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How to test vacuum advance?

3002 Views 5 Replies 5 Participants Last post by  daveoxide
Hi guys,

I have a dumb question. How can I check to make sure my vacuum advance is working properly on my '66 289?

Thanks,
John Thomas

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Even without a timing light, you should hear the engine idle speed increase when you hook the hose back up.

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The timing is supposed to advance as RPMs increase up to a certain point. Check the shop manual for the specs. You can adjust it too but that's not what your asking.

Check the timing with the car at idle. Now pull on the throttle lever next to the carb slowly to make the RPMs increase. Keep watching the timing with the light as you do this. As the RPMs increase the timing will too up to a certain point. After that it will remain at a fixed point.

On the flip side, remove the vacuum line from the VAD and plug it up (a pencil works great). Repeat the test above and you'll notice that the timing doesn't advance with the increase in RPMs.



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I measured my timing as a function of engine rpm with and without the vacuum advance. At about 1500 rpm, the vacuum takes over, and advances it quickly. I made measurements at idle, 1000, 1250, and every 250 rpm up to 2750rpm. Took only about 15 minutes. I set my rpm by adjusting the curb idle speed screw.

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