My '68 111 280 SE engine shakes between 800 to 1100 RPM in park or neutral at idle and is most noticable when warm. When in drive, the shake is more subdued. I have the idle speed set just at 750 in park and 850 in drive. If I adjust the speed below 800 RPM, the engine smooths out with just a little twitch but is now too low and could stall. If I adjust it higher than 1100 RPM, it runs very smooth but is hard on the transmission when I shift into gear.
I have gone through just about everything I know to do as far as tuning and the mixture seems to adjusted correctly when I do the split linkage test. The spark plugs also look good.
My current thinking is now that the problem is with the engine suspension. I have examined both engine mounts and nothing looks seperated and the rubber is resilient. The rubber is bonded to the engine bracket screw and to the sides on the metal plate. The is an arc in the middle of the rubber that is not bonded and that is where all the engine movement takes place. It almost looks like a rubber leaf spring. Does this sound correct?
If it does act like a spring, then you should need to dampen it. I examined the vibration absorber on the front right of my engine and it appears to be original. With engine running and shaking at 850, the absorber DOES NOT MOVE AT ALL. I would have thought with the engine shaking that much, it should move some. Could this be the source of my problems.
I have never seen anything written about this absorber and wonder what is supposed to do. Thank you for your input and sorry for the long post!