Idle Mixture Screw - Which Direction to Lean at Idle

mjleamy · Post by **mjleamy** » Fri May 10, 2013 10:49 am

I want to lean out my mixture at idle - it is obviously rich. I know the Idle Mixture Screw will only make a small change. Right now the screw is all the way tight. Is that the rich setting, or the lean setting?

Thanks! Mike

Mike W. · Post by **Mike W.** » Fri May 10, 2013 10:04 pm

Tight is rich, it's an air bypass, so when it's screwed in it's not bypassing anything so it's rich. The bypass is bypassing the AFM flap, so it's unmetered air. This is the screw in the AFM, the one in the throttle body is only idle speed.

GripGreg · Post by **GripGreg** » Fri May 10, 2013 10:29 pm

Mike; Is this screw adjuster the one you can reach from the windshield side
of the throttle body with a long skinny screwdriver?
Thanx,,,,Greg

mjleamy · Post by **mjleamy** » Sat May 11, 2013 5:26 am

GripGreg wrote:Mike; Is this screw adjuster the one you can reach from the windshield side
of the throttle body with a long skinny screwdriver?
Thanx,,,,Greg

It's the windshield side. Thanks for the responses!

Mike W. · Post by **Mike W.** » Sat May 11, 2013 11:11 am

GripGreg wrote:Mike; Is this screw adjuster the one you can reach from the windshield side
of the throttle body with a long skinny screwdriver?
Thanx,,,,Greg

The windshield side is the idle speed control, not mixture. It's also a bypass but around the throttle butterfly, but affects only idle speed, not mixture. The screw to adjust mixture is in the AFM, pointing either up or down depending on year. It's usually a slotted screw, but occasionally a allen. I've heard some of them came with plugs over them to prevent adjustment, but I've never seen one.

mjleamy · Post by **mjleamy** » Sat May 11, 2013 12:18 pm

If you go the L-Jetronic seminar, slide 12, I am turning the screw located at '5'. This is said to be the "Idle mixture adjusting screw".

http://www.firstfives.org//faq/ljet/jetronic.pdf

Is this not the correct screw? It is indeed a slotted screw, but it is on the windshield side. Interestingly enough, this same slide 12 has location '3' labeled as "Air Bypas (idle mixture)". What's the difference?

Thanks! Mike

Mike W. · Post by **Mike W.** » Sat May 11, 2013 10:45 pm

That is indeed the mixture screw. It may be on the windshield side, but it points up or down. Are you at the AFM or the throttle body? The AFM adjusts mixture, the Throttle body idle speed. Actually what kind of car, 30 or 28? On 528i's the O2 input generally overrides changes at the AFM for mixture. And first thing on 528i's is to check the O2 output for mixture.

CBradio · Post by **CBradio** » Mon Oct 28, 2013 5:49 pm

I have a question related to this thread: my idle is smoothest with the screw all the way down.

In fact, if it could screw down even more I think my idle would be even better.

A little far out, and the car stalls.

I want to be able to dial in for a richer mixture, would adjusting a few teeth do the trick, allowing me to adjust, for idle mixture? Or is the teeth adjustment only done for overall mixture?

CBradio · Post by **CBradio** » Tue Oct 29, 2013 7:24 pm

After thinking about it, I realize the above question is silly---
So disregard please.

After reading some, I think I need to check my fuel pressure and CTS before I go adjusting my AFM.

Falkenberg · Post by **Falkenberg** » Wed Oct 30, 2013 5:08 am

CBradio wrote:I have a question related to this thread: my idle is smoothest with the screw all the way down.

In fact, if it could screw down even more I think my idle would be even better.

That could suggest you have an air leak somewhere in the intake.

CBradio · Post by **CBradio** » Wed Nov 06, 2013 12:50 pm

Just in case anybody has the same issue I had in the future:

I checked fuel pressure, its right on the money.

There was no intake vacuum leak as far as I can tell, I just swapped the head and all gaskets were freshly installed.......

The problem ended up being a faulty AFM - I swapped in a remanned AFM and the car runs much stronger!! {Still not 100% but getting there}

Thanks guys for all your help.
I would not be able to manage without this board!