Engine What is this vacuum gauge telling me about my engine?

Discussion in 'Classic Mustang Specific Tech' started by 95RedCobra, Nov 26, 2012.


  1. 95RedCobra

    95RedCobra New Member

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    I have been trying to correct what I believe is a lean condition with my carb. Problems are a bad bog when accelerating and some bucking when cruising at lower rpms.

    I tried reading up on how to read a vacuum gauge, but it seems there 5 or 6 different possible reasons for a low, erratic vacuum reading at idle.

    I've made a few adjustments to the carb to make it richer and the car is running better - not great, but is starting to smell rich.

    Engine is a 351W with Edelbrock Performer RPM heads, intake, cam, and an Edelbrock 1406 carb - 600 cfm, vacuum secondaries. I think this vacuum reading is lower than it should be with these parts.
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  2. Rmoore45

    Rmoore45 Member

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    Had a similar problem on my 289, turned out to be a combination of vacuum leaks, bad carb tuning, timing, and intake manifold leaking. Get some carb cleaner and spray it on the area between the heads and the intake, if you hear a change in motor RPM then that's your problem.
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  3. horse sence

    horse sence Please don't call me Mom SN Certified Technician

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    cam size will effect the vacume .sounds like maybe a timing problem
    possibly slightly retarded?
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  4. 95RedCobra

    95RedCobra New Member

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    Yeah, Rmoore45, I'm beginning to think I may have a vacuum leak causing my lean condition that I've been trying to tune around. That's not going to work very well. I'll pick up some carb cleaner and see if that can help me find any leaks. I've already tried listening around where the carb meets the intake and the intake meets the heads, but couldn't hear anything.

    horse sence: I know a 'hot' cam will really effect vacuum, but I don't think the Performer RPM cam should lower it that much. The cam specs:

    Duration @ .50
    Int / Exh
    Lift @ Valve
    Int / Exh
    Lobe Seps.
    224° / 234° .496" / .520"
    110°

    I assume by timing you mean ignition timing and not cam timing. Believe the cam shoulc be installed straight up with the timing marks on the gears lining up. As far as ignition timing I believe it is around 14 BTDC, but would have to re-check to know for sure. That is unless my balancer is off or something else is causing me to not be able to set the timing properly.
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  5. woodsnake

    woodsnake Active Member

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    I have that same cam in my 68/289, and as I recall, around 11 inches of vacuum is all I could get out of it. Actually, I am on my 3rd copy of that cam, and will not ever recommend to anybody to buy an Edelbrock camshaft.
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  6. PoppyMod

    PoppyMod Member

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    Hi,
    I'll offer another check item.....re-check you valve lash/pre-load settings. Too tight will result in a lower vac reading.
    Here's an articulated vac trouble shooting link I have in my arsenal of tools:
    http://sbftech.com/index.php/topic,3020.0.html
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  7. 95RedCobra

    95RedCobra New Member

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    Thanks for the replies. Was finally able to spend a little time working on it this weekend. Checked for vacuum leaks with carb cleaner and didn't find anything. Maybe this is normal for this cam - just seemed a lot lower than I thought it would be.

    I'll keep the lifter pre-load suggestion in mind - I'm pretty sure I've got that set properly. Had the rocker arms off about 6 months ago to replace the valve seals and was careful to get the pre-load correct. Spinning the push rod between my fingers while tightening the rocker. Then when I just began to feel it drag, tighten the rocker down another 1/2 turn, then lock it down.
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  8. PoppyMod

    PoppyMod Member

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    What do you know about the cam? Is your PCV system in good working order? Less than 15 on a street cam is venturing into pretty drastic for a street-able ride. If it's an Eddy cam and you know the grind number contact Edelbrock tech. They should be able to tell you what the expected vac should be, obviously, all else being equal. I know the custom grinders can offer the vac numbers.
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  9. bartl

    bartl Member

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    Un-balanced carburetion, improper spark plug gap, poorly seating valve, incorrect ignition timing.
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