Runs better with vac advance unhooked (Help)

[73 C10]

Like the title says, when vac advance hooked up it idles good. When I give it any gas it starts to miss (sputter). The sputter is all through the RPM range. I pulled the vac advance hose off the manifold vac and capped the port and it runs much better. I think I still feel a small miss but almost imperceptible.

I need someone to help me through the rest of the diagnostics. I am wondering if I need to replace my vac can on the distributor (bad diaphragm maybe). I'll be putting a vac pump on it to see next when I get home.

Let me know what I've left out in my signature.

Thanks.

[Bishops Trucks]

Sounds like a bad VA. Have you tried changing which port your drawing vacuum from on the carb?

[73 C10]

Yeah. I tried the timed port. Fine at idle, sputtered under load.

[rich weyand]

Set the timing with the vacuum disconnected to 12* BTDC.

Now hook up the vacuum and check the timing again. Whatcha got?

[73 C10]

The timing is at 12 and with vac straight up. I guess that would be around 24. How much should the va add?

Update:

I checked the va with a vac pump. Found good va can, leaky hose.
I replaced the hose. Still runs bad with vacuum. I realized the trans and brake booster were connected at the #8 runner and if leaking would lean out 8. I found a leak at the hose between the t and the auto trans hard line. I replaced the hose and connected it back to the carb this time. It seems better to lean out everything a little then #8 a lot. I capped off the runner port. I also checked the brake booster check valve and line. Verified good. I connected the va to the timed port to start off. Not much better result. I connected the va to the manifold vac on the carb and it was pretty good.

So I guess I'm good. Thanks guys.

[rich weyand]

Vac advance on a stock HEI distributor will add 15 crankshaft degrees.

Centrifugal advance on a stock HEI distributor will add 20 degrees.

Base timing should be 12-16 degrees BTDC. More is better, but knocking is bad.

Now lean out your idle mixture screws for best rpm.

[73 C10]

Thanks for the numbers.

FYI timing by ear best rpm with va unhooked and vac port capped then check with a light is 14 btdc.

Currently set with light to 12 btdc.

Would limiting the va with a stop to 10 help or hurt.

[rich weyand]

More base timing is good right up to the knock limit. Generally a mild tune should be able to go to 16* BTDC. I'm running 16-17*BTDC with 87 octane and no knocking.

Mixture and timing interact, so you set the base timing, then adjust idle mixture for best rpm, not adjust timing for best rpm. My hot rod mechanic buddies say keep advancing the timing until you get knocking, then back off two degrees, but I think 16*BTDC is plenty. Normally you hit the knock limit on a mild build between 18 and 20. So I tell people to try 16, and if you knock, back it off 2 degrees at a time.

So set the base timing and THEN hook up the vacuum advance and set the idle mixture screws to best rpm, then tweak them back in just enough to drop 20 rpm. This will be just a touch lean, which will keep the plugs from fouling.

Vacuum advance is to correct the timing for slower burn of thinner mixtures due to less pressure in the manifold, resulting in less cylinder pressure at ignition. Smaller vacuum advance will result in best rpm at a richer idle mixture (which burns faster) to make up for it, then you get into all sorts of other issues. Put a 10* stop on the VA and you will probably foul plugs. The stock 15 crankshaft degrees of VA is best for a street car.

[rich weyand]

BTW, be aware that most vacuum cans are specified in CAMSHAFT degrees. The stock HEI distributor on our trucks came with 7.5 CAMSHAFT degrees of vacuum advance, which is 15 CRANKSHAFT degrees.

So when you say a 10* stop on the VA, which do you mean? 20 crankshaft degrees of VA is probably too much but OK, 10 crankshaft degrees is not enough, you'll need to tune the idle rich and will probably foul plugs.

None of which will affect WOT performance at all. Wide open throttle is no vacuum, so no vacuum advance, so your vacuum can does not matter at all. Tuning the vacuum can for performance as some espouse is nonsense. When your foot is in it, it's not doing anything. Race cars don't even have vacuum advance, because their foot is always in it.

Vacuum advance is purely for idle and cruise, and optimizes mileage and driveability.

Just stick with the stock VA. It's fine. Your performance is going to come from getting the base timing up, and making sure your centrifugal advance is working properly.

[rich weyand]

BTW, be aware that most vacuum cans are specified in CAMSHAFT degrees. The stock HEI distributor on our trucks came with 7.5 CAMSHAFT degrees of vacuum advance, which is 15 CRANKSHAFT degrees.

So when you say a 10* stop on the VA, which do you mean? 20 crankshaft degrees of VA is probably too much but OK, 10 crankshaft degrees is not enough, you'll need to tune the idle rich and will probably foul plugs.

It also depends on what your base timing is. If you run base timing of 10-12* BTDC, and have 10* (camshaft degrees) of vacuum advance, then your total advance at idle is about 31* BTDC. You can run the stock HEI at 16* BTDC base timing, get 15 crankshaft degrees of VA from the stock advance can, and still be at 31* BTDC at idle, but when you jump on it, 16* BTDC will way outperform 10-12* BTDC as long as you are below the knock limit. Guys used to tune their vac advance to get 31* BTDC at idle even though they couldn't run at 16* BTDC base timing because they were running modified engines that would knock before that, so they would add 5 crankshaft degrees of vac advance to clean up the idle. If you can get 16* BTDC base timing with no knocking, 15* of VA is all you need to get 31* BTDC at idle.

None of the vacuum advance tuning will affect WOT performance at all. Wide open throttle is no vacuum, so no vacuum advance, so your vacuum can does not matter at all. Tuning the vacuum can for performance as some espouse is nonsense. When your foot is in it, it's not doing anything. Race cars don't even have vacuum advance, because their foot is always in it.

Vacuum advance is purely for idle and cruise, and optimizes mileage and driveability.

Just stick with the stock VA. It's fine. Your performance is going to come from getting the base timing up, and making sure your centrifugal advance is working properly.

[rich weyand]

Here's the full story.
http://www.chevytalk.org/fusionbb/sh...hp?tid/239321/