Howdy,
I recently bought an 86 560SL that had been parked (neglected) for a long time. The neck of the fuel tank was rusted out and leaking and I fixed that with a radiator hose until I get a new one. The engine ran wonderfully for a while and it drove ok too.
A few days later, it really didn’t want to start and when it finally did it sounded horrible, it shook violently, and produced black smoke that left a wet black spot on the ground (doesn’t smell like gas). The spark plugs were black so I changed them (Bosch Copper) and ran it but didn’t fix anything. Actually, the new plugs turned black after a minute of running. The RPM dial dances and the engine revved by itself one time.
Also, I drained the fuel tank and put in some fresh gas just in case I put bad gas last time or something.
How do I figure out the cause? Please help

 

How long was the car sitting without running. Gasoline "varnishes" after a few months, especially if the car was left with very little gas in it. This could have fouled up so much stuff you're looking at a complete engine teardown and clean/rebuild. That's one of my first guesses, the second is that you have a misfire (also caused by bad "varnished" gas). This could be a bad rotor/cap and wires. Is there any wire arcing (spark jumping) in the dark?

 

I put new gas in it the day I bought it before I ever drove it. The gas I drained yesterday was only a week old. I just drained it because I feared I might have put something lower than 93 octane in it during the last fill up.

How do I look for spark jumping wire? could the coil be bad?