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Thread: MFI high idle

  1. #1
    Senior Member Simonjjb's Avatar
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    MFI high idle

    After fixing the oil leaks at the thermostat I took the 71S out for an hour run. She ran really well and the handling is outstanding. I’m getting to the end of dialing her in and completing the last remaining punch list items. The gas gauge was only registering 1/2 so I removed the sender and reinstalled. Now working properly. I also have to check the emergency brake as the lever is coming up too far.

    One item that I need help on is the idle. The car starts well from cold and the idle is about 1K. However, after it is warm the idle increases to 1600. On hot restart it does the same. The cross throttle rod arm is fully seated and the butterflies are fully closed. The cold start lever is fully down and not engaged. The car is stock 2.2 MFI. Engine rebuilt including rebuilt dizzy and throttle bodies. I suspect its been like this since the rebuild.

    What would I start to check ?

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    Last edited by Simonjjb; 07-30-2023 at 02:31 PM.
    1968 911L Coupe - Golden Green
    1971 911S Coupe - Gemini Blue
    1973 911S Targa - Signal Yellow
    1974 914 2.3 - Sunflower Yellow

  2. #2
    Moderator Chuck Miller's Avatar
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    I'd first check the vacuum hose connections (and hoses) from the throttle bodies to the 'T', to the distributor... (checking the plastic 'T' as well)

    Good luck
    Chuck Miller
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  3. #3
    Senior Member Simonjjb's Avatar
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    Quote Originally Posted by Chuck Miller View Post
    I'd first check the vacuum hose connections (and hoses) from the throttle bodies to the 'T', to the distributor... (checking the plastic 'T' as well)

    Good luck
    Thanks Chuck - but I don't think my car has those. I certainly can't see any and there isn't any connection to the Dizzy.
    1968 911L Coupe - Golden Green
    1971 911S Coupe - Gemini Blue
    1973 911S Targa - Signal Yellow
    1974 914 2.3 - Sunflower Yellow

  4. #4
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    yes 69 to 71 MFI cars do not have vacuum advance. That started in 72. I have a 70 2.2S and the idle was the same as yours until I had the engine rebuilt and now the idle is pinned at 1000 rpm all day long. Of course I had 114,000 miles on the car when rebuilt. My throttle bodies were done by Aron Burnum. Since you are in Florida I would send them to Eurometrics on the east coast and have them rebuilt anyway. Just to make sure air is not getting past the butterflies making a high idle. Chris
    1. Chris-Early S Registry#205
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  5. #5
    Senior Member Simonjjb's Avatar
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    These were rebuilt by Aaron also. So I doubt that would be it.
    1968 911L Coupe - Golden Green
    1971 911S Coupe - Gemini Blue
    1973 911S Targa - Signal Yellow
    1974 914 2.3 - Sunflower Yellow

  6. #6
    Disconnect all linkage arms from the cross bar, then disconnect each individual throttle rod from each throttle shaft, now is the idle still too high? if not you've found the problem, if it is, adjust each idle bypass screw until the idle lowers. This is assuming of course that you have the timing set correctly. Since every piece of linkage was disturbed during a rebuild this is almost always the cause. If all the bypass screws are are bottomed out and the idle is still high then it may be air leak past the butterflies.
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  7. #7
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    I had this same problem on my '70E when I first started to drive the car after restoration. The car ran well but would idle at about 1500 RPM. I did everything suggested by Ed but I could only get the idle down to about 1200. Since I did not have an AFR tester, I turned to the professionals who tuned the MFI pump six clicks leaner. This improved the idle and did not seem to affect the overall performance.
    Tom Butler
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    1970 911E
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  8. #8
    You may try to close for example 1/2 turn on the 6 screw you have on your throttle body.
    That should reduce idle rpm.
    It's better to that do with a synchrometer.

  9. #9
    member #1515
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    Logically, high idle can only occur from
    a vacuum leak (air leak)
    sticking distributor or too much advance
    sticking throttle mechanism
    Follow Ed's diagnostics to find the problem.
    David

    '73 S Targa #0830 2.7 MFI rebuilt to RS specs

  10. #10
    Senior Member Simonjjb's Avatar
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    Quote Originally Posted by RSTarga View Post
    Logically, high idle can only occur from
    a vacuum leak (air leak)
    sticking distributor or too much advance
    sticking throttle mechanism
    Follow Ed's diagnostics to find the problem.
    And I would add - Air bypass screw
    1968 911L Coupe - Golden Green
    1971 911S Coupe - Gemini Blue
    1973 911S Targa - Signal Yellow
    1974 914 2.3 - Sunflower Yellow

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