Yamaha F20 4 stroke tiller

[Blue_Runner]

Posting this for a friend who is having a problem.

He says the motor runs but won't run full throttle. Says nothing abnormal when running on the garden hose, but won't go full throttle on the water. It is not much but that's about all I know. He changed the plugs and says the throttle linkage looks ok. Would the next logical step be to clean the carb jets? What else should he look at?

Thanks