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Thread: MFI - Can't set idle

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  1. #1

    MFI - Can't set idle

    All,

    I had the MFI put back into my ’70 911E. The pump was rebuilt and the motor was setup by iSystems and put on the dyno. Unfortunately the motor sat for about a year before getting back into the car. Once in, things ran rough for a while but after getting fuel pressure fixed the motor ran well stationary. The day of the first test drive the car ran rough upon startup. I let it warm up a bit and drove it around the block. The car had no power! Couldn’t get out of first gear or over 2,000 rpms. Didn’t have enough power to get back up the driveway. Checked timing and plugs and things looked okay so we started adjusting the MFI rack. Ended up increasing the enrichment about 10 clicks. The car ran great but it started to surge at idle. I then decreased the idle enrichment about 8 clicks. Still surged. I then disconnected the shut off solenoid. No surging and the car ran great but did idle a little low, about 300 rpms. So we adjusted the idle screw and it shot up to about 1,700. Adjusted back down and would drop back down to 300 rpms. Could not get it set. I know you can go down a rabbit hole with these systems but I didn’t know if anyone had any suggestions as what I could check. The throttle bodies where not rebuilt. There might be a little play but we all felt that they were probably good at the time. Could this be the problem of not being able to set the idle?

    Thanks

    Chris

  2. #2
    Chris, tried to respond to you woes but was timed out and lost what I had typed, will come back later today after the experts weigh in.
    Rusty

  3. #3
    Senior Member
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    I am far from an expert, but my 71 S surged at idle, but only when it was warm. My local mechanic suggested lengthening the throttle rod, which solved the problem.
    Jay
    1946 Willys Army Jeep
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    1969 911T 'Speedster'
    1971 911S
    1974 Ford Ranchero
    1986 911 Carrera Cab
    1993 Audi V8Q
    2001 Mazda Miata 'Daughters Car'
    2008 Audi S6
    2018 Panamera 4S 'Wife's Car'

  4. #4
    Senior Member VintageExcellen's Avatar
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    Feb 2007
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    Quick tip - the idle rack (lower rack) can be turned all the way in (tight) and turned out 28 clicks for a baseline. Top rack - if you loose your starting baseline then you could be screwed as it has infinite movement.

    Best to start from the beginning and check everything. Distributor timing, Pump timing, rod lengths are important - especially the pump rod, all linkage is important and often the problem is one side is opening different from the other and they need full closure - remove the crossbar from the pedal linkage to independently verify the throttle cable is not too tight. All kinds of other things like timing, fuel pressure can affect it. You need 2 airflow meters, one for each side, if you can get all throats even and pulling around 4 or 5 on the number then it doesn't matter if your shafts are a little leaky.

    Engine also won't idle at 300rpm, probably more like at 600-700rpm, you want 900-1000.

  5. #5
    Senior Member
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    Oct 2010
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    Southern Ca.
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    I recommend using a timing light to check the distributor , is it returning to the initial setting ? The springs on the mechanical advance may be weak , and it's not returning to the initial setting .

  6. #6
    Junior Member
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    Mar 2011
    Location
    Australia
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    I recently had a lot of trouble with air getting sucked into a small hole in the fuel line to the pump. Pressure was excellent but the tiny air bubbles caused erratic performance on a newly overhauled and installed MFI. My two cents....don't play with your MFI pump settings until you make absolutely sure of everything else.

  7. #7
    Senior Member frederik's Avatar
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    Sep 2018
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    Get hold of the Check Measure Adjust document and go through it step by step. I can send a pdf to you if you want (PM).

    Also, you really want to use an air/fuel ratio meter to set the part load mixture.
    1970 2.2S Elfenbeinweiss
    1972 2.4T Targa Aubergine (MFI) [For sale]
    2002 996 TT Midnight Blue
    Member #3833

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