VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 vacuum advance, 1949 Chevy 4400

I've made a big deal about the timing situation on my 49 Chevy. I make it run well through a combination of choke, carburetor and timing adjustments. I know I should be able to make it run well without the choke in the equation, if the carburetor is OK (it's new).

I've been unhappy with the timing situation I have been using as a 'work-around' for my 1949 Chevy 4400 (216 with 235 pistons). I've been trying to re-time it so that I don't need so much choke (or any choke). Vacuum test at the manifold/wiper port with engine running tests 15-16 inches mercury on one meter and 16-18 inches on my hand-held vacuum tester.

Testing the vacuum holding ability on the tube connecting the vacuum advance unit to the carburetor with the engine off indicates no ability to hold a vacuum. The vacuum comes up but bleeds down pretty quickly. Disconnecting the vacuum line at the vacuum advance and testing again (essentially just testing the line and the connection to my hand-held vacuum pump) gives perfect vacuum holding ability. So the bleed-down is not due to a leaking hose connection between my meter and the metal tube.

These data indicate that the vacuum advance unit is not able to hold a vacuum - it's leaking. No parts store in town has one. I don't want to wait on an internet store, but I guess I have to. I'm amazed with all the old cars and trucks around South Dakota that a person can't find a vacuum advance in town. Perhaps this is the problem I've been having all along.

Lee

Are there any tricks for a leaky vacuum advance like there is for getting the vacuum wiper motor going again (mercon treatment)?

Lee

The vacuum advance unit has a diaphram that more than likely has a hole in it. Its a good or no good deal.
Don

I put a new one on my car when I saw the dist. not rotating while revving up. It was an instant cure for the driving with the choke applied. Much more responsive in all gears, too.
Now I have to put ANOTHER new one on. Apparently a small pop back through the carb has torn the first replacement.
The original lasted 58 years, the replacement about a month. HMMMMMM.
Dean

Jeepers, Dean. That's bad luck. Did you get yours at Filling Station? That's where mine is coming from. But, probably all retail outlets get them from the same source......

Lee

Dean, I've been thinking about your recent post....

I don't believe a small pop back through the carburetor would exert much pressure on the vacuum advance diaphram.

First of all, the fuel system at or around the carburetor does not 'hold' pressure - there is no resistance to air flow as can be verified by putting a hand-held vacuum meter on the wiper vacuum port coming off the manifold. Attempting to pump vacuum there won't even make the vacuum meter needle move.

Secondly, the line to the vacuum advance as it attaches to the carburetor is perpendicular to the direction that a pop shock wave would travel. So the diaphram will not be hit by the full force of the shock wave. The wiper motor would also feel any force exerted by the small pop.

This is the most important part of my argument - The part of the pop shock wave that enters the vacuum advance line is further attenuated by the fact that the line already has a vacuum in it. So the shock wave, which needs a medium in which to travel, can't effectively get to the vacuum advance diphram. As you know sound can't travel in a vacuum. The vacuum in the vacuum advance line isn't a perfect vacuum, but it is enough of a vacuum (a vacuum that can support a colum of mercury 20 inches high) that sound will not travel well in it.

These are the reasons why I don't believe a small pop back through the carburetor can cause a failure of the vacuum advance diaphram. It was probably a unit of poor quality to begin with. Mine is also coming from Filling Station. I hope the one I get from them is of higher quality than the one you got!

Lee

This link is interesting. It shows the steps required to rebuild a vacuum advance. They cost around $36-$40 so not many people would go to the effort to rebuild one, but it is interesting to see what is inside one.

http://www.dalemfg.com/dale_025.htm

Dean, if you're going to buy another one, you could take the one you have (the new one that quit working), take it apart and see if the damage looks like a pop back through the carburetor could to that damage. Sometimes the poor quality is obvious when you can see it. My bet is that it was defective when you got it.

I'm wondering now if a (partial) failure of the vacuum advance can be due to the metal case leaking and not necessarily a problem with the diaphram.

I keep remembering the poor quality of auto components we get from China, Mexico and India these days. It's hard to find a voltage regulator that wasn't made in China, a 6 volt ignition coil that wasn't made in Mexico and wrenches that were not made in India or China.

Lee