Adjusting the Air/Fuel ratio on a M117

[Lexxani]

guys,
Im in quite a dillema. I messed with my fuel/air ratio (that screw in front of the air cleaner on the contour hose) and my SL-C will not go over 80mph.

Ive played with it enough that I can not find the 'zero point'.


Previously, Ive had her up to 120mph and still had petal travel (to the spec 130.6mph.)

The engine is a M117.985, the car is a '78 450 SL-C.

My fuel economy sucks (not that I care, its a garrage queen, and the top end rebuild has nothing to do with it, for thoes of you who have read my threads in the past). and the daaaamn and my top end is non existant.

Now the races are on saturday night and I want to have her diled in (plus 80mph is wayyy to slow for my liking, though it makes the MB look like a Ferrari compared to my diesel Suburban).

Can someone give me a good point of reference (ie -turns from bottom)?

[Todd Miller]

Warm up the engine with a drive. Disconnect the oxygen sensor(s) if equiped.

With the engine idling, turn the adjustment in the lean direction in small amounts, like a quarter turn at a time, and wait for a minute between each turn in order to let the engine settle in to the new adjustment. Typically on Bosch injection, "inward" is rich and "outward" is lean.

Continue making these minor adjustments until you just hear the subtle lean misfire in the exhaust note....you may also hear/see the engine begin to miss slightlly. At this point, you've found the "too lean" spot. Stop adjusting.

Now turn the mixture screw back "in" in smaller increments, waiting between each movement, until you just notice the miss go away. Stop. That's rich enough.

If the engine already has a lean misfire, then it's probably too lean already, so instead of doing this proceedure by heading toward "too lean" first, start off by heading toward "too rich" until you find the point where the engine suddenly runs really nice....then come back lean until you get the misfire, and then go back in the smaller increments until you're at that first good idle spot.

[ctaylor738]

"I messed with my fuel/air ratio (that screw in front of the air cleaner on the contour hose) and my SL-C will not go over 80mph."

It sounds like you have been messing with the idle speed adjustment. The mixture is adjusted through the air cleaner a little to the rear of center. I believe that a 1980 has an oxygen sensor and a lambda computer. The only way to get the mixture right is to use a dwell meter or multi-meter with a duty cycle measurement. This has been described in many posts on the SL and other forums.

[Lexxani]

thanks todd, im going to give that a shot.

Chuck, you're right on the money about '80 having the O2 sensor and Lambda controls. My '78 is devoid of all that technology.

[ctaylor738]

Well, that was a pretty good senior moment. Nothing like some useless information.

But still, on your car, the mixture adustment is under the air cleaner between the fuel distributor and the air sensor plate, not. There may be a "closing" plug that needs to be removed. You adjust with a 3mm hex key.

The spec for adjustment is .5 to 2.0 % CO at the tailpipe. I can tell you from experience that you cannot get this correct by feel.

Send me an email if you want the procedure off the CD.

[mercmad6.3]

Don't forget the golden rule when working with Fuel injection. LEAVE IT ALONE UNTIL YOU HAVE THE IGNITION PERFECT!