yamaha 15hp tiller twist wont go to full throttle

I got a motor that is a 90s model. The twist throttle will not go all the way to full throttle. I know this

1. from the sound of the motor
2. the diagram on the twist handle

Can you guys get me on track with what this could be? Perhaps a common issue?

I was trying to set the timing on a friends 15 yammy and noticed the same problem. Reading the manual I'm thinking that it wasn't going to full throttle because it was in neutral and there is a mechanism to stopping it over revving in neutral. I'll test this theory tomorrow and let you know what I find out. If yours isn't revving in gear there is a chance it may be a fault with this same mechanism. Failing that there is a chance that the teeth on the two shafts where the tiller bends are misaligned. This would mean undoing the two bolts on the cowling half to separate the teeth and let you re-align the two halves of the throttle linkage. This probably means turning the tiller end back towards idle so that it has more travel again. There are also stop screws under the cowling for idle and full throttle.

Stuart