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Thread: Unstable Idle - Carb Issues

  1. #1
    Join Date
    Jan 1970
    Location
    Australia
    Posts
    15,232

    Unstable Idle - Carb Issues

    My Haflinger has a Zenith NDIX carb that is twin throat carb.

    I am doing a tune on my engine and need to set the idle mixtures. I have the engine warmed and the idle mixture screws wound out half a turn and idle upped to about 900RPM so that I can adjust the idle mixture screws to get max revs.

    However I cannot get a stable idle - in approx a 25secs cycle the revs increase from about 900rpm up to about 2400rpm and then back down to around 900 RPM then back up and down again - over and over. The accelerator cable is not attached and the carb is doing this all by itself. See this youtube vid - you get the idea in the first 50 secs or so. YouTube

    It has been suggested that air may be getting in so I completely dismantled the carb and cleaned everything with carb cleaner and compressed air - nothing abnormal found - all gaskets are good, butterefly spindles are good, no dirt or corrosion so there is no obvious place for air to get in.

    So I am looking to ideas from people with general carb experience on what might be going on here.

    Thanks

    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

    [SIGPIC][/SIGPIC]

  2. #2
    Join Date
    Jan 1970
    Location
    Avoca Beach
    Posts
    11,674
    Another possibility is that the vacuum advance is advancing it, or you have a broken centrifugal advance spring.
    Do you have the vacuum advance disconnected?
    Regards Philip A

  3. #3
    Join Date
    Jan 1970
    Location
    Australia
    Posts
    15,232
    Thanks Phiip - no vacuum advance with these vehicles but I will check the springs on the centrifugal advance. The dizzy is only a couple of years old so should be OK but is a good idea to check. With the timing light on the timing marks you can see the timing advance as revs increase.

    Cheers

    garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

    [SIGPIC][/SIGPIC]

  4. #4
    Join Date
    Jan 1970
    Location
    Central West NSW
    Posts
    22,655
    The usual reason for this is the throttle spindle or its bushes is worn, but you seem to have discounted that. I would be looking for a gasket leak under the carburettor or between the manifild and the head (not familiar with this engine, so can't be any more definite).
    John

    JDNSW
    1986 110 County 3.9 diesel
    1970 2a 109 2.25 petrol

  5. #5
    Join Date
    Jan 1970
    Location
    Australia
    Posts
    15,232
    Thanks John - I am after generic advice as the arrangement except for no vacuum advance is basically the same as any old banger. The carb was fully reconditioned by the only carb specialist in Canberra (Murrumbateman) about 4 years ago and has only covered a couple of thousand km since. As I indicated pulling everything down showed in the internals to be in good condition with good gaskets and the spindle with no slack - i will spray some carb cleaner around where the manifold bolts to the heads to see if there are leaks there.

    I am wondering if I have a float level issue where at revs the engine handles excess fuel but when at idle the extra fuel is causing the revs to rise until it is used up then the revs drop until the excess fuel builds up again - the float level is fixed ie there is no adjustment but I guess brute force could adjust it - the needle and seats were all replaced as part of the reconditioning process.

    Thanks for the input - much appreciated.

    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

    [SIGPIC][/SIGPIC]

  6. #6
    Join Date
    Jan 1970
    Location
    Central West NSW
    Posts
    22,655
    Are you sure you have adequate fuel delivery? Could be that idling (I assume a mechanical pump) the fuel is barely keeping up, but when above idling speed no issues - could be an air leak in the suction line , a bit of dirt under a valve, or a partial blockage.

    Another possibility - have you checked the float for free movement or leaks? If the float or needle valve is sticking it could have this effect.
    John

    JDNSW
    1986 110 County 3.9 diesel
    1970 2a 109 2.25 petrol

  7. #7
    Join Date
    Jan 1970
    Location
    Australia
    Posts
    15,232
    Once again - thanks - it originally did have a mechanical fuel pump but a previous owner got rid of that and I have a low pressure facet style electric pump. It has enough flow to power the engine at full revs and full load but does not have the pressure to over power the float needle and seat.

    When I did have the carb reconditioned the one thing that did not come in the kit were new floats and they fell apart the day after I reinstalled the carb - I managed to get new ones so they have not been in long either. They looked Ok yesterday. I am thinking of rigging up a gravity feed and seeing how it runs just in case it is a fuel pump issue.

    Though the needle and seat was the one thing I did not pull apart yesterday so I might do that now.

    Helpful as always - thanks

    Garry
    REMLR 243

    2007 Range Rover Sport TDV6
    1977 FC 101
    1973 Haflinger AP700
    1971 Jaguar V12 E-Type Series 3 Roadster
    1957 Series 1 88"
    1957 Series 1 88" Station Wagon

    [SIGPIC][/SIGPIC]

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