i own a 19'hurricane deck boat powered by a 1995 yamaha c115 tlrt since 2 years.i start to loose a little power a few weeks ago.i first replace plugs,still the same, it seems like the cylinder number 4 is working less than the others, when the engine is idling,i disconnect the spark plug wire on the cylindre 4, the rpm drop just a little bit, and a lot on the others cylinders.it seems like this cylinder is working but less than the others, right?
before i was able to spin the engine 5300 5400 rpm, now no more than 4900 5000 and the engine sound a little different.
i tried to switch coils, still the same cylinder missing(there is good spark light).
i tried another used cdi unit, still the same cylinder.i checked the compressions 2 times, 120# on all the cylinders.i pulled the heads and exhaust plate for carbon build up and cylinder scuffing : everything like new inside.
light head shaving and everything back together with new gaskets, still the same cylinder missing half
pulled the carburators, cleaning very well all the canals specialy the concerned cylinder, pulled the reeds valves off for checking (like new) ,switching the reeds blocks starboard to port , everything back together with all new gaskets , still the same cylinder missing half.
when i check for lean or rich combustion, the color of the plugs after a 1 minute full blast ride is brown light on all the cylinders.
when i try to "play" with the mixture screws, the rpm at idle is changing on the 3 cylinders working but not on the #4.
the only thing i did not try is to switch the carburators
do you have any idea to fix my probleme?

excuse my french-english