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

  1. #21
    Serial old car rescuer Arne's Avatar
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    This afternoon did not go well. I may well be the poster child for why average home mechanics should not mess with MFI.

    Despite my best intentions of keeping track of where I was and what changes I was making, I believe I am well and truly lost. I suspect that at least once, when I was slowly turning the idle adjuster waiting to feel the tang drop, it had already dropped and was randomly turning the adjuster. I now have no clue where I am, or how to get to some sort of baseline.

    As of now, the car is hard to start, doesn't want to idle, smokes black when at what passes for idle, and pops a lot when held at higher revs (2k to 3k) with no load. It is no longer warm enough to make good adjustments, but isn't running well enough to be able to drive it to get it warm.

    Definitely seems very rich at low RPM, no longer sure what it's like at higher speeds.

    Despite the thinking that the main rack was rich, I'm still thinking that it may have been lean. (Who knows what it's like now.) I say this because while driving it to warm it up to temp this afternoon (before I screwed it all up), it ran great until the thermostat dropped it into full lean. Pulled strong and clean, no popping or bucking, just like I feel it should run. And then when it finally got to temp, it got bad. Popping, bucking, won't pull up a grade, way down on power. Still made me think the main rack setting was lean.

    But now? Who knows where I'm at.

    Any suggestions on how I recover from this and get to some sort of starting point?
    - Arne

    In Progress
    - 1972 911T coupe, now returning to the original Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  2. #22
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    Don't get discouraged . your description of the running when cold and then when the thermostat is warm certainly seems like a lean condition . I think any MFI car that has not been driven for a long time needs to be adjusted
    richer for todays fuel . My '73 E had not been driven since 1980 . I had to turn the screw 10 clicks to get a decent AFR reading . It seems like the idle is too rich , so turn that screw counterclockwise .

  3. #23
    Senior Member jt912's Avatar
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    It is all downhill to Eric Jones. It would make for a handy tech session car.
    John Thompson

    1966 912-first car 31 years ago and still my driver.

  4. #24
    Serial old car rescuer Arne's Avatar
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    Haven't talked to Eric about it yet, since he is racing this weekend and I only messed it up today. He did comment that he hadn't worked on any MFI in a long time...
    - Arne

    In Progress
    - 1972 911T coupe, now returning to the original Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  5. #25
    Senior Member 2.5MFI's Avatar
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    Quote Originally Posted by edmayo View Post
    Push in and turn until you feel the tang drop in the slot, then turn clockwise to go leaner. Go about 4 clks at a time. If leaning the idle seems to help you may need to go leaner on the rack since that leans out the whole system otherwise the idle adjuster may run out of adjustment range. And yes, I use the .024" gap.
    Ed, Arne, The idle mixture adjustment your talking about will go "Richer Clockwise". Leaner is CCW. Now the main rack adjustment is the opposite, CCW for richer and CW for leaner.
    Mark Jung
    MFI Werks
    Early 911S Registry #972
    Carrera T w/LWB, MT, RWS, PCCB
    72 T 66 x 100 MFI Twin Plug Coupe
    R Gruppe #686

  6. #26
    Serial old car rescuer Arne's Avatar
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    Wait, what?

    Oh, crud...

    Thanks for pointing that out, Mark. I actually knew that the two adjusters go in opposite directions. I've got the Lee Rice MFI articles in PDF from the Pelican link. Read them multiple times, so I had the "opposite directions" bit down pat. But I missed that Ed crossed up the direction above, and I kept using that as my guide. Oops.

    On the plus side, this gives me some hope. Basically, everything I've done has been going the wrong way, making things worse. That means there is some reasonable hope of getting back to a decent starting point. If I back out of everything I think I did, I'm going to be far closer than I am now.

    Wish me luck.
    - Arne

    In Progress
    - 1972 911T coupe, now returning to the original Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  7. #27
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    Popping out of the intake or exhaust? Out the intake is likely lean, out the exhaust is likely rich. If you run too long on the rich side you might want to be ready to throw a new set of plugs in it otherwise you could be chasing your tail.

  8. #28
    Serial old car rescuer Arne's Avatar
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    Well, better today than yesterday. After reversing what I did yesterday I'm more or less back where I started, only without the high idle speed.

    But once I got back to that point, I haven't been able to get much improvement. Warm or cold, the idle is quite slow, rough and stumbling. But the other idle symptoms are mixed—exhaust is dark and somewhat sooty, yet it pops and spits in the intakes.

    As for the main rack adjustment, I believe that is still lean. It actually runs great while the warmup thermostat is still enriching the mixture, pulls smoothly and cleanly, no popping, sounds great, etc. If it ran like that all the time I'd be a happy camper.

    But as soon as the thermostat gets fully warm, it becomes bad. It pops, spits and bucks (intake, not exhaust), won't pull under load, just not a happy car.

    Does any one (Mark?) know about how much enrichment the thermostat does on the main rack? It would seem that I need to go rich on the main rack to about that about. Then once that is running clean, try to figure out what to do with the idle circuit.

    For now, I've called it a day. Tomorrow I'm planning to go back and double check all the throttle plate settings and pushrod lengths. I've done it once, but it is worth another look. I'd like to ignore the idle circuit for now and try to dial in the main rack. But since I need to enrich the main rack, that doesn't seem to help the perceived too-rich condition at idle. So I may need to do them in tandem.

    Open to other ideas, too.
    Last edited by Arne; 03-31-2019 at 08:17 PM.
    - Arne

    In Progress
    - 1972 911T coupe, now returning to the original Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  9. #29
    Senior Member 72targa's Avatar
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    I have to ask. Have you studied this?

    https://www.pbase.com/slidevalve911r...docs_bosch_mfi
    Peter Kane

    '72 911S Targa
    Message Board Co-Moderator - Early 911S Registry #100

  10. #30
    Serial old car rescuer Arne's Avatar
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    Quote Originally Posted by 72targa View Post
    I have to ask. Have you studied this?

    https://www.pbase.com/slidevalve911r...docs_bosch_mfi
    Yes, I have. I downloaded a copy some time ago. I have referred to it, but some of the other guides seem more useful to home mechanics.
    - Arne

    In Progress
    - 1972 911T coupe, now returning to the original Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

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