Yamaha 9.9 2 stoke wont rev under load

Hi all, i am looking for some help i have a 1990 yamaha 2 stoke outboard and it wont rev under load, i used the boat for hour or so and ran purfect, then just as i was coming in it wasnt picking up on the plane as quick, started misfireing on idle then would cut out when throttle was released off full when it was ingear and taken a few pulls to start again, fuel filter ect all looks clear. I taken spark plugs out and one looks as it should and the top one looks all black as if its been running on one cylinder? It has 2 coil packs Would rev fine in neutral but not ingear.. Just trying to save some money and not take it to the dealers!!

Many thanks

Regards Tom

All motors will rev in nuetral with a dead cylinder -----------Try a new sparkplug it's cheap.---Thermostat in place and working?-----------Check for spark that will jump a gap of 3/8" on both wires , with a snap you can hear.----------Compression test is next after that.-----------Water pump impeller replaced recently ?

Hi there i will try a new set of plugs in her this week, she had a new impeller at the end of october last year. The spark on the not so good plug is jumping around and the one on the good plug looks as it should be...

Regards Tom