but I dont run a vacuum advance because I have a Mallory race dist with no vacuum advance on it but I have full 36 degree's timing by 1800 rpm and idle with about 24 degrees which is what my eng likes. Have you tuned the car lately ? I only ask because I wonder how the plugs are ? I had a 6 cyl Maverick once that did the same as you are saying and I found out it was burning oil and had a ton of carbon on the plugs and it just would not take any more timing without running bad. Once I put good plugs in it the car ran great and the vacuum advance worked good. Did you test the new vacuum canister to see how much it is advancing the timing ? Ron

vacuum advance is for fuel economy. It allows for the engine to run a leaner mixture in cruise modes. A leaner mixture require more timing to get burning properly.

Some engines won't tolerate or don't like the extra advance provided. If you have a huge cam or a set up that isn't going to benefit much from vac adv, why run it. My .650 roller engine isn't going to suddenly get 25mpg because it has vac adv. It's all relative to the build. Even if it gave me an extra 1 mpg... the hassle may outweigh the benefit.

sounds like the breaker plate is loose & when vacuum pulls on it the point gap gets out of wack

So.....a vacuum advance really does not advance the timing at all, but retards or controls it.
Vacuum chamber is another term used in place of vacuum advance, and is a more accurate descriptor.

I am experiencing a 383 engine running rough at lower rpms and part throttle loads. It's a 4 speed car that "surges" when driving through town in 2nd or 3rd gear. The surges aren't excessive, but they are enough to make me push the clutch in to quite the annoying bucking.

I suspect a distributor issue and plan to find the problem and return to a more comfortable ride.

pbiggs said:

So.....a vacuum advance really does not advance the timing at all, but retards or controls it.
Vacuum chamber is another term used in place of vacuum advance, and is a more accurate descriptor.

I am experiencing a 383 engine running rough at lower rpms and part throttle loads. It's a 4 speed car that "surges" when driving through town in 2nd or 3rd gear. The surges aren't excessive, but they are enough to make me push the clutch in to quite the annoying bucking.

I suspect a distributor issue and plan to find the problem and return to a more comfortable ride.

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"So.....a vacuum advance really does not advance the timing at all, but retards or controls it."
Just the opposite. Advances it a fixed number of degrees during a low load/high vacuum condition. What number is stamped on your advance arm?

turbine68rt said:

"So.....a vacuum advance really does not advance the timing at all, but retards or controls it."
Just the opposite. Advances it a fixed number of degrees during a low load/high vacuum condition. What number is stamped on your advance arm?

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I do not know the number stamped without checking the arm, but this rough running issue just started and one of the things I did do was use a vacuum pump to check for plate movement. The plate moves.
I now have changed the points, new plugs and rebuilt the carb with no improvement. My dwell meter is steady at idle, but I must admit I did not check for needle flutter at 1500 rpm.
That's next on tap

Typically, the bucking problem at idle or low speeds is caused by a lean condition. You might try opening up your mixture screws a half turn or so and see if it improves...