1986 evinrude 150 engine woes,got some questions

[mushman]

I put the boat in on fri,ran great on muffs, took it out last night ran good, no issues.Today we went out opened it up a little more ran great till I let off the throttle a little, then I noticed it was bogging down every once in a while.I opened it up again and it really started to bog and run rough.I noticed the rpms were getting lower and lower, if I pushed on the throttle I got no power out of it.I headed back to the marina at around 2700 when I hit the wake zone at low rpm the engine was shaking alot more than usual.
Yesterday morning I changed the fuel water separator, changed the plugs from ul77v gapless to jjc4 gapped at .30 inches, put in 5 gals fresh fuel on top of the 5 I put in thu, there's about 20 gallons old fuel in there too.
I ran a compression test back at the dock and all cyl are @ 90-94,spark plugs had some w oil, some dry, and one w black crud on it.She starts up easy,but runs rough as I said before.I mix gas and oil, but it still goes through the vro.
Any ideas?.
I have a can of valvetech decarb that I bought last season should I give it a try?, I mix omc decarb(since the engine was rebuilt in 2001),and the green seastar or whatever it is ethanol treatment on every fuel up,along with the evinrude 2 stroke oil.