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Old 07-15-2003, 12:14 PM
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Vacuum Guage Test Results.....? (Holley 600CFM))


Well, I ran home at lunch and took the mustang for a drive. I hooked up the vacuum guage and ran it with extended rubber hose through the passenger side window and took the car for a spin.

I'm sure my results are pretty standard but I am trying to decide if my engine is getting too much gas, or not enough.

The engine seems the most responsive when I feed the gas pendal in gradually ie: 1/4, then 1/2, then full throttle by the mid for some good performance.

If I punch it full throttle from a low speed, my throttle response and powerband is nowhere near as good.

At first I thought that this ment that I am getting too much gas but then I was thinking that at 1/4 and 1/2 throttle my vacuum is stronger then at full throttle when it is 0.

So, if i have vacuum secondaries then how can they be pulled in at full throttle when the vacuum is 0?

Maybe I am getting more gas at the part throttle positions because it's pulling the secondaries open with the vacuum signal, thus giving me more gas for better power?

Anyways, here are the results of the vacuum test:

"Vacuum Guage Test Rresults"

Idle in Park: 12 and Smooth
Idle in Gear: 7-8 and Smooth

Cruising: 14-16 and Smooth

1/2 Throttle: Drops down to about 6

Full Throttle: 0

Note: I have a 6.5 power valve.

What do you guys think...why do I get a crisper, stronger powercurve when I feed the gas pedal in incrementally?

Thanks, Greg.

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Old 07-15-2003, 01:12 PM
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Pump circuit is largely dependate on a quick motion of the throttle to discharge fuel to cover the lean spot on accelerating from a low speed.

When you ease into the throttle, you notice no power loss because the pump circuit isn't needed or used . But when you stomp on it the pump is working at it's hardest and since if can't supply enough fuel to cover the lean spot then you feel it as a loss of power production. Usaully this manifestes itself as a hesitation or bog for just a second or two after mashing it.

You secondary springs may also be to loose, allowing the secondary's to open to soon. Since the pump circiut is already used up and can't help, the car goes lean untill RPM comes up and booster signal is restored. (Vaccum)

With the low vacuum levels you have indicated, I can only assume you have a long cam. Long cams already produce lower then ideal vaccum levels so the problem is pronounced.

What's this mean? The engine went lean.

Change springs in the secondary's before meesing with anything else.

Secondary's rely on vacuum that's created as air moves through the carb, not when the carb is closed, producing vacuum under the carb. You are measuring from the port that is under the carb.

With long cams, vacuum is often higher when RPM's are up and engine isn't working hard because of the overlap period. The cam's getting closer to it power band.
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Old 07-15-2003, 05:59 PM
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You secondary springs may also be to loose

you know, I have always used a feeler guage to set the pump springs.....I set um @ .026 and thats it.......but, thats usualy with a high stall and set for launch.....It may be not very worth while but they have to be set somewhere and thats just what I have found to work best for the perticular applications I deal with....anyone else do this?
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