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Thread: MFI - Can't set idle

  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
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  4. #4
    Senior Member VintageExcellen's Avatar
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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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    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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    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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    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
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  8. #8
    Frederik - I just PM you.

    All - Thanks for the advice. I did check the timing before I started messing with the enrichment but I'm going to run through things again this weekend. I'm going to especially run through the linkage. I may have something too tight or something. It doesn't seem as smooth as it should be.

    Chris

  9. #9
    Senior Member VintageExcellen's Avatar
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    Quote Originally Posted by chris belyea View Post
    Frederik - I just PM you.

    All - Thanks for the advice. I did check the timing before I started messing with the enrichment but I'm going to run through things again this weekend. I'm going to especially run through the linkage. I may have something too tight or something. It doesn't seem as smooth as it should be.

    Chris
    It should be butter smooth, and from wide open throttle when you release it should jump right back to all the way closed. Lots of items can hang up here. I recommend replacing all linkage if the balls are sloppy from wear as most originals are by now.

    You don’t have to run a part load air/fuel test until you can get it to idle and rev and airflow is set. Once you get it to behave then you can start playing with load settings. I have the Bosch CO machine that hangs off the back of a 911 but really these days I can tune just as easily by ear/feel and driving around moving the rack one or two clicks at a time.

    If the car was running properly before then it is usually some dumb little setting and that’s why you always go back to basics and step one.

  10. #10
    I wanted to post an update. So I've made all the adjustments that I can to the throttle linkages. I did find a few things that I thought may be causing a partial throttle. Those are now taken care of.

    So this morning, I fired the car up, with the idle stop screw backed all the way out and no hand throttle. After about 5 minutes, the car would idle on it's own. I took it for a drive and got it up to temp and the car idle steady at 1000 rpms. I was happy. About 25 minutes into the drive the car idle speed ran back up and started to hunt. I disconnected the micro switch and the car idled steady at 1600 rpms. I cannot get it to come down. I believe I have no more adjustment in the idle speed adjustment on the rack. I believe my problem shows up after I've driven for a while so it seems heat related. It got me thinking that could the thermostat have something to do with it?

    Thanks

    Chris

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