does vacuum advance really help ?

[andy2175m4]

I have a 1965 Thunderbird with what is probably not the original carb, it may be newer than a 65, it's an Autolite 4100 and works pretty good, but it has the ported vacuum port for the vacuum advance.

For the 1965 does the vacuum advance vacuum line go to the carb port, or straight to the intake manifold ?


thanks

[georgemii]

I predict that you’ll get a variety of opinions on this.

Mine goes like this: vacuum advance functions as a measure of
throttle setting, which is then used to extend burn time (advance)
or delay (retarded). Following that line of reasoning, I use manifold
(not ported) vacuum; i.e.: let’s tell the distributor what the carb is
doing at all conditions, not just some conditions.

My $0.02 worth.

[jtschug]

Is it possible you have an Autolite 4300 carb? I know you said it was newer. I don't think the 4100 had a provision for timed vacuum.

Anyway, you want to get vacuum for the spark advance from the manifold or at least from below the throttle plates. Vacuum from above the plates was used in conjunction with pollution controls in the 1970s, and when I tried it on my car (with a Holly 4150-style carb), it ran hot and got worse MPG, so no advantage, and some disadvantage.

[bbogue]

I hope someone with a 65 shop manual will answer your question. And, if you think you might have a later model of something else, there are lots of photos on-line at carburetor parts sites, Mikes Carburetors, for instance.

As far as I know, the 4100 has a vacuum port only for ported, some say timed, vacuum. Vacuum is only available above idle so no timing is added by the vacuum advance at idle. Opinions of converting to full manifold vacuum for vacuum advance vary. I did the conversion to full manifold vacuum for vacuum advance as part of an effort to cool the engine better at idle. Initial timing of 15 degrees is supplemented with 5-6 degrees of timing from the vacuum advance. Under heavy throttle, when vacuum is low, timing from the VA goes away to protect the engine from too much timing, the same as it does with ported vacuum. The only difference is at idle. My engine seems to like this arrangement just fine.

Good luck.

Bill