Same problem with getting vacuum at idle, Vacuum advance makes motor hesitate

[67charger440]

Mild 440

Going to ask this again because I’ve done a lot of suggestions from you great folks, but still have the same issue. I’m getting around 10” of vacuum at idle on both vacuum ports. (Ported and manifold) and I CANNOT figure out why. My engine rpm is ~800 and can’t go any lower. I’ve tried three different carbs. A used edelbrock 750 CFM, the stock carb, and I just went to Summit racing and bought a brand new AVS2 and plopped it on and I’m still getting around 10” of vacuum at idle, “ported”. My timing with the VA unhooked is 12 initial. When I plug in the VA at idle, it jumps to ~20 AT IDLE. Again, I’ve tried three different carbs, including a brand new out of the box AVS2.


I’ve tried adjusting the throttle plates to where the plate covers to vacuum port, but my engine rpm is too low and it won’t idle.

 

[BeepBeepRR]

Everyone will say have you checked to make sure you do not have an intake leak.

 

[65-440]

You're getting 10" from ported at idle? That's not right. Your throttle blade is open too much... have you tried increasing your initial timing?

 

[67charger440]

You're getting 10" from ported at idle? That's not right. Your throttle blade is open too much... have you tried increasing your initial timing?

I understand that logical explication is the throttle blade is open too much , however it can’t be open to much on three different carbs

initial timing is 12

 

[Don Frelier]

I already suggested a different distributor.
It should run fine at 20 degrees of advance.
I also attempted to explain that closing the blades wasn't going to solve the problem.

 

[BeepBeepRR]

I understand that logical explication is the throttle blade is open too much , however it can’t be open to much on three different carbs

initial timing is 12

Run the timing up a bit. Try 14 initial. If you get improvement go a little bit more. Try 15 then 16 etc etc.

 

[67charger440]

I already suggested a different distributor.
It should run fine at 20 degrees of advance.
I also attempted to explain that closing the blades wasn't going to solve the problem.

Yes sir I agree that the throttle blades are not the issue. But im genuinely asking, changing the distributor won’t affect ported vacuum at idle will it?

 

[RemCharger]

20 degrees is 20 degrees. No matter what distributor is in there

 

[Moms68]

I vote for a distributor problem. Do you have another one you can try?

 

[67charger440]

I vote for a distributor problem. Do you have another one you can try?

I want it to be that as well, but I just can’t wrap my brain stem around how a faulty distributor causes ported vacuum at idle.

 

[Don Frelier]

It's not running properly so the blades are open farther than they should be.
And if it's a mild cammed motor hitting on all 8 it should idle at 650 no problem.
You are focusing on the wrong thing.
The ported vacuum issue is a symptom, something else is the problem.

 

[65-440]

I understand that logical explication is the throttle blade is open too much , however it can’t be open to much on three different carbs

initial timing is 12

Which is why I asked if you've tried increasing initial timing. Then you'd have to adjust the idle speed down which wouldcose the throttle blades and decrease the ported vacuum at idle...

 

[64fury74charger]

Just throwing this out there because it’s a possibility…. Have you checked the timing chain for excessive slack? Big block’s are hard on timing chains. Don’t ask me how I know. If the chain is loose nothing will be right. Pop distributor cap and grab the belt and see how far the crank moves before the rotor moves.

 

[threewood]

Mild 440

Going to ask this again because I’ve done a lot of suggestions from you great folks, but still have the same issue. I’m getting around 10” of vacuum at idle on both vacuum ports. (Ported and manifold) and I CANNOT figure out why. My engine rpm is ~800 and can’t go any lower. I’ve tried three different carbs. A used edelbrock 750 CFM, the stock carb, and I just went to Summit racing and bought a brand new AVS2 and plopped it on and I’m still getting around 10” of vacuum at idle, “ported”. My timing with the VA unhooked is 12 initial. When I plug in the VA at idle, it jumps to ~20 AT IDLE. Again, I’ve tried three different carbs, including a brand new out of the box AVS2.


I’ve tried adjusting the throttle plates to where the plate covers to vacuum port, but my engine rpm is too low and it won’t idle.

You have to add advance to the distributor. The more you add, the higher the higher the rpms. Then you take that rpm back down by unscrewing the idle adjustment screw (closing the throttle plates.)

 

[AR67GTX]

Or if it has a big cam and you want to get your throttle blade position and ported advance back to intended position and function, you can do what the Holley guys have to do on their older models - drill the primary throttle blades. Start out about 5/32 to 3/16”. Then lower the idle back down to close the blades. But once done it’s only reversible by replacing the blades.

 

[Dan64]

Like these guys are saying, you are seeing the same vacuum off of both ports because the blades are most likely too far open meaning that there is now no difference between manifold vacuum and ported vacuum.Once the blades open there is nothing separating the ported from manifold so they become the same. You have to turn your idle screw out to lower the idle ( which will close your blades) but also advance your initial timing which will in turn raise raise your idle back to where you want it. You could test this theory by turning your idle mix screws all the way in ( you now should have no flow on the idle circuit) .If the car doesn't stall your blades are opened too far. If your car has a cam in it it will want more initial timing than a stock engine. Most important would be to not exceed the total timing the motor wants but you should also want to run as much initial timing as you can.

 

[Dragon Slayer]

Even on manifold vacuum regardless of the ported value. 10" sounds to low for idle. Even at a higher rpm. Something not right unless you have a big cam.

 

[pedal2themetal]

Hi,, Just a thought ... what are you fuel/air mixture screw's set at.. they should be no more than 1 1/2 turns out from lightly seated..
good luck
tim

 

[hangn0ut]

Have you checked your balancer against top dead center?

 

[Paul_G]

I feel for you. You are trying to tune the engine by the numbers. That wont work. Forget about the vacuum advance for now. It does not do anything for engine performance, it is an economy thing. You may think low vacuum readings is a carb problem. and it is, but, it is also a timing thing. They depend on each other to be dialed in to get the highest vacuum reading.

Get the carb tuned well enough for the engine to idle. Adjust the idle mixture screws one at a time. Set them each at 1 1/2 turns out to start. Using your vacuum gauge, adjust one screw at a time for the highest vacuum reading. They may want to be turned in/leaner, or out/richer. Which ever way increases vacuum. Do it slowly giving the engine time to settle in to each small movement of the screw, 10 to 20 second's. Then adjust the other screw for highest vacuum. go back and forth between the screws about three times. You may have to bring down the idle setting as the mixture gets dialed in.

Then focus on the distributor. Using todays fuels, especially with a little hotter cam, headers, maybe some head work, the factory setting of 6 degrees wont work. Someone already stated a ballpark timing number close to 20 degrees at idle with no vacuum advance, and that is correct.

Set the base timing to 16 or 18 degrees to start. That will most likely put your total timing way to high, like well over 40 degrees, but you can deal with that in a bit. Now go drive the car. Take it easy on it, check for off idle stumble, crisp acceleration and and listen for pre-ignition knock. I bet it will be running better. If you do not hear any pre-ignition knock, and the engine runs well at low speed and off idle, leave it there. or try giving it a couple more degrees of timing. It might like it.

Now you are going to have to limit the total timing to something around 34 to 36 degrees while keeping the base just under 20. Easiest way is to use an FBO timing limiter plate. It goes in the dizzy right on top of the advance weights. it limits how much mechanical advance the dizzy will add with rpm increase. If your base is 20, put the limiter plate on 14 and you will have a total of 34, 20+14=34. Get the tuning springs along with the limiter plate. You may find the engine likes the lightest mechanical advance springs in the kit.

Go try this.