F6 Yamaha Backpressure Issue???

[Jim Hawkins]

My 6HP Yamaha runs great in the barrel, in gear, out of gear and throttle up but when I put it in the water on the boat it won't throttle up unless I remove one of the rubber plugs in the midsection where you access the shift linkage which doing so seems to let the exhaust breath.
The engine is Not too low in the water.
Anybody deal with this?
EDIT - I poured water into the hole with the rubber plug and it ran freely out the bottom end with seemingly no restrictions.

 

[99yam40]

check compression, sounds like it is having a problem with the water back pressure when dunked(submerged)

 

[Jim Hawkins]

check compression, sounds like it is having a problem with the water back pressure when dunked(submerged)

I will do that although I want to note that with the rubber plug out the motor runs just fine, gets my little boat up on plane. I would leave it at that but the sound of the exhaust bubbling out is annoying.