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Are my Vacuum Figures ok?

1135 Views 15 Replies 5 Participants Last post by  woodywrkng
OK Ive been looking into why my fuel economy is low, and why my brakes have been getting worse, and Ive just tested my vacuum. Could you have a look at these figues and see what you think could be causing the low vacuum? The engine is mildly built, with a possibly stock cam, exhaust manifolds, single plane intake etc. I know the intake isnt appropriate but I havent had time to replace it. TIA.

IDLE in park with no break is ~750 rpm

Trans In park
RPM Vacuum (inches Hg)
750 9
1000 13
1500 16
2000 18-19

IDLE + Brake applied in park
RPM Vacuum
900 11

IDLE + Brake applied in drive
RPM Vacuum
950 5-6
1 - 16 of 16 Posts
That is way to low.

My 289 gets 19 @ 800 rpm
Those numbers are way low. With a large solid lifter cam I get about the same numbers. For starters the intake needs to go. A single plane intake on a mild motor is a definate no-no.

Checking for vacuum leaks isn't fun. I'd actually do a compression test first though. Your numbers are that low.
Using a can or carb cleaner, spray around the base of the carb and along the intake manifold where is meets the cylinder heads. Any variation in engine rpm is a suspect area.
I would think you need to adjust the timing, the carb, and if your still running points adjust them too. But first when the engine is fully warm does the reading start higher then slowly drop, does it read steady?

If it is a steady low reading:
1. Advance the timing a little.
2. Readjust the idle screws.

If still to low you probably have a leak somewhere around the manifold. Tighten down the carb, or replace the carb to intake gasket and tighten.

Cap off the vacuum lines one by one to find a leaky hose. Do you have power brakes? Pull the booster hose and cap it at the manifold. If your vacuum comes up to specs you may have a bad valve on the power booster. If nothing changes after this you may have a camshaft timing problem.

But first check the timing and readjust the carb.
Then look for a vacuum leak around the carb then the intake.
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Would the intake be affecting my vacuum? I know its no good powerwise but thats what came with the car.
I did the test when the egine was warmed up and the readings were pretty steady. Ill go out now and give your suggestions a try. Yes I have power brakes.
A stock intake should have a minimum of 15" no matter what.
I blocked all vacuum lines at the manifold, and there was no change at all. I didnt pull the line from the carb to the dist, but that looked and felt fine.

The ditributor is an MSD pro billet, with an MSD 6a box.
What is your timing set at? Try advancing the timing and watch the vacuum.
No idea what my timing is at. Do I need a timing light to check? If so I'll have to borrow one of my grandfather.
To tell the truth I raely use a light. Try advancing the timing and see if you can improve your number without the engine stumbling. Plus I would readjust the idle screws. Did you make sure the carb was tight?
I'll give the timing a go when a mech. friend comes around and post in another thread tomorrow. Thanks for your help /forums/images/icons/smile.gif
Before you go crazy, check your reading with a different (new & accurate) vacuum gauge. It could be yours isn't reading properly. Your readings are awfully low. Things to check include ignition timing, carb settings, vacuum leaks at carb, at intake to head gaskets, inside power brake booster and at AT vacuum modulator. See if any of these things make any difference and report back. Good luck.
I used to have a single plane Torker intake on my 289, and have a vacuum gauge mounted inside the car. Upon going back to a dual plane, there may have been a very slight increase in vacuum, but it was certainly never as low as yours. In other words, the single plane intake doesn't lower vacuum much if any at all.
1 - 16 of 16 Posts
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