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#1
08-25-2007, 04:25 PM
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Another rough-idling W126 - '89 500SE
Apologies for being a newbie with an olbie problem, especially one that's been discussed many times previously. I have read many of the relevant posts, and have been taking away as much of the wisdom as I can.
I picked up this car a few months ago, and it's become clear that it's been worked on by gorillas in the past. The more I search, the more tacky and badly executed work I find. I've fixed many things already, each of which has improved the operation of the car. But so far the weird idling - high at about 1,000 rpm in N or P, and going [bomph] every few seconds - hasn't responded to my threats or labour. I'm trying to avoid the FD replacement till I'm sure, though. My question is peripherally related to this: I'd like to know the actual detailed operation of the idling mixture adjustment screw, which in my case appears to have been tweaked waay out of position. As I have no manuals on the car yet, the following is a slightly fanciful diagram I put together of the mass-flow device as far as I can work it out. The details of how the arm and plate actually change the potentiometer probably don't matter yet, but I'd like to know what the screw actually does.  From what I've read and seen, it sounds as though the screw actually contacts the arm, and limits its upwards travel, as shown. Is this the case? That doesn't happen in this car. With the engine off and the plate & arm as far upward as it can go, the screw is still at least 5mm from touching the arm. If I push down on the screw against its spring, I can just get it to touch the arm. Is this a red herring? Is the actual screw operation not related to holding the arm down? Does it control a jet or something? Furthermore, the bolt in the centre of the plate, at the top of the arm's travel, is about 10mm from the rubber buffer on the crossbar. I assume that they wouldn't normally touch, but the distance seems excessive. I guess my dream scenario would be to get a longitudinal cross-section drawing of one of these devices, but a description would do just fine. Any info most gratefully appreciated. CD 
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