86 yamaha 200 hp question

[brianmccaw]

Hey all,
I have an 86v with an 86 yamaha 200hp precision blend outboard. Under WOT the engine sounds and runs perfectly fine. However, while at a slow idle as i increase the power the engine begins to run roughly and vibrate throughout the rpm range until i get to almost full power. All the plugs were changed and i checked for water in the gas tank. Anyone have any idea what this could be???