I had a rather "interesting" experience with my 1970 280SL lately which I would like to share with fellow members since things are not sometimes as straight forward as they seem.
Following my recent total engine rebuilt which included new cylinder liners, pistons, rings, bearings, timing chain etc., I could not tune my car to run as smoothly as usual (prior to the engine rebuild). The car was running ok but sometimes it would misfire or gave me the filling of a wrong mixture setting, although it was set properly. I suspected, timing chain installation, ignition timing, FIP installation/timing, fuel mixture setting, linkage setup. I checked everything again and everything checked out ok. Nevertheless the car was not as smooth as I would have liked it be, idle was sometimes erratic, especially when stationary at traffic lights (auto box). Trouble is that at one moment things were ok then the tune up was messed up, then again erratic idling. As the car was running in with its new engine things were becoming worse. I started to suspect something had seriously got wrong with the engine rebuild.
Fortunately, last week during another attempt to tune up the car I noticed that when the brakes were applied in neutral revs were increasing by 100-150 rpm. I tried to move the shifter to position 4 or reverse the car started to work rough..... released the brakes engine started to run better. My mind went to the brake booster (servo). I checked for leaks at the manifold and servo connections, the servo vacuum hose and check valve having been replaced few months ago. No leaks. In the course of the next few days things became really bad to the extent that the car would stall with gear engaged when held stationary on the brakes. In neutral the car would idle ok and when brake pedal was pressed revs increased and then car stalled.
It was evident to me that the brake booster had failed internally, since brake pedal started to become hard. I ordered a new Ate T52 booster set and had it fitted today. The car runs perfectly now. The booster was the source of all my issues. If the condition of the old unit had not deteriorated so much during last week, so that it was obvious what the problem was, I would still be looking for the source of my troubles... internal booster failure which created a vacuum leak to the engine when pedal was depressed...
So another item to put on checklist in trying to sort out tuning issues.
Hope this occurrence would help some owners sort out
