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

  1. #1
    Serial old car rescuer Arne's Avatar
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    MFI and fuel troubleshooting - high idle, pops and bucks at part throttle

    Without thinking, I posted this over on the Bird site as well. Not many MFI peeps over there, except for some who are here as well. So no need to answer both places.

    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; 04-05-2019 at 11:46 AM.
    - Arne

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  2. #2
    Do you have some idle after the main rack adjustment? You should. Then with the idle rack (below 1500 rpm or so) you can further tune the idle up or down. Then go to the fine tune with the idle air bleed screws. The recommended screw settings are just that, recommended. If you have CMA follow it to the letter. It works. I followed Grady Clay's advice and finally figured it out when realizing the main rack has substantial influence on basic idle.
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  3. #3
    Start with the basics, timing must be set correctly, and then the downfall of most is the basic mechanical linkages. I can guarantee they aren't right. Somewhere there are throttle air valves being held open. And probably the pump rod is the wrong length.
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  4. #4
    Yes and what Ed says.
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  5. #5
    Senior Member NZVW's Avatar
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    Just a quick ask,
    How old is the petrol in the tank ???
    Mark

  6. #6
    Serial old car rescuer Arne's Avatar
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    Thanks for the info so far.

    Quote Originally Posted by NZVW View Post
    Just a quick ask,
    How old is the petrol in the tank ???
    It's fresh. Tank was drained when removed for the front pan replacement.

    Quote Originally Posted by edmayo View Post
    Start with the basics, timing must be set correctly, and then the downfall of most is the basic mechanical linkages. I can guarantee they aren't right. Somewhere there are throttle air valves being held open. And probably the pump rod is the wrong length.
    Timing is good at idle, both with vacuum connected (5 ATDC) and disconnected (~5 BTDC). I'm reluctant to check at 6000 rpm as the engine is still fresh and not broken in. Been trying to keep it below 4500 or so until I get some miles/hours on it.

    I'm not concerned about the function of the thermostat or the warm air supply from the HE, as there is a marked change in behavior as the engine gets up to temp, that seems to indicate the thermostat is working.

    I pulled the pump rod to measure it, and it is correct at 114mm. A little surprised to find that was correct.

    Tomorrow I'll pull all the rest of the linkages, check and set the throttle stops.
    Last edited by Arne; 03-29-2019 at 09:16 AM.
    - Arne

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  7. #7
    member #1515
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    When I used my car every week I never had any MFI problems. Now that I leave it stored for a couple of months at a time I am having some small cracks and pops when I first use it. I run a few tankfuls of fuel with stabil marine and take it for a couple of long drives and the problems go away. Stuff just gets gummed up. I now make sure to leave the tank full and with stabil when I put it away. Stuff gets gummed up.
    Have you cleaned out the idle screens ports?
    All this in addition to all of the above comments.
    Last edited by RSTarga; 03-29-2019 at 10:47 AM.
    David

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

  8. #8
    Serial old car rescuer Arne's Avatar
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    OK, today's topics.

    Checked the pump timing, it was spot on. Not too surprised to find that, the pump was installed and set up while the engine was out and the timing marks were easy to see.

    Took it for a drive to get it up to temp.

    Got back and pulled all the throttle connecting rods off, reset all the throttle plates. The plates were not set too badly, most were pretty close. Started it up and fine adjusted them.

    Started to check and re-install the connecting rods then, found that one of those on the right bank was a touch long and was probably holding #5 open a tad. Corrected that.

    Next went to the 2 throttle pushrods, and found what I suspect was the root cause of the fast idle. The left side rod was too long and was lifting all 3 throttles on that side just barely off the stops. Re-adjusted it to stop that from happening, then rechecked synchronization. Still good.

    Re-checked the pump rod for 114mm center-to-center length. It's right there, or as close as I can measure it. So I popped it back on as well.

    Results of today's work? Mixed. The fast idle is definitely corrected. I don't 100% trust the in-dash tach, but it shows a touch under 1000 rpm, and it sounds right too. But the mixture is apparently a bit worse now. It pops and spits a fair bit just while idling, and the popping and bucking while driving is worse.

    So my next steps are???

    Should I attempt anything on the pump rod, or assume that it is correct and adjustments are needed elsewhere? If I leave the pump rod as is, what should my next step be?
    - Arne

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  9. #9
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    I have the same experience with high idle after warm up. My tech says it could be the throttle bodies are sucking air past the throttle plates because of ware. Have yours been re built? Chris
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  10. #10
    Serial old car rescuer Arne's Avatar
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    Quote Originally Posted by raspy2point2 View Post
    I have the same experience with high idle after warm up. My tech says it could be the throttle bodies are sucking air past the throttle plates because of ware. Have yours been re built? Chris
    I don't think so, but don't know for sure. Indications are that my throttle shaft bushings are not leaking, that was one of the first things I looked at. But based on my experience today, I would warm your car up and then pop off all of the various pushrods and see where it idles with no throttle connections. And then reset all the throttle stops while you are in there. If one or more of those rods are not correct, a high idle can be the result.

    But maybe your tech has already done that.
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

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