Vacuum advance problem ?

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cecil4speed

cecil4speed

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I just hooked my vacuum advance up. Had to put a fitting in the intake to hook it up as there was no fitting in the 750 holley that is on there now. Took it for a test drive and the engine seemed to be hitching as you go along, running quite rough. I unhooked it and everything was ok again. I had already set the initial timing first, with vacuum line plugged off. Have no light so just adjusted it until I had only a very,very slight ping at WOT. Could the vacuum advance have brought the timing ahead too far to cause this roughness or hitching as you drive? When I hooked up the advance the rpm only came up a very slight amount. Should I try to adjust the vacuum advance diaphram? The previous owner never had the advance hooked up, but he raced the car some, and I guess a lot of people don't use advance when they race. I drive on the street quite frequently so I was hoping the advance would give me a slight fuel mileage increase and it should be a little better around town driveability wise. Also could the distributor have been reworked in some way to cause this. I'm not sure what to look for. Thanks guys. Keith
 
Spark advance should be connected to venturi vaccum which rises and falls with the throttle. Manifold vaccum is constant.
 
I put a vacuum gauge in the car and it holds 15" at idle and when you punch the throttle it goes to 0 so I thought this would be ok. It isn't?
 
Manifold works, but ported is better. This subject has come up and been
argued many times.

It would be very good to know what your total all in timing is mechanically
first, then worry about the vac advance. Total of 34 is pretty safe. What is
your inital and total at now?

What carb are you running? There should be a port somewhere that is for
timed spark.

Your vacuum canister will most likely add somewhere in the area of another
15 degrees advance.
 
As I said in first post I have no timing light, and the 750 Holley has no ports at all for timing or anything else. I don't have the carb #'s now but I tried to search what the carb was and it came up to be a marine carburetor. So I don't really know what was done internally to the carb or distributor. Would too much total timing cause the hitching? It didn't seem to spark knock at WOT with advance hooked up. The car drives along great with the advance not hooked up.
 
Does your Holley look like this one....?


The vacuum advance port is the nipple just above the idle mixture screw.

http://www.ghettonet.org/images/mustang/holley750.jpg


Try and beg, borrow, or steal a timing light and determine your initial advance, total mech. advance, and total mech.+ vac. advance. You really need these numbers to dial in your setup properly.
 
Carb looks very simular except not a double pumper. I don't think there is a nipple on mine though. I'll look tonight. Thanks.
Although when I was serching the carb # I think I remember something about it fitting a Mustang.
 
cecil4speed said:
Carb looks very simular except not a double pumper. I don't think there is a nipple on mine though. I'll look tonight. Thanks.
Although when I was serching the carb # I think I remember something about it fitting a Mustang.
Click to expand...

The carb in that pic is not a double pump. Probably a #3310. The big round ,saucer thing w/ a rod out the bottom is the secondary vacuum chamber/diaphragm., but that doesn't matter, as the primary end of the carb will have the same setup regardless, single or double pump (or a 2 barrel, actually). Your primary metering block should have a vac. fitting in the same spot.

Try and get the number on the airhorn off your carb. (or a good pic)
 
Yes, too much timing can cause problems without pinging, but also you can
detonate without hearing it.

If it were me, I'd disconnect the vac advance and not run it for now, that
would be way better than doing permanent damage to your engine.

Also, buy a timing light, you can get a cheap one, don't worry about one
that does fancy advance features, get a cheap light and spend 5 dollars
on timing tape for your damper. Crank up the throttle and see what all in
advance gives you. You do not want more than 34-35 for a small block.
With your vac hooked up, you can get all the way to 50 under no load.
You will not gain that much in mileage honestly. I've been tuning my car
with a wideband air fuel gauge. If you are worried about gaining 1 or 2
mpg, probably not the hobby to be in :)
 
Thanks for all the input guys. I did locate the vacuum nipple above the mixture screw on rh side like 65Val said, but vacumm at this nipple wouldn't work, no vacuum at idle, a small amount of throttle gave about 15", but when you punch the throttle, vacuum would only go to 0" some times, some times it would only go to 7 or 8", it was very erratic, and the car hitched same as before. So I'm not going to try to hook it up at this time. Thanks again.
 
Sounds like its operating 100%. There should be no vacuum at idle at this port, hence the name "ported vacuum". There will also be 0 or near 0 vacuum at wide-open throttle at this port. The ported vacuum port only supplies vacuum (and advance) at part throttle/cruise.
Get a timing light on that puppy and set the initial advance at around 7*. Hook up your vacuum advance hose to that port and take it for a drive. Do some "grandma" starts and see how it feels.Also, load it up in 3rd gear. If it pings, then back off the timing SLIGHTLY 'til it dont ping no more under load. That should be optimum setting ...for YOUR engine...not everyone's.
 
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