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fordmerc

auto-lite vacuum advance

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how can I tell if the vacuum advance works? It is off the distributor now but the protruding arm is rigid and I can't move it with gentle pressure (push or pull). I applied a vacuum cleaner to the vacuum port and again no movement. Are these valid tests? If not, how to test on the bench?

If replacement needed, can I use any vacuum advance from NAPA (assuming the arm is appropriate length)?

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Not a good sign that you can't move the arm either way! The way to test to insure that it will function when a vacuum is pulled is to apply a vacuum to it and see if the arm moves and if it hold the vacuum. Use your mouth over the fitting to see if you can draw and hold a vacuum. No, a vacuum cleaner doesn't produce any where near enough vacuum. If your unit holds a vacuum applied with your mouth (or a little mechanical vacuum pump), then see if the arm moves.

More thorough testing would be to mount it on the distributor and put that assembly in a distributor machine where you can measure the amount of timing change as a function of the vacuum applied.

A generic vacuum advance unit will need to have the specific advance set up for your car's specs; this is done via the spring tension and adding/subtracting shims to get the proper degree of change at various vacuums, again using a distributor test machine. If you get a rebuilt one, you can just transfer the springs and shims from your unit, assuming it was the correct unit in the first place.

Finding older vacuum advance units is becoming problematic; if it's truly old even if NOS or NORS, chances are the rubber diaphragm has deteriorated and is likely to fail within a very short time. There are folks who rebuild the units if you have an old core. Both Kanter and Merritt offer this service for some units.

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You should be able to push the arm in by hand. If you can then put your finger over the vacuum port and let go of the arm. If the diaphragm is good the arm will stay retracted. If it is bad the arm will slowly move back out.

Not being able to move the arm manually is a good indication that the unit is bad.

The best way to check is like Owen said and use a vacuum source to see if the arm will move.

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Thanks for the precise answers. It sounds like I'll need a new one or get this rebuilt. I don't have any significant test equipment but I can and will measure the amount of vacuum I apply, just to double check.

(I suspect I may have an aftermarket device since the words "auto-lite" aren't there.)

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The words Auto Lite would not be on there. More than likely if it was an original or and NOS replacement you would find a A/L type logo. I handle NOS and rebuilt/rebuilding, so feel free to contact me if I can help in any way.

To comfirm what the others said, It's definitely bad if you can't move the arm. Is it corroded? Having one that won't move at all is a little unusual.

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You can also test the vacuum chamber by removing the fitting on the end (where the tubing connects) and then the spring and shims (if there are any). Then, do the mouth vacuum test as described above. Easier to tell if the unit leaks this way; and, easier to do without having to work against the spring.

(o[]o)

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I am in the middle of replacing my vac adv. Max Merritt has them as well as Terrell Mach in Texas rebuilds them. I used a mityvac to test mine. It would not hold a vac nor move the arm. It manifested itself in that the engine would not idle.

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Never head of a car refusing to idle because of a leaking vacuum advance diaphragm. Normally the symptom is balking on sudden acceleration. Perhaps you have yet another problem?

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It would idle at standstill-it is a "coast down stall". I plugged the vac adv tube and went for a ride-it has not experienced the "coast down stall" since. I also adjusted the breaker plate gap/new coil/took out the cermaic fuel filter-each improved the perf somewhat. All during this however I could not get any vacuum reading on the vac adv-it must have a large hole in it-

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