I have reason to suspect my vacuum advance is not working. I first checked it by unplugging it and driving it around. It felt the same as it always does. Then i remove the diaphram and hooked it up, and cranked it up. It did not move no matter how much i reved it. I hooked it onto my manifold vacuum and it went down, but the vacuum off the carburator is not moving the thing. Should i replace the diaphrapm and hope that works? I have a pertronix if that makes a difference, i dont think it does however.

 

The port on the carb for vacuum advance is above the throttle plate so at idle or low rpm you not see any movement. When you hooked up to manifold vacuumyou had a high vacuum source at idle.It is more for economy than performance because it adds a few degrees while cruising. When you accelerate the mechanical advance is the important one.

 

I understand that the vacuum is lower at the carb, i knew that. It still didn't move when i reved it up. Economy? Good, i need that too. Im getting like 11 mpg right now with a totally stock 2v 289.

 

I think that 67Coupe was trying to say that when using the proper port on the carburator, the vacume advance is not supposed to work in your driveway.

If you want to check the diaphram and mechanical opperation of the vac advance parts in the distributor, (pardon me for saying this, but it works) disconect the vac hose at the carburator, (with engine off and cap off of distributor), the suck on the end of the hose, you should be able to see the advance plate in the distributor move. If the hose does not hold a vacume when you suck on it, then the diaphram isbroken, or rhe hose has a hole in it.

 

Oh, I already tried that. It doesn't move, and it is pulling air through it. Thats why i think it's bad. lol.

 

Yea, if air is passing through the tube, then the diaphram is for sure ruptured!

 

easiest way to test the thing is to disconnect the hose on the carb side. engine off.
put it in your mouth and try to suck air. when you put your toungue on the hose to block it you should feel it pulling back.
If you can suck a lungful air through that hose then yes. it's dead.

Another way is to get rid of the distributor cap. do the same thing.
when you suck the contact on the plate should move. And when you put your toungue on the hose to block it the contacts shouldn't move back.
If they do you have a leak. So again: new vacuum advance

kalli

 

Yeah, my vacuum advance is definately shot, It doesn't move, or hold any air. Well, I'll buy one soon. Too bad it wont help performance any.

 

Or, you could apply vacuum to it with a vacuum pump. They are relatively cheap, allow you to supply a known amount of vacuum, and allow you to see what the rate of bleed down is (don't know if that is the correct term). I agree though that the vacuum advance is not terribly important to the operation of the engine. In a car, you would notice it mostly going up a steep grade driving through the mountains, when the car is under heavy load. In a truck, you would also notice when you are towing or when the bed is full (generally not a situation you run into with a Mustang). Now if your centrifugal advance is shot, you would definitely have problems with missing even while just cruising at high speeds.

 

just a thought..I was tearing my hair out over the same thing,,lousy MPG,,,and I 2 saw the same thing with the vaccum advance,,as I understand it,,it will only kick in under a load,,,,,what fixed my MPG thing was the ,, *&^%^$$#@@$^%& carb !! I order'd a new one and while waiting for it I bought a rebuild kit ( had rebuilt it 2 weeks earlier) and grabed a pipe cleaner ( for smoken type pipes) and a wire twist tie from a bread bag and said to hell with it and and hosed everything down with carb cleaner and between the pipe cleaner and the wire tie from the bread bagbeing run thru where ever it would fit the wheels are getting 22 MPG now,,,so it is something to think about