bizarre fault need some help please mercury 50hp blueband

[suzukidory]

hi there i just replaced the mid section on my 80's mercury 50hp blueband electric start [power trim and tilt
and tonite was my first river test and things are not good i replaced all gaskets when rebuilt mid section i changed impellor and serviced engine the fault i have is
the boat starts idles no problem at all and can be run on splash muffs no problem it revs up nicely pumps water with a strong jet everything is fine on splash muffs
when it was in the river tonite when i start it up with choke or without when warm it doesnt matter if i cruise out of the marina at low revs the engine bogs down within 30 seconds and wont rev up at all
but if however i turn the key off wait 5 seconds and then resart engine and immediately go too full throttle flat out it will continue to run at full speed however as soon as i decrease revs it bogs straight away and cannot gain power without turning off waiting 5 seconds and then hitting full throttle again so my friend and i just came back from river test doing 35 knots for 6 miles on a restricted 5 knot speed limit river not good !!!
the engine itself ticks over beautifully and sounds perfect at full speed not missed a beat all night !! can it be anything serious please help

 

[suzukidory]

Re: bizarre fault need some help please mercury 50hp blueband

seams like an exhaust / back pressure fault maybe ??????????????

 

[suzukidory]

Re: bizarre fault need some help please mercury 50hp blueband

its definately not a carb problem !! changed the carbs today for a good pair of tested carbs and fault still present it seams to me as if the exhaust isfilling with water and only runs at full revs from immediate start because it blows the water back out under full throttle does this make sense >??????????????????? help wanted its not good messaging to myself