How can I fix my 1985 K100 which pre-ignites?

I have had this BM for about 22 years and the other week when I took it out for a ride, the hazard lights activated by themselves. It wasn't until I stopped the bike - turned the key off, and the bike kept turning over the starter motor. Even replacing the key in, didn't stop power going to the starter motor. The only way i could stop it was to disconnect the battery. I have since looked over the bike, and even bought a new battery, thinking that may be it, but I haven't been able fix the problem. Any help????