Rich at idle......hoping for guidance.

[Tom in seattle]

You've helped me get it going, now for some finesse.  It starts up thru cold start well, warms up well, runs at cruise temps well, runs at off idle speeds like a dream.  BUT clearly runs rich at idle, wants to stumble after idling at a stop.  It is idling a little slow (600 rpm) and I will bump that up, but I think will still be rich.  How do I lean out the idle without spoiling the otherwise nice running attributes of this dreamboat?  I'm reading about the thumbscrew at the rear of the injection pump.  Should I be fiddling with this?  Yes, I'm the guy that thought the dip switch on the turn indicator column was a defective high beam switch!

[Cees Klumper]

Hi Tom, yes, the large screw at the back of the pump is what you want to manipulate, but before you get there, please check out the linkage tour in the tech manual as there could be other reasons for your rich idle condition and you will want to eliminate those first. Check back with us after you've worked your way through that. Essentially, the large screw determines mixture at idle. Only turn when engine is off: push in until you feel the screw engage the actual screw inside. Clockwide is richer, anti-clockwise leans it out. You can do the split linkage test (search is your friend again) to verify whether the idle is rich or poor. Good luck

[Tom in seattle]

Re my posting above, I found this explanation on a recent post:  If the spark plugs are black then it is running rich. Separate the throttle linkage at the IP rod to the crossover shaft. Move either one of them 1/4" and see if the idle increases. If it increases when you move the IP rod you are running lean, if it's the throttle that causes the rpm to go up then it is rich. With the engine OFF go to the idle screw on the IP and rotate it clockwise if it was lean and counter clockwise if it was rich, maybe 3 clicks (lots of info in this forum). Do this until your idle is stable around 800 rpm. Do the split linkage test and correct the IP idle screw until it's neither rich nor lean or slightly rich. This doesn't mean you are done, you only have a good idle.

Is this the proper procedure to relieve my rich idle?

[ja17]

Looks like a good start. It is ok for the engine idle to increase approx. 200 rpms at idle doing the split linkage test. This will give you the ability to increase or decrease the idle with the air intake screw.

[Pawel66]

Tom, I am not a mechanic, and I was going through what you are going through now. So maybe some practicalities talk, like inexperienced guy to an inexperienced guy:
1. There is never enough of what Cees is talking about. Double check the linkage before you have a go at adjustment. See if both IP and throttle start from their stops, see if you have no play anywhere (bearings, pins). I would also add to check the ignition timing again and the dwell. Idle is air, fuel and ignition timing working together. See DrBenz's posts on ignition timing.
2. I would make sure the engine stalls because it is rich. Split lnkage test and the colour of the plug will tell you that within five minutes. What happens if you open air a bit when you idle at 600rpm? You can open air either by the adjustment screw or by the split linkage at the throttle. If it goes dead immediately, you are lean. If it increases revs, you are rich. Then you have procedure on the forum what to do - playing with idle screw on IP and the air screw to get to 800-850revs.
3. Now look at what Joe wrote - it feels like if you are indeed rich at 600rpm and on split linkage your engine will rev up by ca 200rpm as you add air - you now just have it with air suppressed by air intake adjustment screw. So maybe it is just about screwing it out a bit.
3. Turning the idle adjustment screw - ONLY WITH ENGINE OFF. Left is lean, right is rich. But you may not even get there...

[A Dalton]

 <<<<you now just have it with air suppressed by air intake adjustment screw. So maybe it is just about screwing it out a bit.>>>


This is a very god point.
I always start with the air screw for  a quick look/diagnostics.

 If you open the screw and the rpm's do not increase, you know your pump is lean.
 And if you open it and the  idle rpms go real high, you know pump is rich.

 For fine tuning, The trick is you do want to see this 100 or so rpm increase  when opening the screw...as that equates to a good base line.
The co spec  is 4,5 @idle , so they want it slightly rich.
You do not bring the rpms up with this idle adjustment, rather you adjust for a smooth rpm slighly above 800/900 and then turn the screw in to make the rpms come down to your liking.

Same deal if using a vac gauge...set idle slightly high with the best/steady  vac reading   you can get.

Then turn the screw in and watch for a slight fall on the gauge and a decrease in rpm.
Either of these will get you where you know if the thumb screw is close to where it should be,
and which direction to turn it.
Remember, in most cases, the highest vac reading/rpms  is slightly Lean...that is when you bring the rpms down , thereby slightly riching the mix.

Those with a good ear/experience  will hear this definate change in the exhaust notation..

[Atazman]

First time I have heard this explanation; I like the logic.  Thank you A. Dalton!

[A Dalton]

 You're Welcome

Nice car you have there..

250's are my fav...................

[Tom in seattle]

I'm getting great advice here and I'm making significant progress.  Thanks for the discussion.  One point I want to explore more:   It seems that as I get things set up there is a really big change by small relational changes between increased air or fuel from the IP.  And I notice as I sit at a stop, a little pedal increases engine rpm's more than I would expect.  I believe this is because my ball joints in the IP thru air plenum circuit are a bit too loose.  The first pressure affects the IP first because the throttle valve at the air plenum is closed hardest (if that makes sense, I hope it does).  What to do to tighten the whole mechanical circuit up?  Can I buy new ball joint couplings on both ends of the connecting rods?  Where?  Is this the price of a child's leg?

[Cees Klumper]

The ball joints are still available, I think from most of the regular parts suppliers and MB. They should not be that expensive. As you say it's all quite sensitive and sloppy connections will have a negative impact.

[Pawel66]

As Cees says - the ball joints are available in Mercedes. They come with Norma signature. Please also look at the studs for those ball joints. Some of them are screwed to the levers, some are riveted, so you have to drill them out. They do not cost arm and leg. If money is concern - they are pretty standard item, probably you can order them in any serious hardware store (it seems that in the US you can order anything from hardware stores, in my country they lost the fight with chain DIYs). for sure they are available in SLS.

What I did - I bought some ball joints (need to check left/right) and tried them on the studs. Wherever it was enough to replace the ball, I left the stud alone. You may not want to have a go at the stud in the IP lever unless you have it easily screwed, but do not attempt to remove the lever from IP, I would say.

Then the next thing is the bearings - you have two brass ones in the cross rod. They are also available in MB. On the forum you will find threads on replacing them. So check the cross shaft for play. In my case 0.5mm cross shaft play resulted in good 1.5mm movement of the IP lever.

You also have a couple of other pivots in the system - need to look at them as well.

Sorry, modified the post, forgot to add: grease all the ball joints nicely, clean and grease those that will stay not replaced.

Please note that in the linkage - the start of air and fuel levers travel from the stop and ending both at maximum (just some tolerance there), the 233mm or the "hole in the bracket" story - all of that is no kidding. The levers turn at the same time, but the geometry of their movement is not the same, angles are different.

More experienced members - please correct if you see anything bizzare in what I wrote.

[glenn]

Hi  As I remember MB says to idle about 50 rpm rich, i.e.  idle air screw peaked at 800 rpm, then screwed in to 750 rpm.  Run engine to operating temp 180 F or so. WRD thermo element extended fully shutting off WRD air.  Throttle butterfly shut. Disconnect linkage arm from IP.  Engine should idle at 750 rpm with gas coming only from the IP(about one tenth a cup per minute) and air from the idle air adjustment screw.  No other fuel or air!  Engine should be at 800 rpm.
  Now open the idle air screw slowly.  It should initially send rpm up to 800 then go down as air leans out.   
      If the engine wasn't at 800 rpm, adjust the idle air to get max rpm. If this max rpm is below 800, you need more gas-at the back of IP.  If above 800, you need less gas- at the back of IP.
      Repeat adjusting until you get 800 rpm.  Then screw idle air in 50 rpm.  Reattach linkage and check it.
     This should not be more than a couple clinks on the IP.  Otherwise, sue your STM(shade tree Mech)
Pursuing Pagoda Perfection

[chumps]

I have been following this thread closely (and many others over the years). My car (71 280) runs perfectly, idles very smooth at 700 rpms in gear, jumps to 900 out of gear, has great power thru all gears, shifts smoothly up and down. My problem - if you want to call it that - is when I do the slit linkage test the idle is running extremely rich. When I push on the IP lever just slightly the engine stumbles. When I push on the throttle lever the rpms go up almost 1000 rpms before it starts to stumble. I have done the linkage tour many times over the years and it is set correctly. the CSV is not leaking and is working properly. I have adjusted the thump screw on the back of the injector numerous times. No more than two clicks at a time over a period of weeks up to 12 clicks. No change so I put it back where it was. Plugs look good (nice tan color). Like I said the car runs perfect. I don't think I get as good as I should gas mileage, about 13 mpg. I have never looked into the WRD because the car runs so well I didn't want to cause a problem trying to fix something that does not need fixing. The old saying "don't fix something that isn't broke. Thanks for reading my long winded problem. Any suggestions? Thank You Chumps 

[Benz Dr.]

Three things affect idle speed; air, fuel, and ignition timing. Change any one of those things and you will change idle speed.

Late 280SL USA version is a different animal and not so easy to set up. Frankly, I'm not so fond of that system.

[gcw206]

I just went through this exact same problem with my 280SL.   I referred to this site, made a list of all possible causes, made the adjustments as recommended here, and still had the same problem: Running rich.  Then I checked the Injection Pump Thermostat, and found that the PO had replaced the old style t-stat with the new (oversized diameter) t-stat.  The result was that the new thermostat did not seat down properly in the old style seal ring, which made the thermostat sit w-a-y higher than it should be.  That condition made function of the rod too short, which in turn made the car run rich all the time.  A new thermostat in the proper (oversize) seal ring made a huge difference.   Authentic Classics sells both.