BMW K100RS Will not tick over and power dies when starting to cruise?

I have this 1990 16 valve k100rs i purchased it some 2years ago. The bike had only done about 200 miles in the two years before i bought it. I have been unable to get the bike to tickover unless the choke is out. When you ride the bike it has good pulling power but as soon as you start to cruise the bike dies. It also dies when you change gear either up or down. I have changed the spark plug and put clean fuel in the bike.