Mixture – help to get it into the ball park

[jedwards]

Hi all,
I am attempting to set the Fuel /Air ratio on my early 250SL (#1513) but could really do with some guidance. I have the basic tools but no CO meter. 
I have read a vast amounts on the various aspects of “tuning” on this excellent forum, but I am struggling to get reliable outcomes.  I have only had my car for 3 weeks now and the previous owner had  confused the throttle stop screw for the idle screw, so it was well off the basic settings.  The CSS did not work and the WRD was stuck closed.
I have followed the Linkage Tour with care, have the ignition and timing  correct and have dismantled, cleaned and tested the manifold cold start valve, solenoid on the FI pump and the WRD.  So everything now seems to be OK  but I need a bit of help with setting the mixture.

Q1 - What is the best way to manually check mixture without a CO meter?
I know the technique of disconnecting the throttle rod and depressing the throttle plate slightly. If adding air causes the revs to rise more than say 100 rpm, then its running too rich.   I find I get quite variable results this way. Is there another method?  On carbies, a “blip” on the throttle will result is a stumble if it is rich and a hesitation if its lean. Is there something like this for the W113?

Q2 - Fuel v Air: The idle adjustment instructions in the Technical Manual suggests setting mixture by first adjusting the air bypass (idle adjustment) using a vacuum gauge to achieve the highest possible vacuum and then correcting the idle speed up or down by adding or reducing fuel via the mixture control at the back of the FIP.    I struggle with this method, as it appears to be extremely sensitive to the starting mixture settings. As mine was very badly off once I correct the WRD and throttle setting, I am hoping someone can help get the basic settings re-established.  Any suggestions?
Hint: I currently have some slight “popping” on deceleration. Is that usually a sign of overly rich or overly lean conditions in a 250SL?

I hope you can help.

[ja17]

Hello jedwards,

If the blip of the throttle causes "caughing and popping" through the intake, then the mixture is usually too lean.  Hesitationcan be caused by several problems.

Make sure that the distributor vacuume cell is working and that the distributor centrifical advance is functional. Set the timing at 30 BTDC at 3,000 to start. Check to see where the idle timing ends up at this setting. 

After the engine is warm, do the split linkage CO test at idle, 2,000 rpms and 3,000 rpms for a complete picture. The mixtrure at these ranges can be set separately or all at once  as needed.

Let us know what you find and we will guide you as needed.

[wwheeler]

I don't think many have had much luck with the vacuum gauge method of mixture adjustment. I think one member noted that it may only be effective for new, tight engines.

I think you need to determine what your mixture is at the three ranges using the split linkage test that Joe mentioned. At that point, there are a number of ways to adjust the mixture and the best way depends on your findings. It is possible to be rich at idle, lean at 2000 and rich again at 3000.

I have used the split linkage test from this site many, many times. It is a very useful diagnostic tool to have.

[jedwards]

Thanks ja17 and wheeler.

I understand the concept of the split linkage test and the benefit of isolating the fuel side from the air side, but could really do with some guidance on the finer points of this technique.
I understand that if I add air, by moving the throttle butterfly while leaving the fuel mixture isolated and the rpm rise, then I am running rich.
If I add fuel independent of the throttle butterfly and the rpm rise, then I am lean, so need to reduce the air bypass adjustment screw? Is it CCW to increase air and CW to decrease it or the other-way around? What should happen when I have it just right?
Is that the extent of the technique? Are there any other things that can be done or learned?

Finally, wwheeler suggests doing the split linkage test at 2K and 3k rpm, to check mixtures at medium and high rpm. Sorry from my ignorance, but how do I accelerate the motor if the fuel rod is disconnected from the throttle linkage? will it not simply stall?

My apologies, I have only had the car 3 weeks and am only just starting on a steep learning curve. 

[ja17]

Hello,

The injection mixture is regulated by the "rack"  in the injection pump.  The "rack" is influencecd by coolant emperature (the WRD), engine rpms, and acclerator position.  The air screw on the intake only adjusts the idle air.   The thumbscrew on the back of the injection  pump, will change th mixture up fo around 1700 rpms.  There are some interna screw in the IP to adjust the mixture at dfferent speed ranges. In additon there is a rack adjustment in the fuel injection pump which adjust the mixture at all speed ranges at once.   Removing some thins shims under the barometiric compensator on the IP is also an easy way to adjust the fuel mixture at all speed ranges at once. Read a little about the injection in the Technical Manual and get back with us for more help. 

[wwheeler]

As you can see, Joe knows his stuff about the IP (and many other things of course). The way Joe told me to check the mixture at those engine speeds is by disconnecting the fuel rod ball socket at the linkage. Then with the socket loosely held on the ball, bring the engine up to speed. At that point hold the linkage arm and fuel rod and slowly pull the socket off the ball. They are now seperate and you can move the fuel rod to richen or lean. It doesn't take much movement.

The trick here is to hold both linkage arm and fuel rod so they don't move when seperating. It takes practice but can be done. As far as engine speed, you can use a handheld tach or by ear.

There are gobs of info on this subject and you can also use the search function to find threads that will help. This has got to be one of the most complicated parts of the car if you have never delt with mechanical injection. Expect it to take awhile to figure out how it works.

[jedwards]

Thanks wwheller,

That is the trick I could not work out. Now it all makes sense.
I will have ago and report back.

Everyone's help is hugely appreciated.
Jeff