Confused While Adjusting Idle

[Pawel66]

Please help - it is probably quite common...

Here is the advise: While watching the vacuum gauge, screw the idle air valve out until the vacuum begins to fall (idle gets rough). Now screw the idle air valve in; the vacuum reading should increase for a while then begin to fall again (idle gets smoother/faster, then gets rough again). Now slowly screw the idle air valve out again until you achieve the highest vacuum reading and smoothest idle.

But when I am turning the air valve out, engine revs are just going up - until the air valve is almost out of its seat....

I have a rough idle and the mixture is too rich (smell, the way the plugs look).

Where do I start? The other regulations (dwell, ignition advance, linkage) are done.

Pawel

[Tyler S]

Make sure the car is at operating temp before adjusting anything. If everything else is set up correctly, especially the WRD and the Cold start valve is not leaking. You need to lean out the injection pump to get it in the ballpark via the knurled nut on the back (Engine Off!!). Try one click counter-clockwise at a time then go back to your air screw on the intake to fine tune the idle. Go back and forth until you can get the reaction/satisfactory idle with the vacuum gauge procedure etc..

[Rolf-Dieter ✝︎]

Hi Pawel,

Tyler makes some very valid points, however, also have a read of the Linkage Tour

here ----> http://www.sl113.org/wiki/Restricted/Linkage-tour

Dieter

[Pawel66]

Thank you for your kind attention!

WRD - I did the simple listening test and it is ok. CSV - not leaking, unscrewed the screw, not a drop came out.

Thank you for the link, Rolf Dieter - but this is exactly the procedures I am following, that is where my starting quote came from. I know idle adjustment is the last step and I set everything I should set upfront (dwell, ignition timing, linkage).

Point is I have troubles going through the iterative procedure as described in the manual as I cannot find the point described in the procedures: when I turn out the idle air screw the revs are increasing and the vacuum is increasing. There is no point where idle gets even, it is rough at the beginning, then it becomes even and stays even. There is no moment of, quote: "screw the idle air valve out until the vacuum begins to fall (idle gets rough)".

I think for somebody with experience it should be known and you may know where to start. In the computer you would probably re-start it .

Also: I think vacuum may be a bit too low (on the gauge I have it is right under the generic green area). Also the hand on the gauge is not 100% stable - it moves a bit up and down, very slightly, but it does. But this happens when idle is rough and the revs are not stable - so it may be natural.

By the way: could help on what should be the range of vacuum value (I am using the bottom port in the throttle, same as for ignition distributor for measuring). I have 280SL, march 1970 retard throttle and 009 distributor.

Thank you so far and thank you fo rfurther tips...

Pawel
 

[Benz Dr.]

Assuming you have a euro version, and I don't think it makes that much difference here, when you unplug your vacuum line to plug in you gauge there will be no signal going to your distributor. This will advance your timing and your engine will idle at a higher RPM. Idle adjustments should be done at idle speed around 900 RPM.

If you have all of your adjustments for ignition, throttle linkage, and idle mixture air screw set up properly, and you can't turn your idle fuel mixture screw out any further, then your pump is set too rich and it will need to be leaned out. I usually remove one small shim from under the BC which will lean out your mixture across the entire range - all the way from idle to full load. Sometimes this will lean out your idle mixture too much and you would then need to adjust your mixture screw at the back of your pump CW to bring your idle mixture back to the correct ratio.

By using the split linkage test you can quickly tell if you are too lean or too rich. Idle speed is a combination of fuel, air, and timing. By triangulating those three elements it is possible to get an idle speed that is not only smooth but consistent. If you are unable to obtain a good consistent idle speed, then you probably have a problem with one of those three essential elements and you will need to adjust further.

[Tyler S]

If backing out your air screw results in a RPM higher than 1200, then you need to start over and lean out the Injection pump first. Or find the reason it is running rich. Reason is the governor weights in the pump take over after 1200 rpm and the Idle circuit is no longer being influenced by the small spring in the governor. The IP is adding extra fuel at this point and turning the air screw just "adds air to the fire" so to speak.

[Benz Dr.]

I agree. If he is unplugging the vacuum line to his distributor to hook up a vacuum gauge this is the reason the idle speed would be over 1,200 RPM.

[Pawel66]

Thank you for your kind replies!

I hook the vacuum gauge to the throttle port for ditributor, but I do it via triple manifold (or a T-shaped connector, not sure what is the English name for that) - vacuum is delivered to the distributor and to the gauge as well. I thought this is the way to do it. Is there another port on the throttle where I can hook the gauge? It is the 280SL retard throttle. Please kindly indicate. I have read people wrote about some screw is there, but I do not know which one. i read someone was drilling a vacuum port - but that was another problem...

Yes, the rpm can go over 1200 as I screw out the air valve. And yes, it may feel like adding air to the fire. So this might be about overall IP richness adjustment and you say I should remove the shim. I saw some posts on this when I was doing my homework - will dig out and try to do it, hoping I will not destroy anything...

I am pretty sure the mixture is too rich - the smell in the exhaust, the look of the spark plugs.

I will also check again the other parameters - just to make sure they are ok, I understand their importance and I understand there is no sense of other adjustments if the basics are not set.

Thank you very much for your kind help!

[Benz Dr.]

You hooked it up properly so that's not a factor regarding your rich running problem. Go to BC adjustment to lean your injection pump.

[Cees Klumper]

Before removing shims from under the barometric pressure compensator, have you tried leaning out the mixture by turning the screw at the back of the FI pump counter-clockwise? That may be all it needs at idle.

[Benz Dr.]

Before removing shims from under the barometric pressure compensator, have you tried leaning out the mixture by turning the screw at the back of the FI pump counter-clockwise? That may be all it needs at idle.

I agree Cees. However, if you move the idle mixture screw all the way out and the mixture is still too rich, then you would have to adjust the IP rack or the BC. I generally choose the BC because it's easier to get at and it's possible to turn it in and out to find the sweet spot while the engine is running. As a precaution, I mark the top the BC before I even start so that I can tell how far I've turned it from it's base position. If your engine is really running rich, it can take a half to a full turn before you will see any results and because you can easily loose where your starting point is, it's very important not to over look this.   

[ja17]

Good advice from the Benz Doctor.  I think one turn of the BC equates to (.012")

[Benz Dr.]

If the thread is something like 14 x 1.0 then it's 1 mm per turn or about .040''  I know it's a fine thread but I think it could be more than .012''  However, without looking I'm just guessing.

[Pawel66]

Thank you again!

Now I got a bit confused: I set my mind on shim removal, and now I hear about turning out BC (whatever it is). So which one am I to do?

I will dig into posts, maybe this will clarify it.

I did not mess around with the idle adjustment screw in the IP as I could not get to the point described in the procedure really when I should do that. But I was at the left hand end of the scale of clicks, I guess.

[Pawel66]

These are the shims I should try to remove to lean the mixture, correct? 

Also: I think I may understand your discussion on turning the BC - you gain the turn and length as you remove the shim and vice-versa. Do I get it right?

Just to confirm: removing shim - leaning, adding a shim - enriching. Is this correct?

Do not laugh please. It is all like an eureka to me.

[Tyler S]

If nothing has been previously disturbed and you have to add shims, Then you are compensating for a problem elsewhere. This could lead to a lean condition at other rpm ranges and cause engine damage. Have you checked the fuel injectors? They can leak causing a rich idle condition.
The IP idle enrichment knob needs to be adjusted first before trying anything else. The car also needs to be driven between adjustments.

[Pawel66]

Tyler, you have just ruined the party...

But seriously: the car is now like new. She was stripped to the bear metal and everything was rebuilt. Injectors are all new. The IP was overhauled at Bosch, but they cannot tune it to each car exactly - they specifically say that. So you put in an overhauled IP to an overhauled engine with overhauled ignition distributor and all of the other parts - there is no chance they will all work together at once like a clock.

Thank you for raising that - it is important. I need to make the basic settings. it is easy for me to go to someone with CO2 meter to check it while driving - this is no issue and I will do it to follow your warning. But the guy with the meter will not tell me how to adjust the IP in Pagoda... It is easy to do split linkage test as well - so I will watch for what you are raising.
I think it is a fair attempt to remove (remove to make it leaner - correct?) and try to adjust the idle - if this works, I am half way there - I will just do the rest of the checks.

[A Dalton]

 Baro thread size is 14x1.5

1/4 turn is .015"

I mark them with a C/L so  i can measure the approx  shim size via 1/4 turns......................

[Tyler S]

Well than yes your on the right track. I didn't know the history of the car when posting above.
Yes, remove shims to lean out, Add shims to richen.
 Just my personal preference- You can also use the main rack adjustment screw located under a small slotted or hex cap on the back of the IP pump. It is a small flathead and has click detents like the idle knob. On that adjustment it is clockwise to lean, counter-clockwise to richen. Its a bit more precise and there are no shims to measure or deal with. If your pump has a start enrichment solenoid then a 14 mm head pivot shaft near the linkage arm needs to be backed out about 1/4 inch to give you access to the adjustment screw. Do Not take it all the way out!

[Pawel66]

Thank you all for your kind assistance!

I will report the results back, hopefully the positive ones!

I have the newer IP type, just one solenoid there.

I understand all of the adjustments on IP are to be done with engine off.

[Jowe]

I usually remove one small shim from under the BC which will lean out your mixture across the entire range

I want to do this as well, but the BC i really stuck in place. I've tried to fit a 19 mm wrench around the collar but there is no space to turn it around. Any suggestions on how to remove it?

[Tyler S]

Johan, you can grind an old wrench thinner then remove the IP oil fill cap and WRD or both to give you more room.

[Benz Dr.]

I have found that a short 19 mm wrench will work much better than a regular length one.

[Pawel66]

Look, if may, I have another problem.

In essence: I have not yet tried to remove the shims. I was checking other parameters before doing that and I found out that plugs in cylinders 5 and 6, after some driving after adjustments of dwell, timing etc. were black - had black residue - while other plugs were clean with just a touch of brownish color, very faint. The injectors in cylinders 5 and 6 were the two injectors that were not replaced as they passed the verification. So I am replacing those nozzles first.

Bur what I notuced while working on the car is that on idle, shifter in N or P, when I press the brake, the engine revs go up by 200-300. As I read in Technical Manual this is almost 100% leaking diaphragm (the hose booster - manifold is new OEM with working valve).

It is quite amazing how all of these things depend on one another: brake booster affects idle, affects gear change etc. etc.

[Pawel66]

Just as an update: following many pieces of advise (thank you!) and some time spent bent over the engine bay I think I am getting there... Idle is better, car does not hesitate while acceleraitng. I have still to go through a couple of fixes (plugs and wires replacement).

I have a new problem though... Now my glove box door opens as the car accelerates at the rate it can accelerate now... Must be that the spring does not hold it as it should.

And I had to go through a noisy tacho problem. As indicated on this Forum - when it is noisy and you feel the tacho is vibrating - it is probably a tacho internal problem. It was.

Pawel