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checking engine vacuum on 308

Discussion in 'Technical Q&A' started by BlueMax, Oct 11, 2007.

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  1. BlueMax

    BlueMax Formula Junior

    Aug 6, 2006
    307
    Norfolk, VA
    Full Name:
    JR
    #1 BlueMax, Oct 11, 2007
    Wondering if folks use a vacuum gage much to check the health of their 308. I have a '79 carbed 308. As you know there is no real single point to check the vacuum. I guess you could check each manifold separately.... Is this an effective check?

    Right now at low rpm's I am showing a bit of an erratic reading.

    Thoughts?
     
  2. miketuason

    miketuason F1 World Champ
    Owner Silver Subscribed

    Feb 24, 2006
    15,831
    Cerritos, CA.
    Full Name:
    Mike
    #2 miketuason, Oct 11, 2007
    This is what I do, to pin point where the vacuum leak is coming from, I use an aerosol carb cleaner with the little red tube that's attached to the nozzle then while the engine is idling I start spraying on the vacuum hoses, around the plenum gas, intake manifold, etch... then as soon as you hear the RPM raise up, that is where the leak is. Now you may have more than one leak so continue on until you don't hear the RPM go up anymore. you can verify this by spraying a little bit inside the carb and you will notice the increase in the RPM.
     
  3. BlueMax

    BlueMax Formula Junior

    Aug 6, 2006
    307
    Norfolk, VA
    Full Name:
    JR
    #3 BlueMax, Oct 12, 2007
    thanks, but not really looking for a leak necessarily. I'm trying to determine if I can use a vacuum gage to troubleshoot my engine. ie steady vacuum around 17-22= normal; decrease in vacuum on accel, increase on decel. fluctuations could mean sticky valves, worn valve guides, etc.

    Just not sure how it works on an intake that really doesn't have a plenum (just drops right down into each cylinder....)
     
  4. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,367
    NY
    #4 Steve King, Oct 12, 2007
    I have found the vacuum to be erratic in my 77. Not like most of the other engines I have worked with. I believe it is due to the cam overlap and the fact that on the carb cars there is not a "manifold" vacuum port. I tap off of the small vacuum ports under each carb but still get flucuations even at each port and when I gang them together. You can get a steadier reading if you pull the brake booster hose and make an adapter to hook up the gage. Just go back to the hose or weber gage to do the carb adjustment.
     
  5. BlueMax

    BlueMax Formula Junior

    Aug 6, 2006
    307
    Norfolk, VA
    Full Name:
    JR
    #5 BlueMax, Oct 12, 2007
    I'm glad to hear others have seen the same fluctuations. Under normal circumstances, my readings would indicate a valve or valve guide problem. But since you can only really take a ported reading from a couple of cylinders at a time, it doesn't seem like the old "tried and true" vacuum gage can help much in diagnosing this engine.... (years ago on different engines, if I didnt have my timing light, I used to be able to set my timing fairly well with the vacuum gage!)
     

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