Posting this because it was an interesting problem, and it shows the crazy stuff that people do to our cars and the unintended consequences.
A 1970 280SL arrived at the Faded Star Garage a week and a half ago, belching black smoke and running on three or for cylinders. Two shops had worked on the problem ("running rich") and sent it on its way. The first had replaced the plug wires, coil, and spark plugs. The second replaced the distributor cap, installing it backwards. Neither solved the problem.
I replaced the fouled resistor spark plugs and while they were out did a compression test - 170 psi on all six. The new non-resistor plugs got it to run well enough to warm up on a test drive although it ran very badly. But the warm running device seemed to work. Removed the cold start valve, turned it around and reconnected it - no leakage with the pump running. No current to the valve after a few seconds of cranking. Pump lever on the stops at idle. The idle mixture adjuster moved easily both ways. But splitting the linkage, I could raise the rpm to at least 4,000 by just giving it air. I blew air through the return line, no obstruction.
Hmm. This is getting interesting. Did something in the pump break and jam it full rich? Time to investigate the movement of the injection pump rack. Looking at the front the pump, I could see that the plastic cover for the fitting at the end of the rack was missing. I poked my M5x40 screw into the fitting and felt something rubbery, definitely not end of the rack. I removed the fitting and found it had been sealed (probably long ago) with urethane, which had gotten on to the end of the rack and caused it to stick in the forward (rich) position. I used my screw to hold the rack forward and cleaned the sealant off and made sure the rack moved freely. Cleaned the urethane out of the end fitting using one of those combination wire brushes used in soldering copper pipes, and put it back in, leaving the screw in place to monitor the movement of the rack.
Started the engine and observed that the rack was moving as it should and that engine was running much better on all cylinders and not blowing black smoke. Took a test drive. Just a hint of a stumble at first but that cleared up after a few seconds and it was now a good-running engine.
Attaching a picture using my spare pump to illustrate.
Best wishes for the new year,
CT
