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Thread: Carb air leak causing rough running?

  1. #1
    Senior Member Phill's Avatar
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    Carb air leak causing rough running?

    I'm pretty sure I have an air leak on somewhere on my inlet manifold which is preventing me from setting the carb up properly. When the engine is cold and the choke is full on the engine runs sweet and healthy. But once the choke has heated up and the inlet flap is fully open the engine starts to cough and splutter and refuses to idle without cutting out. No amount of adjustment fixes this. However, if I disconnect the choke completely so that the flap remains partially closed at all times the problem is solved and the engine runs sweet and healthy with a fast idle. I'm assuming this is caused by an air leak and the carb flap remaining partially closed is compensating for the extra air being drawn into the manifold from somewhere else. Does this make sense - am I on the right track?
    "The most beautiful kit car in the world - Motor"

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  2. #2
    Senior Member Brett's Avatar
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    Sounds like an air leak.

    I assume the car ran fine up till now.

    I would check all vacuum lines first. Make sure nothing is cracked and leaking or heat got to a line and melted it.

    Next I would check your vacuum advance. Make sure the diaphragm is in good condition and not leaking. Test it to see if it holds a vacuum.

    If you haven't found any leaks you can take WD40 or a carb cleaner and spray it around the intake manifold mounting points and the carburetor mount. If the RPMs pick up at the spot where your spraying there is a good chance that the leak is there. .

    One spot everyone usually misses when rebuilding a carb is the shaft that the "inlet choke flaps" are on. If that shaft moves around the bushings are worn out and needs to be replaced


    Edit:

    One other thing not to rule out is maybe dirt got into the carb and plugged the jets


    Edit:

    Another thing that could cause that problem could be that the fuel tank is not vented properly. remove the gas cap and see if that fixes the problem
    Last edited by Brett; 05-05-2019 at 05:01 PM.

  3. #3
    Are you sure slow idle jet isn’t blocked?

    sometimes crud, or even part of cut fuel lines block this up and it doesn’t run. Will be masked when on choke, but when off chocke will struggles.

    Can you see inside and see fuel running from the little tube?

    Sometimes just a blow blow through the carb with air line can sort it. If you take it off and play on the bench you can operate and see how the fuel is being delivered.
    All the best,

    Jim

    Club Nova Member, SDBC Member, GRRC Member, Southern Classics



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  4. #4
    Senior Member Phill's Avatar
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    Thanks for the replies. I spent the day checking everything carefully for leaks. Removed the rear valence for better access and checked the tappet gaps for good measure. All was in order and I couldn't find any sign of an air leak anywhere yet still the carb runs rough once the choke flap is fully open.Hoever, it runs sweet with the flap partially closed so I have disconected the choke meanwhile in order to get smoother running until I decide what to do next?
    "The most beautiful kit car in the world - Motor"

    http://stores.lulu.com/rightsigns


  5. #5
    Senior Member BlueNova's Avatar
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    Hi Phill,
    Don't know whether this is helpful or not, but I had a problem with my lawnmower a couple of years ago ... it would always run fine for about 30 mins but then would cough, splutter and cut out. Couldn't start it again till it had cooled down. It was out of warranty but I took it back to the supplier who couldn't solve the problem.
    I then removed the carb and cleaned it out myself ... but the problem remained.
    Long story short, I bought a new carb and the mower's never missed a beat since.
    Are you running with your original carb or the 'value' one you bought last year?
    Alistair

  6. #6
    Senior Member Phill's Avatar
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    I'm back on the original Carb Alistair - but thinking of trying the "Value" Carb I bought again as I do think it's the Carb itself that's at fault but don't have a a great deal of confidence in the other carb
    "The most beautiful kit car in the world - Motor"

    http://stores.lulu.com/rightsigns


  7. #7
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    I would go with Jim take the jets out easy job and give them a good blow with compressed air.

    Dirk

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