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Thread: Low vacuum

  1. #1
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    Default Low vacuum

    84 Ltd lx 347, t5
    Purchased a low mile 347 built by a reputable engine shop. The engine originally had a Holley 750 carb and made 450 hp on the dyno. The previous owner switched out the carb for a Edelbrock Pro Flo 4 fuel injection system. Everything I have read says my idle vacuum should be 14-15. My current system idle vacuum is 7.5, and my power brakes are pretty weak. I have checked everything for a vacuum leak, any suggestions.

  2. #2
    FEP Super Member bwguardian's Avatar
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    ...need more specifics like cam specs play a big part. I don't know anything about that fuel injection system, but does it have jets and/or butterflies like a carb that allow more or less fuel and air?
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    FEP Super Member mmb617's Avatar
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    I have that problem with my car, probably because the cam has a lot of overlap. I installed one of these vacuum reservoirs and it helped. I can't say the problem is entirely gone but there was a noticeable improvement. The brakes have always worked fine other than when the engine is at idle like when backing out of the garage, and the reservoir did help there. I keep thinking I'll just change over to manual brakes some day but I've been saying that for years so it may never happen.

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  4. #4
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    Not sure about the cam, on the build sheet it says custom ground cam. It has injectors on the intake manifold.

  5. #5
    FEP Power Member richpet's Avatar
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    Injectors on the manifold doesn't affect vacuum. It is just where the fuel is introduced. Port instead of common manifold/tbi type injection.
    A vacuum storage canister might be what you need if you are certain there are no leaks.

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