I'm using the Mallory Distributor part#Maa-4748201, which has vacuum advance and should work like a normal vacuum advance i'm guessing. Unhook vacuum - idle drops, Hookup Vacuum - idle should rise right ? Mine don't idle up it stays the same

It will only idle up if it's hooked to manifold vaccum,not ported. graemlins/waving.gif

Originally posted by JRS70LS5:
It will only idle up if it's hooked to manifold vaccum,not ported. graemlins/waving.gif So should it be ported ? And being vacuum advance why doesn't it work like a regular advance ?

Vacuum advance is controlled by the load on the engine. Under light loads the volumetric efficiency is low, and the engine requires more advance to burn the mixture reasonably. As the load on the engine increases and the vacuum decreases, the volumetric efficiency goes up and you need to retard the timing.
Mechanical advance is a function of the engine rpm only. A system of weights and springs controls the amount and rate of advance.
Ported vacuum advance appeared in the early emission control systems, and reduces emissions at idle at the expense of engine efficiency. I believe the reason that most distributor manufacturers recommend ported advance is to comply with the smog rules. You can also use ported advance to make your car idle rougher. The speed shop down the way sends all their cars out with ported advance for this reason.

So what do you guys recommend with my distributor ?

i've already read the other post, but why don't mine idle up like a normal vacuum advance . is it cause its a holley carb , Mallory ditributor, or what , somebody please explain !!

when your vacuum advance canister is hooked up to ported vacuum port there wont be any or very little change in your timming......so therfore your motor will not respond in anyway at idle........now if you was to move your vacuum line to a full port vacuum port at the base of the carb you will find that your motor will respond with a higher rpm. and you will see your timming jump higher on the hormonic balancer.

the amount of jump depends on the vacuum canister in which you are using.......some are rated with 10* and others up to around 25* and if you have an adjustable unit then you can control the amount via an allen wrench......by turning the adjustment screw inside the vacuum canister.

now which port do you hook it up to it just depends.......if you have a big sloppy cam that idles rough then you can smooth out the idle using the full port vacuum. you then will only have the extra timming when the motor is at idle or at cruise rpm......as soon as you mash the gas peddle down the extra timming is void since there is no vacuum to operate your canister with.

even some engine with way bigger cams even at idle may not have enough vacuum to operate your canister so you may not even see a change with either full or ported vacuum.

vacuum canister is a tunning device it can help out to smooth out an idle, it can help out with keeping an engine running cooler at idle and down the road.

for the most part running the vacuum canister on the full port vacuum port will allow the engine to run cooler at idle and smooth out the idle. and help out with fuel economy.

ported vacuum i have yet to see a need for that in a performace application since it will give to much timming when it is not needed. cause it comes on when you mash the gas and you end up with to much timming that may cause missing and pinging.........

"ported vacuum i have yet to see a need for that in a performace application since it will give to much timming when it is not needed. cause it comes on when you mash the gas and you end up with to much timming that may cause missing and pinging........."

Even with a ported vacuum source, vacuum drops to zero when you "mash the gas". Once the throttle plates are cracked, ported and full vacuum are the same. It is only when the throttle is closed that they are different.

Originally posted by Pat Kelley:
"ported vacuum i have yet to see a need for that in a performace application since it will give to much timming when it is not needed. cause it comes on when you mash the gas and you end up with to much timming that may cause missing and pinging........."

Even with a ported vacuum source, vacuum drops to zero when you "mash the gas". Once the throttle plates are cracked, ported and full vacuum are the same. It is only when the throttle is closed that they are different. So giving my set up ,in my sig, i should probably be better off using full vacuum ??? What would you do ?

I would. You should find out how much vacuum advance the canister applies. That will help you figure out if you need more or less VA. My driver, with a 350 and 204/214 @ .050", has 14º plus 10º VA totaling 24º at idle. It likes it.

Originally posted by Pat Kelley:
I would. You should find out how much vacuum advance the canister applies. That will help you figure out if you need more or less VA. My driver, with a 350 and 204/214 @ .050", has 14º plus 10º VA totaling 24º at idle. It likes it. Thanx Pat graemlins/thumbsup.gif I'll give it a shot that way, don't you set total timing with it hooked up, i'm at about 34 or 35 degrees, or so i remember doing it some time ago. Does it really make that much difference which one you hook it to ? I run my vacuum cockpit gauge right out of the intake, i don't want to use this do i!!!!!! Go ahead call me stupid !! graemlins/clonk.gif

Set total timing with it disconnected. Be sure to plug the line to the carb or you have a huge vacuum leak.

You'll get a slightly better idle with full vacuum. If you're moving from ported vacuum, you might have to re-adjust (lower) idle speed.

Originally posted by Pat Kelley:
Set total timing with it disconnected. Be sure to plug the line to the carb or you have a huge vacuum leak.

You'll get a slightly better idle with full vacuum. If you're moving from ported vacuum, you might have to re-adjust (lower) idle speed. But dont go straight off the intake right, sorry man , i got people here telling me all kinds of different stuff

Right. On the 750DP you have, on the front right side, very low, is a full mainfold vacuum port. The vacuum modulator for the trans may be hooked there. You can take the trans vacuum off the manifold or you can tee both from the same port.

Thanx Pat !!! graemlins/beers.gif