Idle adjustment

[Cees Klumper]

Some mechanics not so familiar with our cars will use the intake venturi end stop screw + locking nut as the idle or the idle mixture adjustment.

I had this in mind last wednesday when a group member asked me whether I could have a look at his car, which was freshly restored, engine rebuilt etc etc, but running extremly poorly. So the first thing I checked was the position of the intake venturi flap (which should be almost binding i.e. as closed as possible - as pointed out here before) and, sure enough, it was very much open. The mechanic had set this adjustment and then proceeded onto other adjustments to try to get it to run right, which of course he did not manage. After adjusting the flap, and subsequently the idle mixture on the back of the FI pump and the large slotted air screw on top of the intake manifold toward the front, the car ran well. There was no CO meter around so we could not do fine-tuning, but the performance was light-years ahead of what it had been.

That was wednesday morning. Then in the afternoon, I went to pick up my own car which had been into a garage for its annual compulsory safety-fitness test and, lo-and-behold, the mechanic who did the testing said there was only one thing wrong: the CO-level was at 4% (which is where I had set it) and maximum allowed in the Netherlands was 3%. So ... he adjusted the air somewhat by ... adjusting the end stop screw on the intake venturi!

Twice the same problem in one day. So there are many mechanics who will make this mistake these days. It may be worthwile to have the basic adjustments checked by someone who knows how all this is supposed to be done, if you're not entirely sure about your regular mechanic. There have been many reports of owners who thought their car ran pretty good (sometimes for years), until it was finally adjusted properly.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[Mark in KS]

Cees, thanks for the well informed story.  I am in the process of "cleaning" my engine bay and will note your comments about idle mixture controls to accomplish the proper setup, when everything goes back into the car.  Right now the flap is set properly.  But I would have been one of those wrongdoers had you not made your posting.

Thanks from me and others in this group that lurk on a regular basis.

Mark in KS

[Bob G ✝︎]

Its sound like the air flap is simular to a throttle butter fly ans opens according to the sensors from the fuel injection pump to allow a mixture of fuel and air to burn creating horsepower.
Do you remove the air filter to check this flap and how to do determine if it is set correctly?
Bob Geco

[knirk]

Hi Bob,

Here is how I do it:
Disconnect the linkage rod. Adjust the stop screw so that the throttle valve can be shut all the way without stopping. You should be able to feel when the valve binds in the closed position. The valve should stick slightly, but not bind – so adjust the stop screw as necessary. Then adjust the linkage rod so that the throttle valve is still in the closed position after you put it back on.


Per G. Birkeland
69 280 sl aut 834
Norway

[Gerhard]

Coming feb my car has to go for the yearly APK. I have a problem adjusting the CO level. CO level is over 12% that is the max my meter covers. When i adjust the idle screw on the FI pump I can get the CO level down to the needed 3-4% but after a few minutes the CO levels rise. I found that after adjusting the screw and starting the engine the idle screw rotates with the FI pump. This means that the distance between the inner mechanism and the idle screw is to small. Before I take the whole thing apart I consult the forum for any tips on fixing this error.



Gerhard Radstake
1965 230SL

[Ziggy]

Gerhard, this is a common misconception but cars build before 1-1-1974 do not legally have the requirement to match a certain CO level, idle speed, or anything like that. Don't worry about it, you don't have a problem. If your gets rejected just ask the 'keurmeester' to get his books and show you that rule.

[ja17]

Hello Gerhard,
First screw the knob inward enough to dis-engage it with the inner mechanism, if it is not too late. As you have found if the knob is unscrewed far enough it will back out the inner screw far enough to make it in permanent contact with the adjustment knob so that everything spins. Usually at this point the "click" from the adjusment knob dissapear also and the knob no longer moves inward. If you are able to turn it in enough to dis-engage it, you will need to lean up the injection in another area so you will have proper adjustment range at the idle knob.

Make sure tune-up items are  correct first. Some tune-up items which will make your car run rich are; clogged injection pump air filter, clogged main air filter, improper ignition timing, improper linkage adjustment, too high fuel pressure (clogged return fuel line), leaking intake starting solenoid, stuck warm up mechanism on injection pump, very bad or leaky injectors, and very badly set injection timing (not likely). You should check most of these first before making major changes to the injection settings.

These injection systems "wear rich", in other words over time they begin running richer. After all else is checked, removal of a oval (three hole) shim between the warm up device housing and the injection pump will lean the entire range of the  fuel injection. I like this fix since the shim can be easily returned later if you find this not to be helpful. This will usually enable the adjustment screw  on the injection pump to be  correctly positioned so as to have a proper adjustment range.

Do a little research on this site on any areas you are not familiar with. We will be here for help!

Joe Alexander
Blacklick, Ohio

[Benz Dr.]

Make sure when you do this adjustment at the throttle valve that it's dead cold first. Having it warmed up and then adjusting it so it's closed and binding a bit may surprise you the next morning. It might not ever open again.

Throttle linkage adjustments make the car respond better but aren't as critical as you might think. Starting with this in mind will often frustrate you. Always start at the begining.

Compression - yes, move on. No, set valves or find out why.
Timming - look at valve timming when setting valves.
Ingintion- points, wires, spark plugs ( gap colour ) distributor function etc.
Fuel - volume ( pressure isn't that critical ) fuel flow back to fuel tank, injection pump function, cold start valve, injectors etc.

On non injected engine you learn quickly that you make carb adjustments last as they affect everything else.



Daniel G Caron

[knirk]

I wanted to check the CO level yesterday and looked up in my Haynes manual to check which way to turn the idle speed adjusting screw. In step 1 on page 70 the manual states that the air cleaner should be removed. I haven’t noted this text before and I am not sure if this is ment for the entire procedure, or only the first part where they talk about checking the closure of the throttle valve plate.
It’s a real hassle for me to remove the air cleaner since the AC hoses run above the cover. I can understand if it’s a long time since renewal, but with a clean filter is this still necessary?


Per G. Birkeland
69 280 sl aut 834
Norway

[n/a]

Cees, Can you describe a little better how you adjusted the fuel mix and the idle air mix? Or can you direct me to an already written FAQ covering this? My Haynes manual is not clear enough for me to understand what I am doing with the 2 adjustments. My venturi is closed. The Haynes says if I don't have a CO meter, hook a vac gauge to the venturi. When I hook a vacuum gauge in parallel to the distributor vac advance I get a reading that goes to 0 (no vacuum at all) when I increase the rpm. (Distributor advances fro 3 BTC to about 30 BTC so vacuum advance does work. At idle it reads about 7 in hg. I have a 1967 250 SL, which runs smoothly but the exhaust smells like I am running rich. Thanks for any info. R Hoffman (first gen. non-dutchman).

[hands_aus]

Some simple tests to determine if your car is running rich or lean.

HOT ENGINE, at idle remove link rod to Inj pump
1. Open the throttle rod slowly..... If engine speed increases more than 150RPM the air/fuel mixture is too rich.
2. Increase fuel supply by slowly moving disconnected Inj Pump rod..... If engine speed increases more than 150 RPM the air/fuel mixture is too lean.

Then remove your spark plugs and compare them to the pictures in the inside back page of the Haynes manual.

Black and sooty means too rich.
White means too lean.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Cees Klumper]

R Hoffman - I think you're not supposed to hook up the vacuum gauge to the distributor vacuum line; to get accurate readings, I think you have to plug it into the intake manifold "downstream" i.e. on the main body behind the intake venturi - on mine, there's a relatively small (8 mm I believe) bolt that, when taken out, allows you to hook up a vacuum gauge right in the main body of the intake manifold.
I use an extensive "step by step" instruction on how to adjust the fuel mixture that is residing on Pagoda113.com, here:

http://pagoda113.com/eve/ubb.x/a/tpc/f/3296041521/m/7246004272

This is a page that was viewed already over 3,100 times ...

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[Bob G ✝︎]

Cees:
Can you post a picture of were I would attach a vacuum gauge to my engine to perform your outlined adjustment.

Thank You
Bob Geco

[Cees Klumper]

Bob - I will take a photo today and post it.

Cheers,
Cees

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[dum]

I found this old post and feel I am a victim of exactly this lack of experience from mechanics.

Can anyone show me with some photos where and what we should not do and what we should?

Many thanks.

[Naj ✝︎]

I would suggest you start with the linkage tour and make sure everything else related is in order, especially the throttle valve adjustment.

http://www.sl113.org/forums/index.php?topic=5003.0

naj

[dum]

Thanks for the link, very useful and comprehensive, indeed!