Yamaha help

[colayn]

Bear with me while I share the details. My boat has been in the shop for a month and still no luck.
1999 Yamaha 150 Vmax TLRX Bought the boat in May and it ran great. I never turned it more than 5600 RPM, but it has some throttle left. (maybe slightly under propped)
Around July it lost top end power. Stall and shutter coming out of the hole and once on plane would turn about 4700 RPM tops
I was convinced fuel problem. New fuel pumps no help. I went thru all the carbs and they were in excellent condition, afterward no change. Checked everything from the bottom of the fuel tank to the intake and all checked OK.
Take it to the doctor and he said someone put in the wrong plugs. They changed plugs, decarbed engine and I picked it up.
First time out it would turn 5600 tops, that lasted about 10 mins and then fell back to around 4800.
comprssion is 120 on every cyl except one which is 115

Shop has made one other attempt by changing carb float height and this did not help any.
Been sitting at the shop every since.
I really appreciate any help.
Thanks in advance