86 yamaha pro v 150 low rpm

I have a 86 yamaha pro v 150 on a 19ft 4winns and the rpm will only reach 4100-4200 with engine trimmed up.when I got the boat it ran 5400 rpm at 47mph gps and now it only reaches 36mph..I checked compression and 5 cylinders were at 95 and one was at 50 so I pulled the plug and used some seafoam in the cylinder let sit for awhile and checked compression again and it was 90-95 on all 6 cylinders.its has spark on all 6.i checked fuel and seems ok I pumped the primer bulb at wot and no change and I changed fuel filter and oring.i changed shift and throttle cables..I'm lost on what to do next and I can't afford to take it to a shop to be serviced does anyone have any ideas what to check next thanks,mike