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Thread: 73/3 MFI preheater flapper contraption

  1. #11
    Senior Member NorthernThrux's Avatar
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    Quote Originally Posted by frederik View Post
    I’m missing mine and am looking for one. I did wonder about the noise difference but Boulder Bob just answered that question for me. Like my mechanic said: everyone removed those in the past but somehow they’re very difficult to find nowadays.
    There are at least a dozen on ebay at any time, including now (search Porsche MFI air regulator), usually needing reconditioning and usually $500-$750. I bought a perfect one that had been removed early in a ‘73 car’s life and it was about a grand as I recall. Still went through it and lubed it and checked the thermostat.
    Early 911S Registry # 2395
    1973 Porsche 911S in ivory white 5sp MT
    2015 Porsche Macan S in agate grey 7sp PDK

  2. #12
    Senior Member frederik's Avatar
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    Thanks for the tip!
    1970 2.2S Elfenbeinweiss
    1972 2.4T Targa Aubergine (MFI) [For sale]
    2002 996 TT Midnight Blue
    Member #3833

  3. #13
    @Northernthrux:

    I have a 1972 2.4 T Targa which didn’t run properly, too rich. Turned out the micro switch of the MFI was was broken. When changing it, I had a look at the air regulator and I have the impression the flap is not working properly. When I open the throttle completely by hand while the engine is not running, the flap only opens halfway. Shouldn’t it completely go down and completely block the intake of heated air, or does it mix the hot & cold air by closing only halfway…?

  4. #14
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    I'm in the process of making this system functional on my 72, in order to get the motor warmer here in the midwest. Motor runs great (rich, yes) but takes forever to get up to 180 degrees. Have brand new correct SSI heat exchangers to replace the non-MFI exchanger on the car. I'm heartened to hear that a functioning flapper does help cold running. Here are the relevant pages from the factory workshop manuals.
    Attached Images Attached Images    
    Last edited by 72targa; 09-14-2023 at 01:39 AM. Reason: Photo rotation
    Mike O'Meara
    Reg #1043
    72 911T
    56,59 356s gone
    67,68 912s gone

  5. #15
    Quote Originally Posted by EsGeht View Post
    I'm in the process of making this system functional on my 72, in order to get the motor warmer here in the midwest. Motor runs great (rich, yes) but takes forever to get up to 180 degrees. Have brand new correct SSI heat exchangers to replace the non-MFI exchanger on the car. I'm heartened to hear that a functioning flapper does help cold running. Here are the relevant pages from the factory workshop manuals.
    Hi there, Esgeht,

    Thanks for your post, it explaines a lot! I wasnÂ’t aware of the second, thermostat actuated flap since I didnÂ’t completele remove the housing (could have guessed though..). Reading your post my take on it all is that the fresh air flap isnÂ’t all that important since the actual airmix is primarely done by the flap actuated by the thermostat; only full throttle closes this airstream with the fresh air flap. This is operated by the outside lever that seems a bit of a flimsy construction for Porsche standards in my opinion. On my car it might be a bit worn out: even on full throttle the fresh air flap wonÂ’t completely close of the airstream mixed by the thermostat (only halfway); looking at the description in your post, my guess is the effect of this will be very limited, especially with an engine at working temp when the thermostat is feeding cold air already anyway.

    My car running rich (and backfiring & fuelfarting) must have been caused by the defective microswith, pouring in too much fuel when coasting and/or going downhill with throttle unused.

    I hopeÂ….

    Thanks again!

  6. #16
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    A properly functioning flapper is a new experience for me, too. I was missing the adjustable roller seen in the pics below. I had to buy a new one (ouch). Once installed on the flapper box and then the box attached to the motor, it became clear how the roller rests on the cam of the throttle linkage. My flapper box is in very good condition, and operates smoothly. I used a heat gun on the thermostat and it closes off the hot air flow as designed. I can see how a worn roller and/or worn arm it fits in could lead to incorrect operation, as will a malfunctioning thermostat. I haven't installed the correct heat exchangers yet, so I don't know how well it actually works. Good luck, hope this helps.
    Attached Images Attached Images  
    Mike O'Meara
    Reg #1043
    72 911T
    56,59 356s gone
    67,68 912s gone

  7. #17
    Quote Originally Posted by EsGeht View Post
    A properly functioning flapper is a new experience for me, too. I was missing the adjustable roller seen in the pics below. I had to buy a new one (ouch). Once installed on the flapper box and then the box attached to the motor, it became clear how the roller rests on the cam of the throttle linkage. My flapper box is in very good condition, and operates smoothly. I used a heat gun on the thermostat and it closes off the hot air flow as designed. I can see how a worn roller and/or worn arm it fits in could lead to incorrect operation, as will a malfunctioning thermostat. I haven't installed the correct heat exchangers yet, so I don't know how well it actually works. Good luck, hope this helps.
    Hi Esgeht,

    The roller arm tends to wobble when screwed in to (almost) the max. The slight wobble makes the difference of the flap closing completely or only halfway. Also, the plastic hinging pin on the right in your picture seems to be in an oversize hole. This is also the case in my flapper box and I thought this was wear & tear, but it appears to be an intentional German conceived engineering issue. I therefore blindly assume it serves a purpose unknown to me.

    Never occurred to me to check the working of the thermostat and the attached flap with a heat gun. Thanks for the tip!

    P.s.: would you perhaps have a factory manual explanation of working of the airfilyer housing? At the bottom there is a water drain outlet that is connected by a hose to a small pipe going through the car body with a rubber closure, that much is clear. But there is also a T-shaped outlet at the back. Going to the right it goes through a brass mesh into a hose that probably serves as an oiltank breather, but IÂ’m puzzled by the small outlet to the left. What is itÂ’s function? Does it have a rubber plug (I donÂ’t have it) and does this plug have a smaller opening? That would mean an open connection to the world bypassing the airfilter straight into the engineÂ…. Seems odd to me.
    Last edited by JoGo; 09-13-2023 at 10:01 AM.

  8. #18
    member #1515
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    These were put on to comply with emission standards 72/73 most cars had a version of this. Idea was to quickly add warm air to combustion chamber to lower emissions.
    Compression dropped to 8.5 from 9.8 for the same reason, increased displacement to make up for loss of compression and HP.
    David

    '73 S Targa #0830 2.7 MFI rebuilt to RS specs

  9. #19
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    JoGo one line from the charcoal canister in the trunk attaches to that "T". If I understand correctly, it is the return line. The supply line (again, no expert here) I believe attaches to the engine shroud. So no, that "T" is not open, it receives vapors from the canister. here are diagrams from the '72-'73 parts book.
    Attached Images Attached Images    
    Mike O'Meara
    Reg #1043
    72 911T
    56,59 356s gone
    67,68 912s gone

  10. #20
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    JoGo, Also if the canister is eliminated or missing, that "T" would then be capped. If I'm wrong, experts (Ed) please correct!
    Mike O'Meara
    Reg #1043
    72 911T
    56,59 356s gone
    67,68 912s gone

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