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Thread: mechanical fuel injection

  1. #1

    mechanical fuel injection

    A couple of weeks ago one of our members advertised his restored 73e with Webers. There followed quite a bit of discussion as to the pros & cons of mfi versus webers with the consensus seeming to be that properly functioning mfi is about as good as it gets. I have a 71e with mfi and I just purchased a 70 914/6 w/ Webers. I think I have a problem with my mfi & before I turn it over to my mechanic I want to get your learned opinion.

    The Webers sound beautiful, the motor revs freely, and the 914 pulls strongly in each gear. In fact she reminds me of my Boxster S.

    The 71e, on the other hand, seems to labor, doesn't sound quite on "full song" and seems to stutter quite a bit under 3000 rpms. When I purchased her last summer the motor had been totally rebuilt, the ppi that was done showed very strong numbers in all cylinders & I guess I expected a really strong motor.

    I'm again looking for your educated opinions on what direction I should take. As you might have guessed, I am not that mechanically inclined, although I'm learning more each day.

    Thank you.

  2. #2
    You want to make sure everything is in perfect tune before you start messing with the MFI (or assuming it is your problem). Start by reading "Check, measure, and adjust". The technical info. page on this site links to this MFI topics page with this document.

    Your problem may be as simple as point gap / dwell, timing, plugs, etc.

  3. #3
    Congrats on getting your /6! Before long you'll be looking for ways to get more torque and power out of that 2.0. The chassis, brakes and gearbox will handle a lot more of both than the 2 liter can give.

    1972 911S- viper green
    1970 914/6- tangerine
    1967 Ferrari 330GTC- oro chiaro

  4. #4
    Jared Rundell - Registered User JCR's Avatar
    Join Date
    Dec 2002
    Birmingham, MI
    What kind of exhaust/muffler are you running on the MFI car?

    Changes in backpressure from aftermarket exhausts can have the effect you describe on MFI motors.
    '73 911S #0793
    '69 912_ #0602
    Early S #0454
    RGruppe #0391

  5. #5
    Originally posted by JCR
    What kind of exhaust/muffler are you running on the MFI car?

    Changes in backpressure from aftermarket exhausts can have the effect you describe on MFI motors.
    I'm not going to doubt that statement except to say that a muffler, or lack of one, shouldn't have much of a effect on the performance at 3000 and less rpm.

    Recently, a friend of mine had a problem traced to the belt having slipped due to no teeth left. That would be my first check The rest gets pretty involved as one thing affects another. All the settings and adjustsments are interdependent. Makes for a good chess match.

  6. #6

    mechanical fuel injection

    Thanks guys for your input. I had a Dansk Sport muffler installed right after purchase. This cut the detonations by about 75%. My mechanic checked the plugs this December, found they were expensive platinums but described them as having "cold" settings if I remember correctly. He replaced them with regular Bosch plugs. Thanks again.

  7. #7
    My car has more bottom end with a stock muffler. It might lose a little on top but the driveablitly is better imho. It also allows me to maintain civil relations with my neighbors who already put up with my various motorcyles although none of them are in the league of a sport muffler.

    "Be who you are and say what you feel because those who mind don't matter and those who matter don't mind." - Dr. Seuss
    "Experience is the mother of wisdom" - idiom
    "Let them that don't want none, have memories of not gettin' any"- Brother Dave Gardner

    Early S Registry #235
    rgruppe #111

  8. #8
    If your 71E with MFI is running poorly and the engine is a fresh rebuild with strong/even compression as you state then you have tuning problem. I have been working on these cars since they were new in the early 70s and I can't tell you the nember of MFI systems that I have seen tossed in the trash to be replaced with Webers just because of incompetance/ impatience. Unlike a Weber one can not succeed in adjusting the MFI system by haphazard screw turning There are just to many variable in the MFI equation to hope to succeed with a random/guessing approach. But I have worked on cars that have had all sorts of indignities performed on their MFI:
    1) pumps out of timing spec is a very common one since it is tricky to line up the marks in a dark or dirty engine bay and some folks don't take the time to make a little mirror tool (I guess?). The timing does not have to be dead nuts but with in a 1/2 tooth is close enough.
    2) adjusting the butterfly stop set screws is another area that gets over looked or poorly done and will result in poor low to mid RPM running. If you hear a bunch of popping or "kitty sneezes" under light load you may very well have mismatched throttle plates. The #6 stop screw is very hard to get at due to the shroud masking it.
    3) Adjusting the linkage rods also takes patience to get just right so no loads are introduced into the linkages when the ball sockets are attached. The left and right main rods are critical in this adjustment.
    4) Having clean idle air bleed ports and screws is essential to get a nice idle. Your engine was rebuilt so hopefuly someone attended to this
    5) Mixture is pretty easy to set but gets really screwed up often by random twiddling. The idle mixture is straight forward. You don't need CO anylizer for the main circuit but it makes the task easier. A section of uphill road grade (prefferably deserted) can serve as your own personal dyno instead. Take your mixture adjusting tools along and do the main circuit adjustment on the shoulder between runs. It ain't rocket science!
    This note is not meant as a lesson in MFI adjustment; I'm just hitting a few high point that I have seen often. That info is available elsewhere (Lee Rice has done a nice summary in the tech section of this site)Think of this as a cautionary word. Your E should purr sweetly at idle and pull strong and clean all through the rev range. If it doesn't it is most likely an adjustment problem. These pumps very rarely ever go belly up. I have seem them with 300k miles still working perfectly. They do not like to sit unused for long periods and yours could (unlikely) need an overhaul. This is still a cheper and better solution than Webers. The 914/6 is different story since the MFI adjustment are next to impossible in that chassis.
    Webers are for the weak or impatient.
    Good Luck.

  9. #9
    Scott, I agree 100% with your statements. I've owned my 72 for 20 years and could'nt be more pleased with the MFI. The only repairs done to mine are rebuilt throttle bodies.

    Kevin 72 911T/E

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