Expert: Ben Deutschman - 8/16/2010

Question
QUESTION: hello  i disconected the advance set timing to 5 degrees then reconected advance my question is should i notice a change in the timing at idle right now it stay the same but does advance when you rev the engine it runs ruff at idle and in drive at a stop but fine at speed i need to know if the new vacum advance is defective

ANSWER: At idle, the vacuum pull is steady, so there wouldn't be much of a change in the Timing setting, but any change in engine speed, would in turn affect the static timing, which is one of the reasons initial timing is done with the Vacuum Advance Line disconnected and plugged. If you disconnected the Vavcuum Advance Line, and left it unplugged, you'll notice a change in Engine speed once it's reconnected, because the open Line created a vacuum leak, leaning out the idle mixture.

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QUESTION: thats my question after setting static timing and reconnecting vacuum advance hose i see no difference in timing setting, it stays the same, but when i increase engine speed in neutral the timing moves freely  so i take it either my vacuum advance is defective or theres a problem in carb  at idle

Answer
If the Timing Mark moves smoothly as you increase Engine speed, generally that is the one of the accepted tests for proving that the Vacuum Advance is working. Also, most Slant-Sixes up into the early 1970's, idled at around 550 RPM in Drive, which does not cause the Vacuum Advance to kick in, even with the Vacuum Line connected. The only other way to check a Vacuum Advance Diaphragm, is to connect a Vacuum Pump to the Advance unit, making sure that the Line from the Vacuum Advance to the Vacuum Pump, is air tight, and then draw, and hold a Vacuum on the Vacuum Advance. It doesn't take much, maybe 5-10 pounds of pull to test the Diaphragm in the Vacuum Advance, and once the Vacuum is drawn on the Vacuum Advance, all you are looking for, is that the needle on the Tester doesn't begin to drop after you've reached the desired amount of pull (5-10 lbs).

What else is you Slant-Six doing that made you think of checking the Vacuum Advance?