Using vacuum gauge to tune carb

[bandit]

Two part question. I have read in many articles about using a vacuum gauge to tune the carb. The theory being to get the highest pressure reading as possible at idle.

My 1st question is I can get the pressure reading to rise quite a bit (11-15)by turning in the idle speed screw but my rpms start going over 1500 and the pressure wants to keep going up as I continue to turn it so I stopped. Does this sound right?

2nd question. I am getting very little response from the idle mixture screws. Turning them in all the way does not kill the engine. I have just rebuilt the carb(3310 Holley 750) and replaced the power valve. I can get a little suttle response on the gauge(.5 on vacuum) at right around 1/2 turn out from bottom but that is about the most I can get the gauge to move. I have been very careful not to fully seat the needles but who knows if it was damaged before.

Has anyone run into either of these situations before?

 

[RustyRatRod]

First off, what are we workin on? What kinda cam is in it? If it's aggressive, you may have to live with a low vacuum signal. ...and it is vacuum, not pressure, but I knew what you meant. Nine times out of ten, when there's no difference in the air screw adjustmend as you describe, there's a problem with the carburetor. You may need a different power valve. Just because it's new, doasn't mean it's right. Lastly, increasing the initial ignition timing will raise the vacuum signal. It sounds like though, you may have more than one problem. I would sort out the carburetor first.

 

[coloradodave]

My 1st question is I can get the pressure reading to rise quite a bit (11-15)by turning in the idle speed screw but my rpms start going over 1500 and the pressure wants to keep going up as I continue to turn it so I stopped. Does this sound right?

2nd question. I am getting very little response from the idle mixture screws. Turning them in all the way does not kill the engine.

Has anyone run into either of these situations before?

When you turn the idle mixture screws in and the idle rises over 1500 RPM, you are no longer on the idle circuit, so the idle screws will have minimal effect. Start with the screws about 2 turns out from being seated and the engine warmed up and off the choke. Slowly turn in each screw 1/8th of a turn at a time to reach the highest vacuum. If the idle rises too much, reset it lower and proceed. When you get to the point where the idle degrades or the screw has no effect, back the screws out 1/2 turn and you're done.

 

[bandit]

My bad I meant vacuum not pressure. I'm not sure what kind of cam is in there. The guy before raced it so it may not be stock but have no idea. I actually stepped it down a size on the power valve when I rebuilt it but I was having the exact same problem with the mixture screws before the rebuild and one of the reasons I did it. (thought new power valve would fix it)

What do you set the idle speed to (rpm's) before you adjust the mixture screws? Just standard rpm or should I take it lower to adjust the mixture. I am getting confused on this vacuum thing. If you are supposed to get the highest vacuum reading, and speeding up the engine with the idle speed screw makes the vacuum go higher but over it's specified idle rpm, is that correct? I am getting myself spun around on this. In other words what trumps what. Is staying with the correct idle speed(rpm) more important or the highest vacuum reading?

 

[coloradodave]

For the best efficiency, you want the highest vacuum you can attain at idle. As I explained in post #3, just reset the idle lower if it climbs too much. Idle speed will vary depending on the cam and a few other variables such as the torque converter, but usually hovers around 750-850 rpm with a stick. With an automatic, around 700-800 in gear, as long as it doesn't stall. I usually set the idle where it 'sounds right'. So to answer your question, you want the highest vacuum reading at idle. This, combined with an idle speed around 700-850 RPM will get you close.

 

[bandit]

Thanks. Going to try and finish this all up tomorrow before it gets cold again.

 

[benno440]

dont mean to hijack thread, but i will be doing this tonite with a vac gauge, except i have dual quads do i need to get vacuum of both carbs at same time or do them separately?

 

[coloradodave]

Benno, Do them separately, after initially adjusting both sets of screws to 2 turns from seated.

 

[MikeyT]

Another thing to make sure is to get the carb set at baseline first. I'd set each jdle screw out 1/2 turns from all the way in, then make make sure your front and black idle blades are set right too, and what I mean by the is square off the transition slots. Power valve size will be half of what you get for pressure, so if max idle is13 hg at 850 or what ever idle is then 13 divided by 2, will be 6.5, now if it was 14, you would still use a 6.5 PV, since they don't make a 7 PV. Also for the record my 440 liked increased initial squirters size too.
Mike

 

[451Mopar]

Vacuum gauge just helps set idle mixture. If idle screws have little to no effect, the throttle is likely too far open and into the transition circuit. You may need to drill the throttle plates to let more air in at idle, reducing the throttle blade angle.

 

[MikeyT]

Here you go Bandit!

This will be a good read for you!

http://www.bob2000.com/carb.htm

You'll be teaching us to tune holleys before you know it!
Mike