FINALLY - Vacuum Advance Dilemma Solved

[SuperHart]

Note to Stallion, not to begin beating this issue another time, but 29* of advance (vacuum plus initial) at idle is too much. The Crane vacuum cannister is usually adjustable with a small allen wrench that came with the cannister kit (3/32" maybe?). To adjust your total advance at idle, use the timing light with the vacuum advance plugged in, not blocked off. Blocked off will measure only your initial advance (that which you dial in by turning the distributor). The advance with the vacuum plugged in will be your total advance at idle. This should be around 18-20* . As previously noted, as your total advance at idle increases so does your idle speed. The centrifugal advance usually doesn't begin to kick in until above 1500 RPM so keep that in mind. Repeatedly disconnect and plug the vacuum line at the cannister and adjust the vacuum advance with the allen wrench until your total advance at idle is reduced to the 18-20* mark. You may have to increase the idle speed screw on the carb before you achieve the correct advance setting (as you decrease vacuum advance, the idle speed may drop). To recap, check your initial advance at idle with the vacuum cannister disconnected (usually 6-10* for Ford engines) and then check your total advance at idle with the vacuum cannister connected. Always make sure when adjusting the vacuum advance that the vacuum hose from the cannister is plugged (a golf tee is excellent for this purpose) and repeatedly check after each adjustment until the desired advance is achieved. Then make a final idle speed adjustment on the carb and re-adjust your idle mix screws if needed to get a smooth idle.

[jwd]

Originally Posted by SuperHart

Note to Stallion, not to begin beating this issue another time, but 29* of advance (vacuum plus initial) at idle is too much.

WRONG. I suggest you re-read the article you posted by John Hinckley and pay attention to the third paragraph.

[DanEC]

Originally Posted by jwd

WRONG. I suggest you re-read the article you posted by John Hinckley and pay attention to the third paragraph.

It shouldn't pose any problems at idle but where you do have to be careful is in a highway cruise situation with a fast centrifugal advance curve. Cruising at 2500 rpm with 36 to 38 deg inital and centrifugal advance all in, and 18 to 20 deg vacuum advance can cause some motors to develop a miss or repetitive light stumble from basically firing too early before TDC. My old, aluminum head BB Chevy developed this problem and I had to modify the vacuum can to limit total vacuum advance to about 10 eg which fixed the problem.

[jwd]

Originally Posted by DanEC

It shouldn't pose any problems at idle but where you do have to be careful is in a highway cruise situation with a fast centrifugal advance curve. Cruising at 2500 rpm with 36 to 38 deg inital and centrifugal advance all in, and 18 to 20 deg vacuum advance can cause some motors to develop a miss or repetitive light stumble from basically firing too early before TDC. My old, aluminum head BB Chevy developed this problem and I had to modify the vacuum can to limit total vacuum advance to about 10 eg which fixed the problem.

I hope you meant 6-8 initial. The only problem I've ever seen from having too much advance at cruise speed is a surging situation. Initial timing, mechanical curve/total timing and the characteristics of the vacuum can (" of vacuum that it starts and at full and the total amount of advance) all have to work together. After you've tuned a few hundred, you get the hang of it.

Jim

[DanEC]

Originally Posted by jwd

I hope you meant 6-8 initial. The only problem I've ever seen from having too much advance at cruise speed is a surging situation. Initial timing, mechanical curve/total timing and the characteristics of the vacuum can (" of vacuum that it starts and at full and the total amount of advance) all have to work together. After you've tuned a few hundred, you get the hang of it.

Jim

No - total 36 to 38 initial and centrifugal advance - please read again. That's what my old 427 Chev runs when all the advance is in. Keith Craft ran my stroked 428 up to 40 degrees total advance (initial and centrifugal) on the dyno. That surging sensation is firing the ignition too far in advace of TDC.

[jwd]

Originally Posted by DanEC

No - total 36 to 38 initial and centrifugal advance - please read again. That's what my old 427 Chev runs when all the advance is in. Keith Craft ran my stroked 428 up to 40 degrees total advance (initial and centrifugal) on the dyno. That surging sensation is firing the ignition too far in advace of TDC.

Okay, I was reading it as 36-38 initial plus centrifugal advance. Now it makes sense. I know what causes the surging. When I tune a car and encounter it, I change the vacuum can to one that had a little less total advance. Usually only a reduction of a few degrees is needed.

Jim

[DanEC]

Originally Posted by jwd

Okay, I was reading it as 36-38 initial plus centrifugal advance. Now it makes sense. I know what causes the surging. When I tune a car and encounter it, I change the vacuum can to one that had a little less total advance. Usually only a reduction of a few degrees is needed.

Jim

Oh - yeah, I can see now how it could be read that way. I understand.

My BB Chev runs low vacuum due to the big, old-school cam and I had to purchase a special vacuum canister that starts to pull in at 6 inches vacuum. There might be one out there like it with less total vacuum advance but it was easier to make a little bracket that mounted over the arm and had a tab that stops the rod from advancing the entire way. That way I reduced the advance that was kicking in while cruising on the road and stopped the surging. Cruising down the road that surging/missing can be really annoying even if you understand what is going on.