Vacuum advance question

H

hoss73ford

Senior Member
Bought a '74 F-250 supercab and it runs great. Later when I popped the hood I noticed there was no rubber hose connected to the distributor vacuum advance. yet it runs great. I thought with it missing it wouldn't run at all.
No it is not an Edelbrock or any aftermarket set-up (dealbreaker if it was, I only want all Ford on my Fords)
 
I'm not really familiar with that specific truck but in general if it's original Ford parts, Yes the hose should be hooked up and it would run even better. The hose, or where it should be, is probably capped/plugged to prevent a vacuum leak which would make for a lousy idle.

I'd first suspect that the dist vacuum diaphragm is bad and that's why it's disconnected.

A quick & dirty way to test the advance diaphragm is put a vacuum line on it long enough to reach your mouth then with the dist cap removed suck on the hose to see if the points plate moves. If it does, quickly put your tongue on the end of the hose to see if the vacuum holds or leaks. If it doesn't hold the vacuum the diaphragm is shot. If it holds the vacuum but doesn't move the points plate, the plate needs a clean & lube.
If it has electronic ignition there will be a sensor where the points would have been, but the vacuum advance mechanism works the same.

FYI, the initial timing is usually set with the hose removed from the dist & plugged (any official 'tune-up procedure' specific to your truck will have that in the instructions) then the dist takes over the timing advancing duties (while driving) after it's hooked back up.
 
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Maybe someone recurved it to run with just the mechanical advance. That's how I set up the dist. in my Yblock. If you have a timing light check to see how much total advance you get when you rev the engine. Should be in the neighborhood of 35* with initial advance at 10*.
 
I drove a 73 model car with a factory 302 and a custom curved distributor. It ran well with no vacuum advance and passed emissions for that model year.
 
Actually the vacuum advance is a overlap from mechanical advance . Vac is high when rpm is low and vac is low when rpm is high.

Lets say the mechanical advance is 24 degrees (engine) at 3000 rpm and the vac is 24 degrees at 19 inchs of vacuum and the engine is running 1500 rpm for 12 degrees of mechanical and vac is 15 inches for another 12 degrees . The engine would then have the needed or desired 24 degrees at this position....

Vac advance is approx 2-3 mpg on most engines . If you have them both you should use them........
Questions.........???????:eek::)
 
My understanding of vacuum advance is that it is ported vacuum which increases with airflow through the carburettor Venturi. There is little or no vacuum applied to the vacuum canister whilst idling. You don't need max advance when vacuum is high, the vacuum advance is there to give maximum advance when engine revs are high, this is certainly how the vacuum advance works in conjunction with my Holley 4160 carb.
 
By hooking up a timing light and watching the timing marks while revving the engine you should be able to see if it is advancing enough to get to the tune-up specs for that engine.
Since it won't be operating under any load the timing will probably indicate slightly differently than when driving. But there should be additional advance created when the hose is hooked up, if the vacuum advance mechanism is working.
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Vaccum advance in its stock form is generally for fuel economy, like mentioned before if you hook up a timing light and a rev gauge bring it to let's say 2000rpm which would simulate cruising speed ( ie no load) and if on ported vacuum from the carb the advance should be added at the end of the curve so check the timing with it connected and not connected, it should be about 3-5 degree more when connected, if it's manifold vaccum the vaccum advance is added at the start of the curve ie at idle this is why you would set the timing with the vaccum hose disconected so you could get the base timing.
 
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