Manifold Vacuum

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Manifold Vacuum

Postby monza355 » Sat Oct 30, 2010 4:10 pm

How many inches of vacuum should you have at the manifold port on a Holley 750, or in general?
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Re: Manifold Vacuum

Postby cjbiagi » Sat Oct 30, 2010 6:22 pm

That's going to vary by the level of engine mods, especially the cam. Stock cams can generate around 20 inches or more at idle. The more radical the cam the less you will have. Many performance cams may be down around 16-17, race cams even less.
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Re: Manifold Vacuum

Postby chevy art » Sat Oct 30, 2010 10:45 pm

the 355 in my drag car has between 5-8 inches. thats low art
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Re: Manifold Vacuum

Postby 72 SS PNL » Sun Oct 31, 2010 10:36 pm

my elco with a fairly mild cam is about 12-14 depending on idle speed and timing , my 04 gto im guessing is under 10 its got a pretty nasty huge cam
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Re: Manifold Vacuum

Postby monza355 » Mon Nov 01, 2010 10:46 am

Thanks guys. I was a little concerned when I was only getting 13. Just wanted to be sure nothing was way out of whack.
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Re: Manifold Vacuum

Postby 72 SS PNL » Mon Nov 01, 2010 1:06 pm

how big/mild of a cam do you have ? is the ready steady or move around slightly ?
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Re: Manifold Vacuum

Postby monza355 » Mon Nov 01, 2010 6:09 pm

Comp Cams XE268H. The needle moves up and down slightly. No drastic movements.

My issue is the engine idles fine and revs fine but when you put it in gear and try to drive, it sputters and misses. I'm going to play with the timing a little and see if I can clear it up some. It smells very rich. I tried adjusting the carb with a vacuum guage to get it a little leaner but it didn't seem to change.
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Re: Manifold Vacuum

Postby starfire383 » Wed Nov 03, 2010 10:27 am

13s low but shouldn't be dying in gear. Everytime that symptom happens to me it's a leaking intake manifold gasket. :bang:
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Re: Manifold Vacuum

Postby monza355 » Wed Nov 03, 2010 12:59 pm

That's what I was thinking. I was going to get a can of carb cleaner and spray around everywhere to see, unless anyone can suggest a better way to find a vacuum leak.
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Re: Manifold Vacuum

Postby starfire383 » Wed Nov 03, 2010 1:49 pm

If it leaks into the valley that troubleshooting method won't do ya any good and will cause permanent harm to engine paint. Mine never leak externally either :bang:
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Re: Manifold Vacuum

Postby monza355 » Wed Nov 03, 2010 2:42 pm

I may go ahead and pull the intake and put new gaskets anyway just to make sure.

Thanks for the info.
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Re: Manifold Vacuum

Postby monza355 » Mon Nov 08, 2010 6:06 pm

It sure is amazing the difference made when the choke butterfly is 1/8" less than wide open compared to wide open. Runs great now. I figured wide open was good. NOT. I'll be installing an electric choke now.
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Re: Manifold Vacuum

Postby starfire383 » Tue Nov 09, 2010 10:29 am

Needing choke is a sign of a vacuum leak. All the choke is doing is compensating for the lean-ness of the vacuum leak by enriching the air passing thru the carb.
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Re: Manifold Vacuum

Postby monza355 » Tue Nov 09, 2010 11:28 am

I don't know where else a leak could be coming from. I replaced the intake gaskets and the carb base gaskets. The only other possibility would be the carb gaskets themselves.

So the choke horn butterfly should be open 100%?
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Re: Manifold Vacuum

Postby 72 SS PNL » Tue Nov 09, 2010 1:15 pm

starfire383 wrote:Needing choke is a sign of a vacuum leak. All the choke is doing is compensating for the lean-ness of the vacuum leak by enriching the air passing thru the carb.


can also be the jetting is way off for what he needs also

sounds like it needs bigger jets , how do the plugs look color wise ?
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