How to adjust the altitude compensator on a W113 fuel injection pump

[ryanpe56]

I have a very sluggish '71 280SL at my altitude of 7500 - 8000 ft. The Bosch pump has been rebuilt by Marty at H&R injection in Bohemia, NY. He did a great job and the car now runs very well after 15 yrs of storage in a barn. Marty suggested I adjust the altitude compensator by adding /removing shims. I'm wondering if anyone has performed this adjustment and did it make any changes in performance. Thanks in advance for your help.

[Benz Dr.]

I've done it many times. The trick is to put a mark on the top of the compensator so you will be able to tell how much you've turned it from your starting point. The BC can be very difficult to undo but yours may move easily because it was recently apart.
I've found that most need a 1/2 to 3/4 turn in or out to get them in the right range. Run your engine until fully warmed up before you make any adjustments. If your engine seems flat under throttle up turn the BC out until your engine pulls well. If it bogs down turn your BC in a half turn. Once you get on to it you will see how easy it is to do.

None of this will work if the BC is defective. I live at about 600 feet above sea level so the compensator doesn't have much to do around here. I've been up to 7,000 feet with my car and it ran OK but I bet lots of them won't if they stay at the same basic altitude.

[wwheeler]

Adjusting the BC will change the fuel/air ratio for the entire range. It is very sensitive and even a .003" shim will make a difference. As the DR. says, the compensator has to be functional in order to change the mixture. There is info here in the tech manual to see if yours is still operable.

Someone suggested using a rubber hose to temporally jam to compensator against the #6 injection line. Doing so allows you to make small incremental changes and determine what is the best setting. Once that is determined, you can find a shim that puts the compensator in the same ideal position. I did that and worked out well. Or you can adjust the full range screw in the back of the pump and do that same thing. There is also information in the tech section about how to do that.     

[BlackForest]

The altitude compensator is designed to adjust fuel flow for different altitudes automatically.
In other words, you don't adjust the compensator with shims to adjust for a different altitude.
It does it for you.
If the pump was calibrated correctly, and the BC was tested and works properly, then the running issue you are experiencing is not related to the shimming of the BC.

Wallace was correct in saying you should look to adjust the full load range by adjusting the rack. (NOT the BC)
The full load adjustment changes the entire range from idle to full load by adjusting the basic position of the plungers and barrels (1:1 ratio).
What we would expect to adjust in the instance for ryanpe56 is that the rack should be adjusted lean to compensate for the higher altitude.
This would make the idle leaner and equally the mid and full load range leaner.
(If we had the calibration specs from Marty I could more accurately advise on what adjustment you should make relative to his calibration)

Adjusting the shims on the BC does not change fuel/air ratio in a predictable manner. It changes the min/max that the pump can deliver fuel at any given position on the 3D cam (fuel mapping) as transferred to the follower roller or stylus.

Correct calibration:
{(lean.............rich)}

BC adjustment lean:
{(lean........rich)     }

BC adjustment rich:
{    (lean.........rich)}

This is a simplistic explanation but serves to illustrate that you lose the full fuel adjustment range.


I will do my best to explain the reason for shims on the BC.
The shimming of the BC is a crucial step in the calibration of the pump.
Bosch describes this calibration operation as the adjustment of the "transmission ratio" of the follower roller (reading the 3D cam) relative to the control rack (which ultimately moves the P&Bs rich or lean) via the rack reversing lever.

Basically you are centering the stylus on the 3D cam so that you receive the entire range of fuel mapping from fully lean to full rich at any given speed or load. {.........}
When you change the shims on the BC to the direction lean you are making the fuel mapping lean as several members have noted in the past. {(.........)     }
What has not been discussed is that you have now skewed the relationship of fuel mapping in a way that you will not get the enrichment at mid or full load that the pump was designed for.
You have taken it completely off the map which was not designed as a 1:1 ratio like adjusting the full load rack.
Calibrating the pump on a Bosch test bench allows the user to observe and adjust the full range of fuel flow relative to the fuel mapping of the 3D cam as precisely set by properly adjusting the crucial transmission ratio.

Follow this link to observe the testing of a BC:
https://www.youtube.com/watch?v=NlFLLG0DxuM

Regards,

Michael

[stickandrudderman]

Michael,
I just watched a few of your videos and it's not often I have shop envy!

[mauro12]

I would like to ask to Blackforest , since he is an expert in rebuilding these injection pumps , his personal assessment regarding the use of a different bench test to measure more precisely the quantity of fuel , injected  to each cylinder . My injection pump has been rebuilt from a German rebuilder , very well known around German forums , with the use of a frequency converter controlled machine which allows more precisions and faster measurement . For what I can understand , you have the possibility to set each cylinder as close as possible with the quantity of fuel in it .  I’m definitely not an expert in this field , so correct me if I’m wrong . Thank you

[thelews]

I've seen Michael in action setting up the injection pump on ORIGINAL Bosch testing and calibrating equipment.  I don't know how one could be more precise than the calibrations afforded by that setup.  Further, Michael can make minor tweaks, based on his experience, to improve engine performance on our old engines and how we currently tend to use the cars.

[BlackForest]

Hi mauro12 and thanks for your inquiry.
I am familiar with the website www-oldtimer--technik-eu for the rebuilder you are referring to.

There are a lot of issues with what he says on his website.
Basically he is trying to create a "new technology" to make up for the fact that he does not have Bosch test equipment and tools.

-I am by no means saying you cannot accurately test and calibrate these pumps on non-Bosch test equipment.
What I am saying is running a self-made bench with a 1kW electric motor is not new technology.
In order to control the speed of the motor you would need some sort of frequency converter.
That is not something novel or somehow more accurate, it is simply how you control an electric motor.

So lets break down his claims:

1) "Modern, frequency converter-controlled drive technology not only allows the respective test speeds to be maintained precisely, but also allows these speeds to be reached very quickly within a few milliseconds."
The maintaining of speed on a Bosch test bench is done in 3 parts:
      a) A 3-phase AC motor ranging in power consumption from 4-14kW
      b) The AC motor is connected directly to the pump of a hydraulic transmission
      c) This drives a 2 or 4 speed transmission
The amount of mass this setup creates is like comparing a lawnmower to a freight train.
What Bosch needed to accomplish was high torque at low speeds. A 1kW electric motor directly driving an injection pump is not going to do that.

The second part of his claim is that reaching the required RPM for a given test is done very quickly.
This claim is absolute nonsense because there is no benefit or increase in accuracy in getting the test bench up to any given speed for a test measurement.
The whole point is to maintain the given speed during the 1000 strokes of the pump during the test.
Just to be clear, the pump is not tested during an acceleration of any sort.

2) "Paired with a powerful drive of over 1 kW, falsification of the measurement results due to the slower acceleration processes of older test benches can be avoided."
-As stated above, nothing is avoided because the acceleration of the bench to a test speed has zero bearing on said test at any given speed.
Also stated above, please note Bosch test benches operate at 4 to 14 times the power of his setup. To call it powerful is at a minimum a false statement.

3) "More than half a century after the development of mechanical fuel injection pumps, the number of functioning test benches is naturally very limited. Documents on their construction or the necessary test equipment are either non-existent or remain in the company archives. The reaction of the market economy to a scarce commodity is well known."
-Test benches are reasonably available, if you are in the pump business you can easily source one. Documents from Bosch are out there, he just does not have them.
Here is a link to the restoration of one of our Bosch test benches: https://www.blackforestllc.com/taketwothesecondefep385a

4) "Our modern test bench technology allows more; we are able to double the accuracy of the uniform delivery of the pump elements compared to the manufacturer's tolerance or halve the variation."
-Running a pump on a self-made bench setup powered by a frequency converter has absolutely nothing to do with the delivery accuracy.
The accuracy of uniform delivery has many factors I could go into detail on if people care, but his claim is patently false.

There are many other incorrect statements on his site but I will leave it there.

It occurred to me that for how unbelievable his cutting edge test bench is, why not even 1 photo of it?
-How does it control the cut-in and cut-off of fuel delivery?
-How does it control and maintain fuel pressure?
-How does it control and maintain test oil temperature?
-How does it create vacuum?

I hope this does not come off as combative or defensive. I have been in the exclusively Mercedes-Benz only business for over 25 years and I take it very seriously.
We have invested a ridiculous amount of time, energy, and money to do what we do.
I guess part of that requires one to "pay to play."

I await any further inquiries and/or lively discussion.

Regards,

Michael

[tcj]

Hello Michael/Blackforest,

Thank you for your insights, but perhaps you have not fully understood how Hans from www.oldtimer-technik.eu overhauls the injection pumps and what great results he achieves with it. This is probably also because he doesn't describe everything on his website.

Here in Germany, word has long gotten around that the pumps he overhauls run best.
I know some old Mercedes drivers who first had their injection pump overhauled elsewhere, then had it overhauled again by him and are now happy.
Same for me, I've been driving my W112 for >100000 miles now and didn't recognize the car after the injection pump was overhauled at his place, so smooth it runs.

Without being able to judge your work, I am sure that you are doing an excellent job.
However, I can't imagine that you do it any better than Hans, because it simply doesn't get any better than that.

Best regards,
Thomas

[mauro12]

I have absolutely zero knowledge to replicate to you . What I can say is that my pump is working very well after his overhaul. I also had an issue that my car ,250sl had an r22 pump typical of 280se car . In order to not replace the entire pump , he put the space cam of the r18 pump and set the pump according to r18 specifications. I guess is not easy work . There are plenty of rebuilders around the world and each of them do an excellent work . This is my personal opinion .

[BlackForest]

Hi Thomas,

As I responded to Mauro, I have no idea how he calibrates injection pumps.
I did initially state that you can successfully calibrate a pump without Bosch test equipment.
Which is to say at no point did I opine on the quality of his rebuilds.
I am happy to hear of the results you have received from his work.

Mauro asked a specific question concerning my "personal assessment" of his test equipment.
I answered that question to the best of my ability and welcome an exchange of thoughts.

Mauro,

Again, I am happy to hear you received the quality of work you had hoped for.

This post started with the reason for shimming the BC, but I guess we got a little off course.

Michael