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Possible vaccum leak??

Discussion in 'Technical Q&A' started by marks308GTB, Jun 11, 2005.

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  1. marks308GTB

    marks308GTB Karting

    Jun 6, 2004
    114
    Los Altos Hills, Ca.
    Full Name:
    Mark S.
    #1 marks308GTB, Jun 11, 2005
    Carbed 308 recently started having slight hesitation and sometimes popping on medium acceleration. No symptoms on very light acceleration or heavy acceleration above 3000 rpm. Idles perfect. Above 3000 rpm is perfect. Dizzys rebuilt 1 year ago. Carbs tuned about the same time. Ran well for the year.

    Of interest: On a very hard turn in either direction, the engine will almost want to die....like fuel starvation.

    I can modify the symptoms slightly by adjusting the mixture screws on the left rear carb a little richer. But also seems to change when I do the same on the other carbs.

    The vacuum hose for the power brake booster is original. Am wondering if this hose has developed a small leak. I know the possible fuel starvation problem is probably not from a vacuum leak, but the other symptom might be.

    So the question: Is it possible that a slight vacuum leak from the power brake booster hose could cause this type of symptom?

    Thanks,
    Mark
     
  2. marks308GTB

    marks308GTB Karting

    Jun 6, 2004
    114
    Los Altos Hills, Ca.
    Full Name:
    Mark S.
    #2 marks308GTB, Jun 11, 2005
    UPDATE

    I have been able to reduce the symptoms just by enrichening the adjustments on the left rear carb.

    Anyone know the exact inside diameter of the brake booster vacuum hose so I can buy some rubber hose and test this hypothesis?

    Mark
     
  3. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #3 Birdman, Jun 12, 2005
    Mark, if you run the car for a minute and rev it a bit to get a good vacuum in the line, then shut the car off and unplug the one-way valve in the vacuum line, you can tell if the line on the far side of the one way valve from the engine is working. It will have such a vacuum in it that it's hard to get the valve out of the hose. When you pull it free, it will "pop" like you uncorked a bottle of chanpagne. If that happens, you know it's holding a vacuum. That leaves just the short peice of hose from the manifold to the one-way valve to check or replace. (Easier to just replace it).

    I would also check the small hose that goes from the front bank of intake manifolds to the charcoal canister on the right rear of the engine bay. That one gave me problems!

    Also, were the carbs synched properly? Typically the carb with the vacuum line to the brakes on it needs a tad more air bypass.

    Birdman
     
  4. marks308GTB

    marks308GTB Karting

    Jun 6, 2004
    114
    Los Altos Hills, Ca.
    Full Name:
    Mark S.
    #4 marks308GTB, Jun 12, 2005
    Birdman,

    Excellent pointers. I will go ahead and do the pop test. Also will check the cannister lines. In fact, now that I think about it, there was a problem with the cannister lines about a year ago which I thought was cleared up.

    The carbs were synched correctly about a year ago. We definitely had a special problem with the carb that has the brake booster line on it and I thought that we had it set up correctly.

    But...another year passes and who knows what happens to this old rubber.

    Will let you know what I find.

    Thanks,
    Mark
     

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