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I tried to follow the Chilton guide in tuning up my ‘68 Mustang, 289 bored out to a 302. It said the advance should change with an increase in engine RPM. It does. It said the advance should change differently with the vacuum hose disconnected. It changes the same at all RPM’s whether the vacuum hose is on or off. When I disconnect the vacuum hose and put my finger over it I can feel a vacuum from the carburetor.

Do distributor vacuum advance diaphragms fail? If so, can they be replaced or can the vacuum advance be replaced or do I need to replace the entire distributor?

What happens if a drive with a failed vacuum advance diaphragm?
 
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you can replace the vacuum diaphram and arms. BUT, make sure your vacuum line is hooked up near the air horn on the carb. Less vacuum at idle and more at higher rpm. if you put it lower on the carb you get more vacuum at idle. 10-12 deg BTDC w/o the line hooked up. it should advance a little with a bit of throttle w/o the line connected (i know its vacuum advance but i think they had a slight mech advance built in that kicked in just off idle). hook up the vacuum line near the air horn and give it gas, should advance quite a bit now, maybe 20-30 deg. good luck ::
 

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The VA is easily replaced, and readily available at most auto parts stores. You can check the VA by removing the distributor cap and observing the breaker plate while sucking on the hose to the VA. It should move, and the diaphragm should hold vacuum put on it. If not, replace it.

What gmiles was talking about is that the VA is connected to "ported" vacuum at the carb. It should have little or no vacuum at an idle. If it has full vacuum, it is either the wrong port, or your idle is so fast that the throttle plates are opening too far, allowing excessive vacuum to the port. For this, and other reasons, the VA is always disconnected when setting initial timing.

I've heard some say that you can just connect the VA to manifold vacuum and it works great. If you stop and think about it, you then lose any benefit from the VA, and just as well disconnect it, plug the ports, since it will be on full advance at startup! The VA allows more advance at off-idle conditions before the mechanical advance takes over, with total timing maxing out at 3000-3500 RPM.
 

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289 bored to 302? That would be .080-.090 overbore, do you mean stroked to 302 with 302 crank.

Beside the vacuum diaphgram unit, check also inside the dizzy, sometimes the parts get stuck.
 

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The vacuum advance doesn't have anything to do with mechanical advance. A vacuum pump (Mightyvac) helps alot in a situation like this.
Mechanical advance is controlled by weights and springs mounted underneath the plate the points are on. Mine had a problem and when I checked, someone had replaced one of the springs with one that was way too strong, elimnating my mechanical advance.
 
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