I was cleaning the throttle valve assembly from my '75 450SLC today and noticed that there was a lock-nutted adjusting screw that limits how closed the throttle place can go. Mine was set to a sliver, i.e. less than 1mm. I've searched my manual but find no mention of how to adjust this screw. I'm thinking that a perfect seal wouldn't allow the engine to run at all.
Right now my car has high idle and the large slotted idle adjustment screw can't bring it down enough. Wondering if I need to tweak the idle speed stop. Which isn't fun as it seems to require removal of the throttle valve to do so. EDIT: And now I've found another thread here (re a 280SL) that says to never monkey with the idle stop adjustment. OK. I won't.
You shouldn't have to adjust that adjusting screw. However, the throttle position switch should be adjusted to that there is a 0.4mm clearance when the throttle plate is fully closed. Make sure the linkage is not preventing throttle from going fully closed. Section 07.4-300 of engine manual does cover adjustment of the linkages.
High rpm at idle usually means an air leak somewhere. AAV is usual but not the only source. Get engine fully warmed up and set idle using idle adjustment screw at say 700-800rpm. Then squeeze the el shaped tube that goes from AAV to idle valve. If you can't squeeze it, remove one end and block the idle valve side with your thumb. If idle speed drops, then your AAV is leaking by. There are complete threads elsewhere on this. But basically first option is to try cleaning and check it while out of car. It should be almost 100% closed at 150F. I used hot air gun to cycle mine. New AAV is expensive. Some of dismantled them with mediocre results!
Other sources of leaks we can discuss later. Just make sure all vacuum hoses are connected.
I had the AAV off the car recently to test it in some hot water and it works as it should. I also had the TPS off the car, cleaned it, and verified it was working with a VOM. Reinstalled so idle position closes the appropriate contacts.
Idle on this D-Jet is still high.. probably 1500+. My tach is still whacked after my Pertronix install so I can't tell exactly what RPMs are. Need to buy a vintage 'engine analyzer meter' from ebay. That or an MSD ignition box with a proper tach out signal as another BW'er recently did.
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