Full load adjustment - Injection pump

[Raymond]

I have chased down about every lead I can and still have a slight problem with the smoothness of the engine and a big problem with fuel mileage.  I have the low speed idle sorted out and under hard acceleration the car is sweet and strong.  However at steady lower speeds or gentle acceleration, it seems to hesitate. I'm getting about 12.5 to 13 mpg.  I had the FI pump and CSV rebuilt by Hans right about 7,000 miles ago.  I am sure the ignition is good as the problem did not change with a rebuilt distributor or subsequent change to a crane ignition.  I've changed plugs three times and the car is definitely rich.

So, I want to adjust the full load screw one click clockwise.  However, I removed the socket head screw (#10 in the BBB)and cannot feel an adjustment screw inside the hole.  How long does the screwdriver have to be?  OR, do I have the wrong hole?  It feels as if there is a weak spring inside the hole that wants to push the screwdriver back out.  

Or, is something else wierd?

Ray
'68 280SL 4-spd Coupe

[mdsalemi]

Raymond,

I'm plagued by the same apparent issue; whether it has the same cause, I don't know.

Under very hard acceleration, all is fine.  On the highway, very nice, and I pull very strong from 70-90 MPH.

Under the mild acceleration you describe, I seem to misfire as well; particularly in 3rd gear but as soon as it lands in 4th it seems OK.

When you have any clues let me know...I will be watching the post.

Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America

[ja17]

Hello Raymond,

Tread carefully here, a freshly rebuilt injection pump does not normally need this kind of adjustment.

Make sure the ignition timing is correct, since retarded ignition timing will cause a rich running situation and be down on power.

Improperly installed injection pump (injection pump timing) will cause some minor running problems.

The full load screw will adjust the entire range of the injection pump. It actually adjust the position of the "rack" which controls the quantity of fuel injected to all the cylinders. This full load screw can be accessed through a 5mm allen screw on the back of the pump(near the solenoids). After the 5mm allen screw is removed, inside is a small slotted screw, and depending on the version a thin slotted screwdriver with a blade about 4 inches long must be used.  Sometimes the slot is hard to engage because the head of the screw is worn. Turning this screw counterclockwise richens the mixtrure which is just the opposite of the idle mixture adjustment knob on the exterior of injection pump.  

An alternative is to remove a shim from under the "WRD" warm running device on the injection pump. This will also lean the entire rpm range of the injection pump with less difficulty than adjusting the rack. It is also easier to put go back if you find the problem somewhere else.

The "injection pump tour" shows some of the internals of the injection pump.

Joe Alexander
Blacklick, Ohio

[Raymond]

Thanks Joe.  I've read your tour, most, if not all, of the posts, and the BBB.  I really think I have left the load screw to last resort.  As I said, I want to try "one click clockwise" to lean the whole thing.  I have played with the WRD several times.  My thermostat is still the very old style with the adjustment lock nuts on the thermostat shaft.  I have adjusted it from very, very rich to so lean that the engine backfires and back to what seems good now.  I feel that the low speed mixture is okay now.  The ignition timing is set at 2-3 degrees BTDC at idle and advances nicely about 30 degrees with rpms.  

I tried to be very careful about the timing when last installing the FI Pump.  I guess I could take it out and try again, but the fittings are such a pain.  I haven't found a good flare nut wrench that's thin enough to fit between them.  I had the 5mm allen screw out and my thinnest screwdriver wouldn't reach.  I'll have to buy a skinny one that's long enough.  

One clue I left out, the warm idle often drops below 600 but not consistently.  I have done a methodical check of the linkages and I'm confident they're correct.

At this point, given the abysmal mileage, the intermittent hesitation, and the correctly operating ignition, would you still take the FI pump out to check the timing or give the load screw a "one click trial"? And, am I correct in assuming that I'll then  need to adjust the WRD again?

Ray
'68 280SL 4-spd Coupe

[ja17]

Hello Raymond,

If you are fairly certain you did not install the injection pump 180 degrees off you are probably fine. You'll have to see how the cold running is before you adjust the WRD again.

If you ever need to remove the injection lines again, start with the back one and slide the fitting up the line and out of the way before moving on to the next one and repeat until all are finished. With this method you can use just an open end wrench!

It sounds like the timing is fine.

Yes, the weak spring you feel is the "rack" pushing back at the screwdriver.

I am not sure your hesitiation or miss is caused by rich running, you can also cause a temporary leaner running situation by mis-adjusting the linkage. (for test purposes only). Do this by slightly shortening the rod between the pump and crossover rod. This will alter the idle but it is a temporary measure so you can determine if the car runs better when leaner.

If you adjust the full load, just keep track of your adjustments so you can return it if needed.



Joe Alexander
Blacklick, Ohio

[Raymond]

I have 140 miles on brand new plugs and read them today.  They are all starting to turn chocolate milk brown. They all had a deposit of soft black soot on the metal base of the plug which wiped off with a shop rag.  The number 4 plug was different than all the others with soot on the ceramic and electrode as well.  :?:  

Given that Hans tested all the injectors at the time of the FI pump rebuild, should I suspect something else, or is 140 miles just to short to tell anything significant?

I did try to turn the screw on the rack, but I could not seem to engage the slot.  Is there a technique that can be described, or just keep trying by feel?

Ray
'68 280SL 4-spd Coupe

[TheEngineer]

It is difficult to engage the screw because the lever that pushes on the rack sits on top of it. You must back out the shaft a little. The shaft is on the screw that has an 14 mm hex head and is under the control arm. backing it out 1/4 inch is enough. But like Joe says: Removing or adding shims under the altitude compensator does the same and is easier. Caution: Make one adjustment only at one time, then drive the car for a while to really determine the effect of the adjustment. These cars do not sense the air density and you will find that the adjustment can be set leaner in the summer when the air is warm. It is also important that all cylinders work about the same because the pump can not sense if the air volume ingested is the same for all cylinders. Also, check for fuel leakage at the starting valve: You can just remove the solenoid and look at the backside of the valve with the fuel pump running. Disconnect the coil to prevent the points from burning.

[Raymond]

The cold start valve was rebuilt at the same time as the FI pump.  I checked for leakage by removing the closing nut on the side of the valve and didn't get a drop.  Previously, I have adjusted the WRD to the point that the car popped and backfired and backed it off one from there.

The car acts like an older Harley in that if it is run slow for a while or idles, or after being shut off and re-started hot, it misses like fouled plugs.  Gunning the engine or running at over 2500 RPM smooths things out for a while.  I'm getting 12.9 mpg and the tail pipes look like they are lined with black felt.  

I backed out the 14mm bolt as suggested, but still can't get past the lever to the full load screw.  I am out of airspeed and inspiration.  

I have tried everything else everyone has suggested. Should I suspect poor workmanship on the rebuild?

Today I added a bottle of Techron Fuel system cleaner for want of a better idea.  


Ray
'68 280SL 4-spd Coupe

[ja17]

Hello Raymond,

Try removing the two slotted screws holding the WRD (warm running device) on the injection pump and remove any shims between it and the pump. This will lean all speed ranges and you can always re-install them if needed. You do not need to disconnect the water lines going to the device.

Joe Alexander
Blacklick, Ohio

[J. Huber]

Joe, since I have you tuned in here, can I sneak in a question?

On the Injection pump, where the throttle linkage rod connects to the pump lever? (excuse the possible wrong lingo), there is a large spring. On my car it is sort of mashed and squished looking. Is this normal? a replaceable part? -- can it wear out and cause troubles? Reason for asking is I am about to tighten up the linkage via new sockets and the accelerator bushings. But I notice play at this juncture too, which I would imagine is causing some of my brief inconsistant idle whenever the car is placed in gear. Thanks, sorry for butting in guys...

James
63 230SL

[glennard]

JH, My R11 FI pump has coil spring about 1" dia. with 3 coils visible(might be more under the edge of cap housing).  It firmly returns the linkage to the stop.  It is currently one the few things working on this frozen puppy!

[Raymond]

Joe,

I have been the route of removing shims from the WRD until the car started popping like it was too lean, and I couldn't keep it running.  I then added the shims back in to stop that behavior.  I'll go out there and try that again.  It acts as if it's got two mixture personalities one for below 2500 RMP and one for above.

I know it's gotta be me doing something wrong, but I don't know how many times I can go back to square one and do this all again.  I have printed out several posts about linkage, WRD, etc. and follow them to the letter. I'm starting to feel like the definition of insanity... doing the same thing over and over again and expecting a different result.  Tonight I'm going to try burning incense and sacrificing a chiken.

Ray
'68 280SL 4-spd Coupe

[hands_aus]

quote:Originally posted by Raymond

Joe,

I have been the route of removing shims from the WRD until the car started popping like it was too lean, and I couldn't keep it running.  I then added the shims back in to stop that behavior.  I'll go out there and try that again.  It acts as if it's got two mixture personalities one for below 2500 RMP and one for above.

I know it's gotta be me doing something wrong, but I don't know how many times I can go back to square one and do this all again.  I have printed out several posts about linkage, WRD, etc. and follow them to the letter. I'm starting to feel like the definition of insanity... doing the same thing over and over again and expecting a different result.  Tonight I'm going to try burning incense and sacrificing a chiken.

Ray
'68 280SL 4-spd Coupe

Ray,
There is a HOT ENGINE  air/fuel mixture test,
often called the Joe Alexander test.

with a hot engine idling, disconnect the top of the link rod that goes down to the injection pump.
increase the air supply, if the engine speed increase more than 100RPM the mixture is too rich. If it increases a little then dies, it is about right.

Increase the fuel supply, if the engine speed increases more than 100rpm, it is too lean. If it increases a little then dies, it is about right.

I hope this is useful.


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