'76 SBC diagnosing vacuum leak

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  #1  
Old 07-13-14, 06:43 PM
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'76 SBC diagnosing vacuum leak

Hello, trying to look for some input on taking action on a vacuum leak that i think i have on my '76 chevy small block 350. Off the intake manifold vacuum port at idle i have 5-8" of vacuum. I found this out when i was trying to adjust my idle mixture screws on my holley 650 DP. From my research leads me to an intake manifold leak.

Now, the intake is a edelbrock performer, and it has a comp cam with a .500 lift but that's all i know about it, it was already in the engine. Now i've been thinking two things...

The cam shaft might be messing with the vacuum but i've read of guys using more aggressive cams than what i think i have and can still achieve 10" or better idle vacuum. But i don't know if thats true or not...

The second is i did a reseal and gasket of the whole motor, and just used a standard fel-pro sbc kit. After doing some research and a phone call, edelbrock tells me that i cannot use the standard fel-pro intake gaskets because they don't seal up, i have to buy their edelbrock gaskets. Which is fine, but that's a lot of screwing around taking that intake manifold back off.

I've sprayed the motor with carb cleaner, took vacuum lines off, plugging lines, checked the pcv valve, brake booster, and so far no luck. Any ideas or should i just go ahead and take that intake manifold off? I hate to do all that work and that still not fix my problem.
 
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Old 07-14-14, 03:23 AM
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If you sprayed all around the intake and didn't notice appreciable engine rev, what makes you sure you have a vacuum leak? Is the engine running well, or not?
 
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Old 07-14-14, 11:04 AM
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i have two main concerns..

first is being i have so little idle vacuum, i can't get my carburator idle mixture screws adjusted, turning them in or out should affect my vacuum and help set whether i run lean or rich (i think?). Right now i'm only getting 11-13 mpg and i should be doing somewhere between 15-18 or better. The car idles ok, but wants to die if held in gear at a stop light.

second is because of so little idle vacuum, my brake booster basically doesn't work. So when in gear at idle i have to smash the brake down to keep the car put. Mainly right now i just slip the tranny into neutral which is fine, but i was in the process of trying to sell the car and well, try explaining why you have to shift to neutral at a stop light to some people doesn't work .
 
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Old 07-14-14, 01:05 PM
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I can't remember, but does that year model have a pcv valve and an EGR? They both will affect idle vacuum if not in good shape.

You are seriously expecting better than 18 MPG with that set up? I've been out of the loop too long.
 
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Old 07-14-14, 09:11 PM
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Donnoo. For me, under no leaks, bad brake boosting was 1st sign of low IN CYLINDERS compression. They simply do not develop enough suction. Run compression test, not that hard to do.
 
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