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Engine diagnosis 2 low vacuum

Discussion in 'The Hokey Ass Message Board' started by deerejohnb, Sep 9, 2012.

  1. deerejohnb
    Joined: Dec 29, 2007
    Posts: 112

    deerejohnb

    Hey guys,
    This post is an extension of a post I created 2 months ago. See link

    http://www.jalopyjournal.com/forum/showthread.php?t=709893&highlight=engine+diagnosis+please+help

    The issue I am having occurs at idle as it puffs or skips out of the exhaust when it is running. I noticed it after the cam was replaced but it may have been doing it before. The cam was ground back to stock but they said they added a very slight amount of lift and duration but not enough to make it idle like a dragster. The truck has great power on the road and I can get up to 55 mph with no problems. In fact, it has more power than its ever had.
    I connected a vacuum gauge to the carb vacuum port that goes to the distributor and I am only getting 11". According the vacuum diagnosis charts it could be either late valve timing or a vacuum leak. I have used propane all around the intake and I cannot locate any type of vacuum leak (I get no change in rpm or the way it runs). I tried another carb off a known good truck and I have the same issue. When The misfire/puff occurs the vacuum gauge drops ½ inch Hg.
    I don't think it would be late valve timing because wouldn't it have a lack of power?
    Here is what I have done to the engine.
    All new ignition(even another distributor and coil)
    Different carb
    Compression is 90+ on all cylinders
    Replaced all valves and ground the valves
    Timing is set to 0 degrees
    Valves adjusted to .018 Cold
    A good running engine should have vacuum between 16-21 but does anyone know about what these engines run at.
    Here are some questions I have for you to ponder.
    1.Could I have a vacuum leak between the intake and exhaust manifolds internally.
    2. Wouldn't late valve timing cause lower compression and a lack of power?
     

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