My Yamaha model 6/8 (6C), 6HP two stroke twin outboard starts OK in the start position but needs momentary choke assistance even once warm. Momentary means that I have to get the choke back in quickly before it stops. Now it will rev up but will not run slowly i.e. tick over or idle. If I close the throttle below the start position she just dies. I can't use it like it is. The motor is 6 years old but has been little used and looks like new. Any ideas please?