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Thread: PMO carb tune + hesitation at 4k RPM

  1. #1
    Senior Member zenithblue71T's Avatar
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    PMO carb tune + hesitation at 4k RPM

    hi all,

    My 1971 911T has a freshly rebuilt 2.2S with solex style hot cams, 40MM PMO, stock Bosch CDI, Bosch dizzy, Pertronix Ignitor, SSI heat exchangers and and stock Dansk exhaust.

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    You can see my PMO settings in the attached image:

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    My latest dyno run is here from Turbohoses:

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    I have two problems I need to tune out:
    1. It's running lean.
    2. I have an annoying hesitation and stumble at 4,000-4,300 RPM I want to tune out. I can accelerate through it no problem but can't hold it say when I'm going 85mph in 5th gear. It stumbles at 4-4300 RPM, especially when warm.

    But first, I want to address the motor running lean and popping so it's been suggested I swap in:
    1. 1.30 main jets
    2. 1.70 air correction jets.


    I bought them from CB Performance Products.

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    My first question is a dumb one, I have the new parts and can't figure out in the diagram where to install new main jets and new air correction jet.

    Weber diagram as I couldn't find PMO exploded diagram:
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    Any help would be appreciated.
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  2. #2
    Forget the Weber diagram,,,,,PMO's are based on IDF design, not IDA. Right next to your air horns is a jet stack, unscrew that and all your jets except idle are in that stack. Get a Weber manual that covers the IDF series. Probably get one from PMO or Pierce Manifolds right there in CA.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  3. #3
    Senior Member zenithblue71T's Avatar
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    Thanks Ed!

    Found the manual on eBay


    This Weber Tuning Manual comes with a 12,000 mile, 12 month warranty. ;p
    Last edited by zenithblue71T; 06-21-2019 at 03:30 PM.
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  4. #4
    James I have the identical engine in my car and use the below jetting. It runs perfectly in all conditions and makes great power. No popping or flat spots. Really happy with the setup.

    Venturi - 32mm
    Idle - 55
    Main - 140
    A/C - 180
    Emulsion – F26

    The carb guru who rebuilt my 40IDA's said that I could use 130 mains if I switched to F3 tubes.

  5. #5
    James I should note that I'm running Weber 40IDA's, not PMO's. No idea if this is an apples-to-apples deal when it comes to jetting.

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    subscribed

  7. #7
    Senior Member zenithblue71T's Avatar
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    From Joe:

    Hi 71T, I'm not new here but have a new account so I can't post just yet.
    I just wanted to tell you I had the same problem in my 73T 2.5 with 40 PMO's.
    Everyone tried to figure it out but Jeff at Aase sales after a few days of trouble shooting fixed it.
    He found 3 cracked floats.

    I'm not a mechanic just thought I'd share.
    He thinks I could have been from Arco gas not tear one.
    Hope this helps,

    Good luck.
    joe
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  8. #8
    Senior Member zenithblue71T's Avatar
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    from my engine builder John Holleran:

    I put in a smaller main jet. Went from 125 to a set of 120's I found an issue with the plug wires so I put a new set of them on. I'll have it wrapped up today. I do have to discuss one drivability issue that is just inherent with all carbs on 911 engines. It has to do with a 200 rpm window above 4k rpm. I have dealt with this anomaly many times before. It is a very difficult thing to tune out. It has to do with what is called fuel reversion. Its where fuel at a certain rpm blows back out the intake path. It's a situation that isn't fixable and it just has to be driven through. If you accelerate through the 4 to 5 k range you never feel it but if you stay at the 4100 to 4300 its noticeable. But better than before by a bunch.


    hmm, the diagram says 130 so now I guess my main jets are 1.20, maybe that's why it's running lean.
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  9. #9
    Senior Member zenithblue71T's Avatar
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    Quote Originally Posted by Noah View Post
    James I have the identical engine in my car and use the below jetting. It runs perfectly in all conditions and makes great power. No popping or flat spots. Really happy with the setup.

    Venturi - 32mm
    Idle - 55
    Main - 140
    A/C - 180
    Emulsion – F26

    The carb guru who rebuilt my 40IDA's said that I could use 130 mains if I switched to F3 tubes.
    Noah, very helpful. What cams are you running? I have Solex style cams and single plug.
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  10. #10
    Quote Originally Posted by zenithblue71T View Post
    Noah, very helpful. What cams are you running? I have Solex style cams and single plug.
    I'm using the Web-cam 104/102's, which as I understand it are Solex cams on a 911E (102 degree) lobe separation angle. Very nice street cam -- great midrange and top end power. Although I've compared lift and duration specs and they're closer to early S cams on a 102 LSA.

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