Hmm ...

I wonder if it is running rich and has fouled the plugs.

If you are getting 59%, your mixture is rich and the lambda system is trying to bring it down.

I would start by checking the cap and rotor, putting in a fresh set of plugs, and adjusting the mixture towards lean in small steps (1/4 turn at a time, ccw) to see if you can get the ratio around 50%.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe