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Testing Vacuum Advance

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Frank C.

Frank C.

Jedi Hopeful
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I think my vacuum advance isn't working properly but I don't know how to test it. Lately I get some pinging when the engine lugs down slightly like climbing a hill. If I ease back on the gas it goes away. I haven't change petrol grade (Sunoco 93) for three years so I don't think it's that.

Any advice?

Thanks as usual.
 
[ QUOTE ]
I think my vacuum advance isn't working properly but I don't know how to test it. Lately I get some pinging when the engine lugs down slightly like climbing a hill. If I ease back on the gas it goes away. I haven't change petrol grade (Sunoco 93) for three years so I don't think it's that.
Any advice?
Thanks as usual.

[/ QUOTE ]
The vacuum advance would not likely cause pinging as engine load is increased. Just the opposite. It should give advance (in addition to the static plus centrifugal advance settings), only at light engine loads. It would retard as engine load is increased & vacuum decreases. Easing back on the gas would cause the vacuum advance to increase & increase pinging. The advance can be tested by using one of the various hand vacuum pumps/gages. The point mounting plate should rotate clockwise as applied vacuum is increased & spring return (rotate CCW) as vacuum is reduced.

There is some leeway in actual octane ratings of a given gas type from batch to batch. You may have gotten some with a slightly lower octane rating than usual. Or your timing may be near the maximum usable advance. There could be other reasons for pinging, but check the easy things first, such as changing fuel & retarding timing a bit.

I would try turning the small timing adjuster nut on the distributor a couple of turns in the retard direction. There are small arrows on the adjuster, near the vacuum cannister, to show A & R directions.I believe the retard direction would be turning the adjuster nut's top away from you, toward the distributor body, as viewed from the top. This will retard the static timing a couple of degrees.

A full timing check of initial plus centrifugal advance, vacuum disconnected, should show a maximum advance of no more than 35 to 38 degrees at around 4500 rpm.
D
 
You can still test your vacuum advance. The test may actually point out other problems!
Remove the distributor cap. Put a length of hose on the advance module - 1/4" fuel line works well for the threaded type, but clamp it on. Otherwise you can just disconnect the vacuum line from the carb/manifold suck on the open end and watch the points plate rotate. You should feel vacuum build in the line. The plate should turn clockwise. As you put your tongue over the end of the line/hose it should hold vacuum. When you release the vacuum the plate should rotate CCW back to the original position. /ubbthreads/images/graemlins/blush.gif
 
Dave and Jeff,
Thanks for the education and advice. Looks like I know what I'll be doing this Saturday morning.

Thanks again,
 
The only operable comment here is "Suck it up" Have waited a long time to say that.
 
Update ...
I "sucked it up" and the the advance seemed to be working fine. I retarded the timing a few turns (as suggested) and it was much better on a beautiful 140 mile ride yesterday. I'll tweak it some more next week.

Thanks as usual for the advice and guidance.
 
You are now qualified to say "Suck it up" to the next poor sole, lol.
 
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