1993 Evinrude 120- acting fuel starved- what am I missing?

[DayOff]

I have an old Lowe deck boat that hasn't been right all summer. Motor is a 1993 Evinrude outboard. We've had this boat for about 12 years or so.

Early summer 2018- ran pretty good up and down Lay Lake in central AL. we use gas with ethanol routinely and have for a number of years (10+) without issue. This boat is used from May to Oct about 6-10 times a month.

Now it stuttered (felt like an electrical thing) a little last summer (2017), so we replaced the PowerPack then and that made it run perfect. Last summer it was cranking fine, not smoky at all.

so mid summer 2018 it would not run hardly at any speed above 3000 rpm ... but it did crank right up every time --with a lot of smoke. We MIGHT get a few minutes of good power in the boat then it would get starved down and quit.

So this summer we have tried a number of things: we replaced fuel lines and added a new fuel/ water separator filter, cleaned the carburetors, replaced the plugs, cleaned the fuel intake screen (it was NASTY...Once we cleaned the screen it was a little better), added SeaFoam to the boat tank. then finally replaced VRO fuel pump. We thought maybe the old fuel tank was full of crud so now we are running it on a portable fuel tank with fresh gas (tried ethanol free and ethanol) mixed after the VRO pump was replaced.

Now it cranks immediately and consistently, not smoky, and it runs great at about 5000 rpm for about 3 to 5 minutes then it always loses power for a few seconds then resumes full power does this over and over again till you back off the throttle to about 4000 rpm and seems to run fine, but when you pick back up to speed (5000 rpm), it does the same thing. It is fine at 4000 rpm but you cannot go above 5000 rpm. this behavior is consistent.

I hope I've explained this well, please let me know what else I might add. HELP!

[boats.net]

Run a compression test to see if all cylinders are even, try bumping choke when loses power to see if helps regain rpm's, try pumping fuel primer bulb to see if helps. Use a timing light to see if any cylinders are losing fire when it drops power.

[DayOff]

Run a compression test to see if all cylinders are even,
how would I do that, please?

try bumping choke when loses power to see if helps regain rpm's, ok will try.

try pumping fuel primer bulb to see if helps. This doesn't help. the fuel bulb is full/hard when we first take off, then it goes soft. No amount of squeezing builds that pressure back up. The bulb is soft when this happens, it's not full/hard like it should be. squeezing it rapidly doesn't do anything to fill it back up. Now that's what it did on the bulb to the main tank, I'll definitely test this over the weekend on the portable tank.

Use a timing light to see if any cylinders are losing fire when it drops power. Hmmmm....Not sure how to do this while running down the river. It cranks and runs fine sitting on the trailer with the ears on. I am not sure how to do this in motion on the water.