Vacuum advance not working

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Dec 17, 2014 | 08:10 PM

quakerj

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Joined: May 2013

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From: Flaherty, KY

Vacuum advance not working

I was out tinkering with the truck today, just a general tune-up was in order: plugs, wires, cap, rotor, etc. I was fiddling with the distributor and decided to check the vacuum advance.

I was surprised to see that the rotor is not moving when vacuum is applied, and the diaphragm or whatever is inside there is not holding vacuum. There is a lot of resistance (meaning it's not as though I was sucking through an open straw), but the rotor wouldn't budge and it would not hold any amount of vacuum. The hose was not the culprit, because if I plugged the end of the test hose I was using, it would hold vacuum just fine.

How does one go about replacing just the vacuum advance, or does the whole distributor get replaced? How would this affect driveability or the truck's performance / gas mileage? I've owned the truck a couple years, and it may not have been working the entire time I've owned it and I would've never known. I don't have a great deal of experience with mechanical stuff, my expertise lies in computer controlled fuel injected stuff.

Any help would be greatly appreciated. Truck details are in signature.

Dec 17, 2014 | 08:52 PM

Tedster9

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Joined: Apr 2002

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From: Waterloo, Iowa

How many miles on the engine? Vac advance can be replaced. It will improve fuel economy and driveability some because the best mileage is realized just short of ping or spark knock.

Dec 17, 2014 | 09:47 PM

Gary Lewis

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Joined: Jul 2010

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From: Northeast, OK

There are two screws holding the vacuum advance in, but there's usually a C-clip holding the arm to the mechanism. I've swapped the advance out with the dizzy on the engine, so it is doable and you won't have to move the dizzy so won't change the base timing.

Edit: I should have said that the Crane adjustable vacuum advance would be my recommendation. And, as Ted said, adjusting the vacuum advance until you are just shy of pinging will net you the best MPG.

Dec 18, 2014 | 07:35 AM

1986F150six

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Joined: Jul 2009

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From: Sheffield, AL

In my quest to "peel the layers of the onion", per Gary, I have records of running with and without vacuum advance. The mpg of my 4.9L engine increased by 2-3 with the vacuum advance working.

Dec 18, 2014 | 08:07 AM

Franklin2

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Joined: Jan 2001

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From: Virginia

I don't know if sucking on it with your mouth is enough to move it. Find a port on your engine that has vacuum at idle. Unplug the dist hose and with the engine running at idle, plug the dist in the port temporarily that has vacuum all the time. If the engine speeds up when you plug the hose in, it's working.