Should the fuel injection pump timing / pump main rack ever be adjusted on a 1969 280SL? Is it difficult to do properly? Are special tools required?

I'm getting conflicting advice on this: one mechanic wants to adjust the fuel injection as part of a regular tune up, the other says the fuel injection should never be adjusted, and will be difficult to correct once changed.

Thanks in advance for any insight.

Steve
1969 280SL

The pump is initially timed on installation at 20 degrees after TDC relative to # 1 cylinder on firing. This is the only timing that there is. There is no adjustment. As for adjusting the control rack this would only be necessary if the individual pump elements were delivering different amounts of fuel. This is not an easy task and unless you have the right equipment and nohow then leave it alone. There are other adjustments that can be made but this should only be done by someone who has experiece with these pumps. Good luck.

As to timing; I doubt that it even matters. As I was learning MBs, 27 years ago, I worked next to a tech who had been putting pumps in for years at any point. This of course wouldn't work in a diesel, but if one looks at the various electronic injection systems the early ones fired all the injectors as a group. The fuel just waits a little longer in the intake on some cylinders.

As to mixture adjustments, my belief is that you do what is necessary. All adjustments need to follow a thorough testing of the warm-up mixture device. Most changes in mixture are a result of sticking or reduced movement of the thermostat/airslide.

If due to wear and engine changes the mixture needs correcting, then there are a number of corrections. Linkages should be verified for proper adjustment first. The knurled knob at the lower rear adjusts the idle mixture. It does this by tightening or loosening the tention on the weakest of three springs opposing the movement of the centrifugal governor. The two stronger springs cover upper and lower partial mixture control and are exposed by removing the large nut that the knurled knob goes through. Its unlikely that these adjustments can be made without a dyno and a lot of experience.

If the range of mixture is widely skewed rich or lean the whole can be corrected by a RACK adjustment. This does NOT involve changing the individual cylinder settings. It is done by changing the point of attachment to the governor. This is located behind an access plug at the rear of the pump. Adjusting this setting makes huge differences. It is universal. If the idle is rich and cruise is lean this is not the adjustment necessary.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician