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Thread: MFI troubleshooting - high idle, pops and bucks at part throttle

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    Serial old car rescuer Arne's Avatar
    Join Date
    Aug 2011
    Eugene, OR

    MFI and fuel troubleshooting - high idle, pops and bucks at part throttle

    Car - 1972 911T 2.4 MFI, engine all stock

    Background - Years ago (at least 3 owners ago) MFI was swapped to carbs, but parts boxed and retained. Previous owner had engine rebuilt (all stock spec) and MFI put back into service. Project stalled immediately thereafter. I bought it as a running car, but with only 2-3 hours on the engine, and no road testing (due to bad brakes).

    Now that the car is road worthy again, I'm looking at some of the basics in order to correct running problems. Main problems are high idle speed (~1300) and popping and bucking at part throttle. Smooths out and pulls nicely at wider throttle opening. These problems only surface when the engine is fully warm. When cold or during warmup neither symptom is evident.

    Keeping the old Check-Measure-Adjust doc in mind, the ignition and engine bits are good. New air filter, plugs, points, plug wires, good compression, fresh valve adjustment. Timing appears good, and the vacuum retard is working as expected.

    So I need to look at getting the idle speed down, and perhaps richen(?) the part throttle on the pump.

    So first I look at the idle. And it is my understanding that the idle is adjusted with the 6 air screws on the stacks, and then the synch is re-checked.

    But there is my first failing. No amount of changing on the air screws appears to have any effect on the idle speed. The screws were all over the place when I started, after trying to drop the idle speed for a bit with no result, I stopped the engine, screwed all of them in to the stop, then backed each out 2 full turns to have a starting baseline.

    Still idles at 1200-1300 when warm. All of them check fairly close on my synch tool, but still no idle change.

    Any thoughts?
    Last edited by Arne; 11-22-2019 at 12:17 PM.
    - Arne

    Almost done
    - 1972 911T coupe, Silver Metallic, Fuchs, Appearance Group, F&R sway bars. Soon to regain Frankfurt and factory A/C.
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

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