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Thread: MFI tuning: space cam adjustment?

  1. #1
    Senior Member frederik's Avatar
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    MFI tuning: space cam adjustment?

    Hi everyone,

    As I've detailed here, I acquired a new to me 2.2S in the beginning of this year. The engine ran reasonably well, but seemed to lack the top-end sparkle that the 2.2S should have, so I brought it to a mechanic I trust (who rebuilt the engine in my 2.4T Targa with MFI a few years ago) in the beginning of May.

    He drove around with an AFR meter in the exhaust and needed to adjust the MFI pump, but it turned out it didn't respond to any adjustment. So the pump went out and was checked by a specialist in the Netherlands. The centrifugal governor was stuck and didn't work at all, so the pump was properly fixed and verified according to specification.

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    Unfortunately, when installed back in the car, the engine still doesn't run properly. It now makes good power at full throttle, but is impossible to set up correctly at partial load, running too rich and it's not possible to adjust it without making it run poorly elsewhere. My mechanic has painstakingly gone through everything in CMA and everything is as it should be. The pump rod at 114 mm, he checked all the angles with protractor rods, and of course valve timing, ignition, thermostat, leakdown test, he even verified the cams have the S profile by measuring the valve depression while rotating the engine. The pump has been checked again.

    Everything on the engine looks stock including the intake and the exhaust but it has been rebuilt around 1998 and we don't know 100% sure what's in there unless we take it apart. I've been able to contact the person who rebuilt it back then and he says everything is fully stock.

    According to the MFI specialist the problem is that at partial load, say doing a steady 50 or 60 mph, you should be at around 12 degrees at the pump lever, but in my car it's at 20 degrees instead, so the pump is enriching more than it should. Somehow the engine works differently than in 1970, but we don't know why -- he kind of shrugged and said it might have a different exhaust, and fuel today is different than 50 years ago, etc...

    Since everything else on the engine has been checked, the only solution he sees is to modify the space cam to make that area leaner so it starts to work well with the engine. Unfortunately, although he has lots of experience with MFI (Bosch, Kugelfischer, Spica, etc) and has modified Kugelfischer space cams, he has never modified a Bosch space cam before so it will be a first for him, and he doesn't think it will be ready before Spring 2024. The process he envisions is to take out the engine, put it on a dyno (which he owns), map it completely with the current space cam to find out exactly what modifications are needed, and CNC mill a new space cam based on that.

    My questions for the MFI wizards around here...

    - Is it at all common for a "stock" 2.2S engine to require this kind of space cam modification?

    - Is there anything we might have missed to check/verify on the engine? I keep thinking that might be something simpler and that modifying the space cam seems too much of a "last resort" solution. Does the problem I described above sound familiar to anyone? Anything we can try?

    Thanks in advance!
    1970 2.2S Elfenbeinweiss
    1972 2.4T Targa Aubergine (MFI) [For sale]
    2002 996 TT Midnight Blue
    Member #3833

  2. #2
    Righteous Indignation 70SATMan's Avatar
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    Injector spray patterns??
    Michael
    “Electricity is really just organized lightning”

    -Dusty 70S Coupe
    -S Registry #586

  3. #3
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    Does the car have the original CDI box and original black Bosch coil?
    1. Chris-Early S Registry#205
    2. '70 911S Tangerine
    3. '68 911L Euro Ossi Blue

  4. #4
    Interesting…on my car I had to play with the black screw inside the pump to have a good behavior at partial load below +/- 4000 rpm.
    How about your idle ?

  5. #5
    I'd contact Mark Jung on this board and ask him. I question the pump rebuild having so much throttle angle.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  6. #6
    Senior Member frederik's Avatar
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    Thanks for the responses everyone!

    70SATMan -- Injectors have been checked, one replaced so they're all good now.

    raspy -- Ignition has been on my mind. Engine has a black coil and original CDI but we don't know if and by whom it has been rebuilt. We're going to swap the CDI with a known rebuilt one and do one more test.

    SFE -- The pump needed to be set a few clicks richer from its "factory" setting to get the idle and the full throttle to be right (rich enough), but that caused partial load to be way too rich (lambda 0.8). Not sure if you can tweak the curve in this way inside the pump.

    edmayo -- Would be great if Mark could chime in. I just called my mechanic and he agreed that altering the space cam doesn't seem right at this point. Our current plan is to check with a different CDI; if that doesn't help we're going to send the pump to Eisenbrandt in Germany for a second opinion.
    1970 2.2S Elfenbeinweiss
    1972 2.4T Targa Aubergine (MFI) [For sale]
    2002 996 TT Midnight Blue
    Member #3833

  7. #7
    Senior Member 2.5MFI's Avatar
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    I hope I can help you on this problem. Although it looks like your mechanic and MFI specialist have covered most of the engine and CMA trouble areas. I suspect the engine camshaft timing was also checked.

    I agree with Ed on the throttle angle. Why the cruising throttle is at 20 degrees instead of the 10 to 12 degree area as it should be. So if the space cam has an over rich area from around 12 degree to 20 degrees at cruising speed @ 3000rpm, you would need the 20 degree throttle to add more air to the mixture. So somethings not right as you know.

    I have dealt with this problem only a few times where I had to re-mapp the space cam by richening up the Top End and Low End area of the space cam so the Mid and Cruising area stay the same, then re-adjust the main rack to lower the fuel flow so you end up with a leaner Mid and Cruising area. Lot of work to do this but better than making a new space cam.

    No, It’s not a common problem with the 2.2S space cam. It was a problem with the early 2.0S space cam.
    - Is it at all common for a "stock" 2.2S engine to require this kind of space cam modification?

    Try another 2.2S space cam? Maybe a worn area on your space cam in the cruising area that would cause a rich AFR. But I’m sure your MFI specialist has looked at this already?
    I wish I had a good answer for you.
    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

  8. #8
    Senior Member 62S-R-S's Avatar
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    Never heard of anyone attempt... with mfi off the car, and webers in their place, that would isolate the fuel delivery as a test comparison for drivability and stumbling. Of course, not everyone has the parts sitting on the shelf for a swap like that. Something fun to try.

  9. #9
    Senior Member frederik's Avatar
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    Hi Mark,

    Thanks for your thoughtful reply. Interesting that a too rich partial load area may lead to the too high angle on the pump when cruising, I hadn’t considered this. It would certainly explain the relationship between the two. According to the MFI specialist, the space cam isn’t worn. And the pump delivers fuel according to spec at the prescribed angle / RPM combinations so it should work in theory.

    I hadn’t considered that you could enrich the idle and top end areas by filing down the cam and adjusting the entire rack, that is definitely easier than making a new space cam. Did you ever do this on a 2.2S or only 2.0S?

    Unfortunately I don’t think we have easy access to a known good 2.2 space cam.

    I’ll ask my mechanic to double-check cam timing via one of the valves. One way or another, I’m sure we’ll eventually get to the solution!

    @62S-R-S: I think the idea would be to map the engine AFR using the existing space cam, then work out the difference that is needed and apply that to a new cam. (Or file down the existing cam.)
    Last edited by frederik; 07-28-2023 at 02:04 PM.
    1970 2.2S Elfenbeinweiss
    1972 2.4T Targa Aubergine (MFI) [For sale]
    2002 996 TT Midnight Blue
    Member #3833

  10. #10
    Senior Member
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    definitely check your CDI - i chased MFI issues for a while only to find my CDI needed to be replaced!
    Bill

    Early 911S Registry Member #4087
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    '72T hotrod 210 0228
    '82SC Targa
    '97C4S (sold - and regretting it)

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