Bogging at Speed 2008 Mercruiser Alpha 1, Gen 2 - 5.0L, 220hp, V8

[Grant Brown]

2008 Bayliner Discovery 210, Alpha 1, 5.0 220hp
I have a situation where the boat is very slow out of the hole with the leg tucked in. It gets up to about 36 mph at WOT and then after about 1-2 min, will surge ahead and gain another 7 mph up to about 43. If I stop the boat and idle awhile it will repeat the process. Seems like fuel to me.
The boat was not well maintained before I got it and sat for a year. I have not yet run a full tank of gas through it so I know it's early days. \
I am thinking possibilities are:
- Clogged fuel filter
- Gummed jets in carb
- Clogged fuel vent?
Anyone else have any suggestions?

 

[Grant Brown]

Replaced fuel filter today, no change.

 

[Grant Brown]

I took my boat out tonight after having been away for 5 days at a conference in Portland. The boat has not been run in that time. I warmed it up, idled about 10 min through a no wake zone, dropped the leg right down and hit WOT. The bow came up and she surged and slowly came up on plane. I mean slowly. I heard it cough or backfire a couple of times too. The boat and rpm increased slowly almost as if it was in steps.
I am planning to hook up the 5 gal tank from my Lund and 30hp Honda outboard to the Mercruiser to see if the fuel starvation is in the main tank or something else.
I intend to replace the water/fuel separator filter again and clean the inlet screen (again) in order to eliminate them as a possibility.
Two questions:
1) How would I best install the external tank? My plan after looking at it tonight is to use the Honda primer bulb and hose. I would remove the outboard fitting end and simply work the hose over the barb on the Mercruiser water separator fuel filter. BTW - I will have 2 fire extinguishers with me. I don't screw around with gasoline and intend to take every safety precaution possible.
2) Do electric fuel pumps usually go bad slowly or do they just quit? It occurred to me that maybe the fuel starvation was a result of a bad fuel filter not being able to keep up. The engine fires and runs fine at idle but just does not have any power out of the hole and a reduced top speed.