89 yam 70hp

[boater21]

My 89' 70hp yam is missing at full speed and I know nothing about the motor, I bought new plugs hoping it was something that easy. It starts fine and runs fine except at full speed and only gets around 3400 rpms any ideas before I have to bring it in.

[boats.net]

Check compression, could be dirty carbs or ignition breaking down, might try cleaning carbs first.