vacuum advance help needed

68GEETEE · Apr 17, 2004

I set the engine to TDC, and re-installed my distributor with the rotor pointing at #1 terminal. I set the timing to 10 degrees btdc. The engine runs great, smooth, idles good, no longer runs hot and accelerates without hesitation. The only problem I still have is that she does ping while going uphill at highway speed. If I give it alot of gas, the pinging seems to subside. I rechecked the timing with the vacuum advance disconnected, and she is about 28 degrees at full throttle. When I reconnect the vacuum and accelerate , the timing jumps over 40. I know this isnt good.

The question is I have heard you can shim the advance so it doesnt go over 38. Does anyone know how to shim the advance?

zookeeper · Apr 17, 2004

While I don't have any experience with shimming the advance to change the amount of total advance, you could try an adjustable advance canister. I've installed a Crane adjustable on a car with an HEI ignition and it was a snap to install and allowed total adjustablity of the advance curve. While I don't know for sure that they make one for Ford distributers, I'd bet they do. Check with Summit, I think they're only about $20 or so.

fordman54935 · Apr 17, 2004

Are you using manifold vacuum or ported vacuum? I'm not positive if this would make a difference, but in my experience anyway I was able to cure an off idle stumble on my 71 Torino GT by switching from ported to manifold vaccuum. Apparently it advanced the timing enough to cure the stumble. Anyways, you could check to see if you are using manifold vaccuum, which may be bringing in too much timing advance at the RPM and load the engine is under while the problem is occuring.

68GEETEE · Apr 17, 2004

fordman54935 said:
Are you using manifold vacuum or ported vacuum? I'm not positive if this would make a difference, but in my experience anyway I was able to cure an off idle stumble on my 71 Torino GT by switching from ported to manifold vaccuum. Apparently it advanced the timing enough to cure the stumble. Anyways, you could check to see if you are using manifold vaccuum, which may be bringing in too much timing advance at the RPM and load the engine is under while the problem is occuring.

I am using a ported vacuum source. So you are saying that manifold vacuum may be lower and reduce the pressure on my advance unit?

68GEETEE · Apr 17, 2004

zookeeper said:
While I don't have any experience with shimming the advance to change the amount of total advance, you could try an adjustable advance canister. I've installed a Crane adjustable on a car with an HEI ignition and it was a snap to install and allowed total adjustablity of the advance curve. While I don't know for sure that they make one for Ford distributers, I'd bet they do. Check with Summit, I think they're only about $20 or so.

I will look into the adjustable, that may be the ticket.

D.Hearne · Apr 18, 2004

Manifold vaccuum will not cure a pinging problem. Ported is what you want. Either try the adjustable vacuum advance or buy better ( higher octane) gas. Also a cam swap can also cure the pinging.

351LX · Apr 18, 2004

Read this link http://www.gofastforless.com/ignition/advance.htm There seems to be a lot of misconceptions about the types of advance and whether one is better than the other so the link will answer them.

Ozsum67 · Apr 18, 2004

Stronger springs will keep it from advancing too much.

68GEETEE · Apr 18, 2004

Ozsum67 said:
Stronger springs will keep it from advancing too much.

I thought the stronger springs would only be used if your mechanical advance was too far forward? Without vacuum, I am pulling only 28btdc at wot, which I think is within limits. Its when I connect the vacuum that the timing goes over 40. I am under the impression that the vacuum is the culprit, not the springs.

68GEETEE · Apr 18, 2004

D.Hearne said:
Manifold vaccuum will not cure a pinging problem. Ported is what you want. Either try the adjustable vacuum advance or buy better ( higher octane) gas. Also a cam swap can also cure the pinging.

Running 93 Chevron from the same gas station. This is only a problem when the vacuum advance is connected, so I am thinking an adjustable may help.

Would a different cam affect the timing at wot?

68GEETEE · Apr 18, 2004

351LX said:
Read this link http://www.gofastforless.com/ignition/advance.htm There seems to be a lot of misconceptions about the types of advance and whether one is better than the other so the link will answer them.

Thanks for the link, the end paragraph seems to say what my condition is, meaning you have to get the vacuum to limit how much more advance it forces on the timing, I am thinking my stock unit is probably not adjustable, so as previously suggested I may indeed try the adjustable advance and see how that goes. Thanks everyone for the good info!

D.Hearne · Apr 19, 2004

68GEETEE said:
Running 93 Chevron from the same gas station. This is only a problem when the vacuum advance is connected, so I am thinking an adjustable may help.

Would a different cam affect the timing at wot?

Yes, I've got a stock pistoned 68 390 in a Merc Monterey that would ping it's butt off with the stock cam, but now after the overhaul, with a Crane 272 degree Energizer cam in it, it runs fine on 87-89 octane gas. Still the same pistons and heads, slightly less compression ( about a 1/4 point lower) with thicker head gaskets. ( 10.25 now vs 10.5 to 1 stock)

68GEETEE · Apr 19, 2004

D.Hearne said:
Yes, I've got a stock pistoned 68 390 in a Merc Monterey that would ping it's butt off with the stock cam, but now after the overhaul, with a Crane 272 degree Energizer cam in it, it runs fine on 87-89 octane gas. Still the same pistons and heads, slightly less compression ( about a 1/4 point lower) with thicker head gaskets. ( 10.25 now vs 10.5 to 1 stock)

Being that I am a novice at engines, I wouldnt have thought about the cam aspect. Thanks for the heads up.

Yesterday, I stuck an allen wrench inside the vacuum advance nozzle and turned it about 7 turns counterclockwise. Then re-checked the timing, she was now reading 39 total advance, which is lower than it had been. I took the car out for a good road test, and got smooth acceleration with no pinging at all. I think I finally got this thing figured out!

Thanks again everyone for your help.

vacuum advance help needed

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