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

  1. #91
    Senior Member 2.5MFI's Avatar
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    I would keep the cold start circuit hooked up. It will speed up the RPM's during the cold start and that will bleed out the air faster. If you have a battery charger (50 amps or more) hooked up to the battery that will help speed up RPM's also and keep the battery a little more charged.
    Mark Jung
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  2. #92
    Serial old car rescuer Arne's Avatar
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    After a few more minutes cranking, it finally purged all the air and fired normally.

    Took it out to get it fully warm, and THE PROBLEM is still there. Perhaps a bit less severe than before, but still there. When fully warm, pulling under a load between 2000-3500 RPM feels like running out of fuel. But it is apparently getting fuel all the time, as it runs better if I can keep the RPM above 4000. Still runs better cold than warm, although that has changed as well, it "feels" and smells a bit richer than before when starting cold.

    I feel I've done all I can with fuel and fuel delivery. The mixture with the calibrated pump is close enough now to leave until later. The plugs after the test drive are no longer black and sooty, but instead have a nice gray-tan color.

    So I do not plan to touch the MFI settings until I can track this down, and then will fine tune it properly using an AFR meter.

    In the meanwhile, I'm going to go back and recheck some of the other basics, most of which I've checked at least once, some several times. Going to take a long look at plug wires and other ignition bits. Also plan to re-check valve lash.

    Other thoughts and advice are welcome.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  3. #93
    Check cam timing,,,,been fooled by that once. Car had come from dealer after new tensioners installed, (this was around 1975 or so ) On the theory of go back to the last thing done I checked cam timing,,,,sure enough it was wrong. Nothing you can do will make it run right ! This has become part of my to do list when the problem won't act normally. Only takes a half hour to check, and can save hours of grief.
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  4. #94
    I’m sorry for your frustration but this thread has been interesting reading.

  5. #95
    Serial old car rescuer Arne's Avatar
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    Cam timing seems unlikely, Ed, since it runs so well cold. But I'll take a peek at it just to be sure. Any gotchas I need to be aware of with the Carrera pressurized tensioners that are on it?
    Last edited by Arne; 05-07-2019 at 05:15 PM.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  6. #96
    Serial old car rescuer Arne's Avatar
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    Just thinking of things tonight. I once had a problem on a 240Z with spark breaking up at higher RPM - felt a lot like fuel starvation, but wasn't. Turned out it was an ignition condenser that was way bad. I don't recall seeing any condensers on this car, did the CDI eqipped cars not use condensers?
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  7. #97
    No condensers used on CD ignition. Seems to me in previous postings you had done some swapping with known good pieces from another car. The Carrera tensioners won't be problem in checking cam timing. I was going to ask if it had solid tensioners,,,I've seen those get so loose the chains whipped around radically changing the timing with engine speed. Measure the rotor resistance HOT just for grins.
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  8. #98
    Serial old car rescuer Arne's Avatar
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    Yes, swapped a known good CDI box and brown-top coil in, made no difference. So I'm comfortable that those two items are OK.

    Cap and rev-limiting rotor are both Bosch, look to be fairly recent, presumably from when the engine was rebuilt. Points too. But plan to revisit all of that soon, along with valve lash and now cam timing.

    Plug wires are suspect. Now that I have taken a fresh, longer look at them, I suspect they are rather old. The ends are Bremi brand, don't know whether that's a good or bad thing. Anyone have any favorite brand of wires they like for a driver? Not worried about original brand or look, just function.
    - Arne
    Current - 2018 718 Cayman, Rhodium Silver, PDK

    Sold - 1972 911T coupe, Silver Metallic; 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  9. #99
    Just measure total resistance of plug wire from end to end, about 3,000 ohms tops, that will make sure there are no open circuit ones. For example, on the rotor, you can't say "they look to be recent" you will have to verify one way or another.This will be a matter of slowly eliminating possibilities until you find the problem,,,,and when you do find it I can almost guarantee it will be simple. Those are the ones that drive us nuts!
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  10. #100
    Senior Member Simonjjb's Avatar
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    Quote Originally Posted by edmayo View Post
    Just measure total resistance of plug wire from end to end, about 3,000 ohms tops, that will make sure there are no open circuit ones. For example, on the rotor, you can't say "they look to be recent" you will have to verify one way or another.This will be a matter of slowly eliminating possibilities until you find the problem,,,,and when you do find it I can almost guarantee it will be simple. Those are the ones that drive us nuts!
    Its amazing how that works out on many things...sometimes its something simple. I hope so for Arne
    1968 911L Coupe - Golden Green
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