Vacuum Tester for Holley

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Jul 18, 2005 | 09:50 PM

foundvettelifeisgood

Thread Starter

Melting Slicks

Joined: Apr 2001

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From: Pittsburgh PA

Vacuum Tester for Holley

My brother in law gave me his dad's old vacuum tester to use for tuning my carb. I believe it's running too rich. The instructions with the tester were chewed on by some sort of insect. The Holley carb instructions tell me to just hook the vacuum tester hose on any vacuum port on the carb and turn the Idle Mixture Screws until I get the most vacuum. Is there more to it than that?

Jul 18, 2005 | 09:55 PM

Perfomer

Instructor

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Thats about it , It will also tell you what power valve to use. If the needle jumps around alot it could be a bad valve, worn guides or big cam with alot off over lap.

Jul 19, 2005 | 06:22 AM

gerry72

Safety Car

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From: San Antonio TX

An analog vacuum gauge (you know, the round dial with the needle kind) will read steady regardless of the size of the cam. The only way you’ll ever see the true pulsing in the intake is with an electronic gauge and an oscilloscope. Then it becomes very apparent that the vacuum in the intake is not steady, but pulses with every intake event regardless of the cam size. This is a valuable diagnostics tool to evaluate your ignition events and the intake events to track down a lazy cylinder. For an analog gauge, a lot of overlap will make it read low but the reading will still be steady. If the needle bounces, then you have severe pulsing in the intake from something funky in the valve train. If it drifts, it would likely be a carb issue.

But to the original question, yes, you adjust to the point that you reach the highest vacuum. But highest vacuum has a very broad range between rich and lean idle if the carb is functioning properly. You really want to turn the screws in one at a time until you see the needle drop, then back it out again slightly to where you have that highest vacuum reading. This will get you the best lean, stable idle.

Jul 19, 2005 | 07:39 AM

foundvettelifeisgood

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Melting Slicks

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From: Pittsburgh PA

Does it matter which port I use? Do they all read the same? Is there a number to shoot for?

Jul 19, 2005 | 07:54 AM

gerry72

Safety Car

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From: San Antonio TX

Quote:

Originally Posted by foundvettelifeisgood

Does it matter which port I use? Do they all read the same? Is there a number to shoot for?

You want to use a port that shows manifold vacuum at idle. One port should show no vacuum at idle. On a Holley, this port is located on the front metering block on the choke side. The manifold vacuum ports are on the base plate, usually under the front fuel bowl.

A normal reading depends on the engine. Compression, cam, and ignition all affect what "normal" is. From what I can tell, you have a base engined '72. If everything is relatively stock, you should pull between 18-20" at idle or very close to that.

Jul 19, 2005 | 08:27 AM

jeaux

Racer

Joined: Mar 2004

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From: Centerville La

I have two holley books and niether one says any thing about adjusting with a vacuum gage other than checking your power valve size
I'm glad to hear this something else I can use for adjusting my new Holley

Jul 19, 2005 | 09:29 PM

foundvettelifeisgood

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Melting Slicks

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From: Pittsburgh PA

I was going for the port Gerry mentioned when I found a vacuum port coming right out of the intake at the rear of the carb that had a capped Y connection, so I went with this and it worked. I highly suspected my car was running rich, because I've had some soot on the bumpers and backfires deaccelerating in gear. The instructions said to turn the screws in to make it leaner and turn them out to make it richer. Both screw channels were pointing to 1:00, so I started by turning them clockwise or in and the vacuum started to drop. I took it back to the starting point and started to go counterclockwise to about 12:00 and got the best vacuum which showed up at 17.5 on the gauge. This has me just barely in the "late ignition timing" zone on the gauge. Any thoughts on this? This took the idle to 1k. I had to back down the idle set screw to get down to 800 rpm at idle. What is the spec idle for a 4 speed?

The car is running smooth and no longer backfires. I'll have to check for soot on the bumpers after I get it clean.

Thanks for all your help.

Jul 20, 2005 | 01:26 AM

cardo0

Le Mans Master

Joined: Oct 2002

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From: Las Vegas - Just stop perpetuating myths please.

Good work.

Congradulations as u have enough patience for that technique as mine is always just close enough and give up.

Back and forth with the idle speed seems like i'm chasing my shadow. BTW the idle speed for my 4sp '74 is 900rpm but its different for many yrs and hp ratings - sb i'm talking here.
The Late Ignition Timing Zone? Sounds like a problem at NASA. But 17.5" vac is very good and all ur vac accessories should work great.

Now remember ur just setting the idle mixture here and when the carb butterflies open past off idle the mixture is controlled by several things - enough that they write and sell books for this.

cardo0