Vacume for HEI distributor

[cherokeejohn]

I just installed a new HEI distributor on my 350, I have it all wired up correctly and the timing set "dead on". I'm not sure where to run the vacume line from??? Do i just use the existing line from the points set up, or should it be from another place? Anyone have any info. suggestions, or pointers would be greatly helpful.

[geezer#99]

Hook it up where the other distributor was before.
You say your timing is 'dead on'!
What's your initial timing set at?

[cherokeejohn]

I dont know the actual number or degree, i just know that it was at TDC on #1 cylinder, on the correct stroke, and the mark on the pully lined up perfectly with the degree gauge. please forgive my terminology, I'm a body tech/ painter, not a mechanic. my mechanic set the timing and everything is lined up as it should. He was just unsure of wether to use the original vacume line or if it needed a different vacume. I cant seem to keep it running for more than 10 seconds at a time, witch is why i replaced the distributor to begin with. $250 later, I'm still having the same problem. I'm going to check the fuel lines this afternoon. The gas tank, sending unit, fuel pump, carburator, gaskets are all new, and it was running fine, then suddenly it stalled everytime i gave it gas. i was told my old points were cooked and I was better off replacing thew whole distributor with an HEI so I did but still at square 1

[Lee H]

And what does your mechanic say about the new distributor not solving the problem?

[LSUMurse]

I am by no means a distributor expert, however here is what I know from my experience.

There are two different types of distributor advances, ones that use a vacuum all the time, or ones that use the vacuum as the engine revs higher and needs to advance the timing. My HEI distributor advance is connected to my carburetor (Edelbrock 1406). I am probably not completely correct on this.

So, for the experts on here to be able to help you, they're going to need to know what carb, distributor, etc you have. More info = more better answers.

[cherokeejohn]

OK so we got it running steady again, there was a vaporlock issue with the fuel tank. I'm still having a problem with the vacume line to the distributor. I reused the original line from the front of the carb, but it is a constant vac line. I need a vac line that only has vacume when the RPMs increase due to the vacume advance on the HEI "so I'm told". if it ever stops raining here I will take some pics and post them up.

[cherokeejohn]

my carb is a stock/ rebuilt Quadrajunk, the distributor is a Mallory HEI for either small or big blocks

[Lee H]

Originally Posted by cherokeejohn

I reused the original line from the front of the carb, but it is a constant vac line. I need a vac line that only has vacume when the RPMs increase due to the vacume advance on the HEI "so I'm told".

That is called manifold vacuum and is just fine if you truck is running good. You can try the other, called ported vacuum if you like. Most Chevy motors run better on manifold, constant, vacuum but some do like ported.

[geezer#99]

To clear a couple things up there's only one distributor vacuum advance. Uses manifold (sometimes called full time) vacuum to operate it.
The vacuum is supplied to it from the carb. The supply ports on the carb are either manifold or ported ports. Normally any port below the throttle plates is manifold and any port above the throttle plates is ported.
The manifold port supplies vacuum all the time. The ported port supplies no vacuum when the throttle is closed fully (like at idle). If you follow the ported passage it goes into the venturi above the throttle plate. The throttle plate acts like a valve, shutting off the vacuum which is present below the plate.
So at an idle your vac advance pot can receive manifold vacuum and advance your timing or receive no vacuum from the ported port.
Which to use is decided by how your motor is set up. A stock motor (or slightly modified) that idles nicely at 600 to 700 rpm could use ported vac to the distributor. Slightly modified means a better intake, carb, headers and an rv type cam. When you move up a step to a more radical cam, bigger carb, higher rpm type intake and bigger headers then you might need to use manifold vacuum.
It all depends on getting your motor to idle with the throttle plate closed enough to keep the transition slot in the venturii covered. The transition slot is the port that supplies the motor with fuel so it will idle when the throttle plate is closed. If the throttle is open too far, the transition slot is completely uncovered and stops supplying fuel and the carb starts to work on the power circuit and supplies too much fuel and idles rich.
In order to get your throttle closed back up you need to speed up your motor by giving it more timing. Manifold vac to the vac pot will do that. When your idle comes up you can slow it back down by closing the throttle plate, covering the transition slot, making your mixture screws work again and not idle rich.
So which port to use all depends on your set up. Experiment. See what your motor likes.
Keep the throttle plate closed, your mix screws working and your motor idling nicely.

[cherokeejohn]

Lee; thank you, ported is the word i have been looking for, lol i could'nt remember it to save my life.
Geezer; thank you thank you thank you, thats the info I've been searching for. I just got home from hell, I mean work, lol and I will be messin with it first thing tommorrow. I had it hooked up to the manifold
(
full time vac) and it idled good, set timing and idle at 700 RPM, but when i hit the gas abruptly it wanted to stall out. so I will try it hooked to a ported vac noiw and reset timing and see if that helps. my motor is all stock, this distributor is my only mod, so far

[geezer#99]

Whether you run ported or manifold vacuum, set your initial at 12 to 14 degrees first.
Hitting the gas abruptly can be timing related (retarded) or accelerator pump related.
To check the accelerator pump look inside your carb and stroke the throttle (motor off) and look for strong shots of fuel each time you stroke it.

[BrianG]

My motor is built fairly good. There was no vacuum at all going to the distributer. The guy I bought it from didn't set up the motor, but said that some race car guys did. It ran REALLY strong before I took it out for the body rebuild job. Is this a normal practice on some motors? If so, do I just set my initial timing and not worry about advance timing? Thanks

[geezer#99]

Apoligies for jackin' your thread Cherokee!
Like I said, you gotta experiment. Some motors run better with no vac advance at all. I ran a bbc (L-88) like that. It depends on how your combination is set up. Mine had 16 initial with a quick curve in the dizzy. Timing came in quick and was all in (34 degrees) by 2800 rpm. That was in a light car (72 nova) running an m-20 and 4.56 cogs. First gear was a wash. Six grand for a couple heartbeats and into second. I tried some limited vac advance (10 degrees) but got a bit of pinging in second. That motor didn't spend much time in cruise mode so vac advance wasn't needed.
The best way to decide if you want manifold or ported vac is how your carb acts. If you can screw either mixture screw all the way in and the motor (at 700 rpm) doesn't quit then you know you're idling on the power circuit and you need more idle timing which increases your idle allowing you to close the throttle enough to get adjustment back on the mix screws.
Adding more timing at idle isn't the only way to get your mix screws working right though. Most holley carbs have an adjustable secondary throttle plate which when adjusted a little more open speeds up your idle then you can close the primary throttle to get mix screw adjustability again. Eddy, carter afb and quadrajets don't have that feature but I've seen them modified to work that way.
If your happy with your motor like it is, enjoy it.
If you want to wring every available horse out of it and still cruise around comfortably then try different things. Keep notes. Change one thing at a time and don't get too frustrated. It takes time for perfection.