90 560SEC, Whats advancing ignition timing?
 

Hello all, my 560 has seemed to advance its ignition by itself overnight. When trying to start she dosent fire until the key is released and then she backfires turning the engine in reverse blowing exhaust fumes out the air intake. She eventually starts after about 20 attempts and 5 when hot, then runs quiet well except under heavy acceleration when she stumbles.Have replaced fuel pumps and filter, rotor,cap, leads and plugs. The plugs are very white which seems to indicate advanced ignition. Anyone got any ideas what has caused this.??
Thanks in advance
Mike