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Thread: Idle Mixture Adjustment

  1. #1
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    Idle Mixture Adjustment

    Hi all

    I have a Kjet technical question which I hope I can ask as clearly as I can. When the Delorean is at idle I understand that the idle motor controls the amount of air that the engine can draw. The amount of fuel is regulated by the position of the plunger which can be adjusted with the CO 3m screw. What I am trying to work out is that on my car if I remove the CO plug bung my car will either start surging or die completely. My question is if the idle motor controls the volume of air at idle why would removing the CO bung have any bearing on my idle ? I guess a more technical question is to what extent is the air plate ďmeteringĒ air while the car is at idle with the idle motor operating? Could it be my throttle plates or brass screws are not closed so removing the bung adds more air at idle?

    Hopefully this make sense

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    Actually I have been thinking about this more and I think I might have answered my own question but want to cross check. Is it the case the while at idle the air plate is not metering any air but it is metering fuel. So if I remove the bung then some air that is being metered by the idle motor is not flowing though the air plate (because itís flowing through the open mixture screw hole) then the air plate will rise up slightly and restrict the fuel flow (therefore causing a lean condition and in my case killing the engine).

    Am I right here?

  3. #3
    Administrator Ron's Avatar
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    Quote Originally Posted by Jonty View Post
    Hi all

    I have a Kjet technical question which I hope I can ask as clearly as I can. When the Delorean is at idle I understand that the idle motor controls the amount of air that the engine can draw. The amount of fuel is regulated by the position of the plunger which can be adjusted with the CO 3m screw. What I am trying to work out is that on my car if I remove the CO plug bung my car will either start surging or die completely. My question is if the idle motor controls the volume of air at idle why would removing the CO bung have any bearing on my idle ? I guess a more technical question is to what extent is the air plate “metering” air while the car is at idle with the idle motor operating? Could it be my throttle plates or brass screws are not closed so removing the bung adds more air at idle?

    Hopefully this make sense
    Removing the plug allows air to enter the system without it being 'measured' (by the plate). This is why you must plug the hole and allow the engine to run a few seconds when taking a CO/dwell reading.
    It is odd that removing the plug makes the engine die. The brass screws may have something to do with that...? Anyway, the brass screws are supposed to be closed on a K-Jet system that utilizes the Idle Air Control system (like a DeLorean).

  4. #4
    Administrator Ron's Avatar
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    Quote Originally Posted by Jonty View Post
    Actually I have been thinking about this more and I think I might have answered my own question but want to cross check. Is it the case the while at idle the air plate is not metering any air but it is metering fuel. So if I remove the bung then some air that is being metered by the idle motor is not flowing though the air plate (because it’s flowing through the open mixture screw hole) then the air plate will rise up slightly and restrict the fuel flow (therefore causing a lean condition and in my case killing the engine).

    Am I right here?
    Yes.
    (FWIW- I never liked the use of "air metering plate" here, often used for some reason. As a verb, "meter" = "measure", especially for the British...The manual calls it an "air sensor plate", which "regulates" fuel ;-)

    Air drawn into the engine forces the plate down, which causes the fuel distributor (FD) plunger to raise, increasing fuel flow. Meanwhile, the control pressure (from the CPR/WUR) forces the FD plunger down. The result of these two counteracting forces against the plunger (and the lambda system) determine the fuel flow, idling or not.
    At idle, the idle speed ECU rotates the idle speed motor to obtain the correct RPM. The fuel system will react to any change in air it, or,anything else, makes. (Try viewing it as vicious circle where desired results are achieved by setting boundaries and only reacting when one is crossed ;-)
    The amount of air entering the system through the adjustment hole would throw things off; but I seriously doubt that alone could cause the engine to die.

    Shut the brass screws!
    Re-set the timing.
    Check the CO/dwell (ALWAYS done last!)
    If no luck, check pressures.

  5. #5
    Senior Member Bitsyncmaster's Avatar
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    Removing or plugging the adjustment hole will make a small change to the idling engine but the working idle system should quickly compensate. That is if everything is adjusted properly. Your mixture adjustment may be far off and a cold engine is not adjusting the mixture dynamically via dwell changing.

    1) Is your idle RPM holding at 775 RPM?
    2) Is your frequency valve buzzing? It should buzz with a cold or hot engine.

    You only want to adjust your mixture on a warmed up engine. That is when the lambda system is working to hold the mixture at 1.00 lambda (14.7 AFR).
    Dave M vin 03572
    http://dm-eng.weebly.com/

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    Quote Originally Posted by Bitsyncmaster View Post
    Removing or plugging the adjustment hole will make a small change to the idling engine but the working idle system should quickly compensate. That is if everything is adjusted properly. Your mixture adjustment may be far off and a cold engine is not adjusting the mixture dynamically via dwell changing.

    1) Is your idle RPM holding at 775 RPM?
    2) Is your frequency valve buzzing? It should buzz with a cold or hot engine.

    You only want to adjust your mixture on a warmed up engine. That is when the lambda system is working to hold the mixture at 1.00 lambda (14.7 AFR).
    Not only must the engine be warmed up, every other adjustment MUST have been done (and done correctly) and any worn, old, tired parts should have been replaced. The mixture adjustment is only then able to be set correctly. Never try to adjust away a problem like vacuum leaks, worn spark plugs, dirty fuel injectors etc. A tiny adjustment has great effect and if you don't know what you are doing it is probably better to leave the mixture screw alone, it's probably closer to correct then you will be turning it to. Especially if the tamper proof plug is still there.
    David Teitelbaum

  7. #7
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    Thanks all

    To answer your questions my idle is 775 and is very stable (open loop at least). I always measure on a warmed up engine. My FV is buzzing away and working fine. I have found since day one whenever I remove the mixture bung the engine dies. I had the system tuned with an gas analyser so I was fairly sure the mixture was right. The off thing is that the dwell swings from 45 down to below 20 when in closed loop. I made a very rich adjustment and it simply raises the idle to 1000 and no impact on dwell. All a little odd. It has been fully smoke tested and no leaks. The WUR, idle motor, distributor and fuel pump are all no more than 5 years old.

  8. #8
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Jonty View Post
    Thanks all

    The off thing is that the dwell swings from 45 down to below 20 when in closed loop. I made a very rich adjustment and it simply raises the idle to 1000 and no impact on dwell.
    That is swinging to much if your seeing it swing 45 to 20 up and down. It should be half that or less. Have you replaced the O2 sensor? A very small adjustment normally makes a large dwell change. I have also seen problems setting dwell if the exhaust is restricted (plugged CAT).
    Dave M vin 03572
    http://dm-eng.weebly.com/

  9. #9
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    Yes itís a fairly new O2 sensor. All a bit frustrating. Iím starting to wonder if it might be something like timing or a valve adjustment.

  10. #10
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Jonty View Post
    Yes itís a fairly new O2 sensor. All a bit frustrating. Iím starting to wonder if it might be something like timing or a valve adjustment.
    I don't think timing or valve adjustment would stop the ability to adjust dwell.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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