Rev drop on 40hp Yamaha

[roji]

I have a 6h4 40hp yamaha. 3cyl, oil inj,.elc start. Some times it runs at full revs other times about 1/2 speed, runs steadily at either position. New plugs, have done tanks.new fuel, fuel line new. bowl filter cleaned. Doesnt change revs by squeezing bulb. Would appreciate some help, have manual and tools. Jim

[jb123]

clean your carbs.

[Bay Catt]

Try a de-carbonizer treatment.