9.9 won't rev at full throttle

[Skidoonick]

Have a 9.9 force runs great on muffs, took out on the lake and runs and idles fine but when in gear it won't rev up. Pull motor out of water in gear and it revs fine, but put back in and same thing? Has spark, using Mercury oil w premium fuel. Any idea what else I can check?

 

[RCO]

Make sure both cylinders are firing. Running on the muffs doesn't tell you much as it isn't even close to regular operating conditions. Pull one plug wire at a time with insulated pliers while the engine is running in gear on the boat, or at least in a barrel.

 

[jerryjerry05]

Welcome.
​Year??
Check the fuel pump diaphragm.
Clean the carb.
Check the hose connections. They leak and suck air.
The primer or push button can be bad?

The linkage operating the throttle right?