Suzuki DT30 not running well on water but perfect on muffs

[Time To Boat]

I have a DT30 (year 1991) oil injection that runs perfectly on muffs, but when I put engine in the water, it runs not well at all:
Symptoms:
-not getting to high RPM at all
-more difficult starting, almost not staying on idle (shuts down)

Already done:
new sparkplugs
clean carb's
new fuel

What more to do?
(I suspect that it might be a problem with waterpressure "on" the exhaust while in the water, but do not know what to do...
Any help?

 

[99yam40]

have you checked the compression?

need good compression,good spark at the correct time(timing) , and correct air to fuel ratio.
test what you can and then work on the fuel if needed

carbs, reeds, vacuum leaks or flooding(too rich)