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Mixture Control 1800 1970

posted by  Dave B on  Mon Apr 7 04:20 CST 2003
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last visit: Fri Sep 26 19:07 CST 2014
[RELATED]

I almost have it. Previous owner installed a big bore kit and cam and the car simply would not run with the 1970 Brain Box. Seredipity led me to try a 72 brain box and things got under control. Fuel pressure stays at The manifold pressure senseor also had to be cranked way rich (a whole half turn) and now the car runs beautifuly and gets aceptable 20-25 gas mileage). Except....

It continues to idle rich (which is something I'm definitely not!) This becomes a problem in stop and go traffic when the plugs start to load up, the idle drops and becomes lumpy.

I've thought about tandem pressure sensors (I have a spare) and setting one to factory mixture, switching automatically to that one at idle. But that's complicated and inelegant. I know there is a circuit in the brain box dedicated to idle mixture and I've played with that. My situation seems to have overwhelmed its capacity to compensate. Could I add some resistance to that, or substitute a different rheostat? What about that thing IPD used to sell. Would that help me out? Am I missing something simple, like timing?

Appreciate comments from people knowledgeable of D-Jet electronics.








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    Mixture Control 1800 1970

    posted by  Dave B on  Thu Apr 24 10:33 CST 2003
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    last visit: Fri Sep 26 19:07 CST 2014
    [RELATED]

    http://members.rennlist.com/pbanders/index.html

    I visited this site, which was recommended above, and corresponded with the guy who put it together. He must have a brain the size of planet. What an incredible analysis. Anyway, it seems I had the wrong pressure sensor (going by the Bosch P/N applications he provided) but, Voila, a spare I'd picked up at a junk yeard had the right P/N. So I fitted that un-tampered-with unit and, per his advice, cranked up the fuel pressure (to 34 lbs). Car now runs much more smoothly. it sitll lugs a bit at mid range, but I think that is attributable to the cam...again, as he suggests.

    Some further tinkering with timing, valve adjustment and the throttle valve switch should help diminish this minor problem to a non problem. One thing I won't attempt again is to "adjust" the MPS. I thought it was turn one screw...but it's much more complex than that...they don't put that impossible epoxy on there for nothing.

    For anyone with a non-standead engine configuration and D-Jet, I'd highly recommend this site. I purchased two books on Bosch Fuel Injection Performance mods (the one from IPD and another I found) and both basically dismissed the subject of D-Jet, saying you couldn't modify it. This guy knows whereof he speaks.








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    Mixture Control 1800 1970

    posted by  Viper1800e on  Mon Apr 7 13:45 CST 2003
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    Make sure the throttle switch is adjusted to go to 0 ohms when released. I have been reading alot on the Djetronic system.

    This site has very technical stuff on the system and adjusting, ECU, MPS etc.

    Porsche related but should help.


    http://members.rennlist.com/pbanders/index.html

    Keep me advised on your adjustment saga as I am learning to.

    Greg Bodner
    1970 1800E







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    Mixture Control 1800 1970

    uid 4846's pic
    posted by  JohnMc on  Mon Apr 7 10:04 CST 2003
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    last visit: Thu Oct 23 07:42 CST 2014
    [RELATED]

    My '70 E used to act quite funny around idle, and off idle. I finally just disconnected the vacuum retard and the cars is much more tractable for it. Before the engine would slowly pull down to a slow idle, then when you tried to get going again you'd have a delay with little engine power until the timing advanced again. Probably unrelated to your problem, however.

    Perhaps a better approach to getting the system to handle the extra 130 cc would be to increase the fuel pressure slightly, and leave the MAP sensor in the stock calibration.

    The big bore B20 in my PV was run for many years in a D-jetted 145E wagon. But we were using an IPD box (which I currently have on my 1800E) and didn't need to manipulate the fuel pressure.







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