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Thread: PMO carb tune + hesitation at 4k RPM

  1. #11
    Quote Originally Posted by zenithblue71T View Post
    from my engine builder John Holleran:

    I put in a smaller main jet. Went from 125 to a set of 120's
    I am hardly a carburetor expert, and maybe things are different for PMO's than Webers, but 120 mains may be too small on a 2.2. The 2.0 911S engine used 130 mains.

  2. #12
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    Are you using the modified accelerator cams? I think all PMO's come with them now as standard. I had a similar issue with my 46mm PMOs (3.2ss engine). I increased the idle jets from 55 to 60 and changed accelerator cams to the new profile. Its still not perfect, and I still have a minor issue on the transition from idle to accelerator jets, but is an improvement.

    Paul

  3. #13
    Senior Member zenithblue71T's Avatar
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    Hmmm, I bought the PMO 40s new last year. I will increase mains, then look at E Tubes. Can you detail what you mean by changed accelerator cams?
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  4. #14
    Senior Member zenithblue71T's Avatar
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    Ok so I changed the main jets from 120 to 130 and airs from 180 to a richer 170. The hesitation might now be worse with an occasional bucking under load in a steep incline. I checked the throats with my sync tool and all 6 throats measured equal at idle and 3k RPM.

    Any ideas on how to tune out this pesky 4K RPM hesitation?
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  5. #15
    Find someone with a local Dyno maybe?. Take a few sets of different size Jets and venturies.

  6. #16
    Emulsion tubes have a lot to do with tuning and should be considered.

  7. #17
    Senior Member zenithblue71T's Avatar
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    I found this on the PMO tech bulletin page. Looks like this is my next setup.

    "The latest feedback on jetting 40mm PMO Carbs on smaller street engines (2.0 – 2.4) is as follows:
    32 venturies,
    125 main jets
    180 air correctors
    60 idles with
    130 idle airs
    F11 emulsion tubes

    We’ve also closed off the fourth transition port. The fourth transition port (the top one) would often cause an overly rich condition on cruise because it overlapped the main circuit at the light throttle position. We are offering a free plug kit to close the fourth transition ports on 40mm PMO Carbs and a free jet and venturi exchange. The latest production run of 40mm PMO Carbs (2010) have three transition ports as the modification works on larger engines, also. A lot of the new information has been from shops using the new chassis dyno’s that give air/fuel in real time. The portable units, like the AFX Powerdex carried by NGK, have also become much more sophisticated and are useful for fine tuning under actual driving conditions."
    Last edited by zenithblue71T; 08-23-2019 at 01:12 PM.
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  8. #18
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    Quote Originally Posted by zenithblue71T View Post
    Hmmm, I bought the PMO 40s new last year. I will increase mains, then look at E Tubes. Can you detail what you mean by changed accelerator cams?
    Here's a link to a picture of an accelerator cam. PMO changed the shape slightly a few years ago to improve the transition from idle to accelerator jet. However, as you bought yours new recently, this is unlikely to be your issue.
    https://www.partsklassik.com/p-3239-...weber-ida.aspx

  9. #19
    Senior Member zenithblue71T's Avatar
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    Correct, I’m going to switch carb parts to get her inline with PMO tech bulletin then check timing. I have a pesky 120 jet stuck in the bottom. The f11 pops out jet less. I see the main jet in there though. Any thoughts on efficient removal? Likely have to tear carb down another level.
    James Hurst #1638
    R Gruppe #514
    911T with 2.2S Ps and Cs, Solex grind Cams and PMO 40mm

  10. #20
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    A little grease on the end of a q-tip or the jet stack should retrieve that jet. You can spread them a bit when you put the stack back together to keep them from coming apart in the carb.

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