I have been restoring 454 Mag (385hp) on Formula 252. Recently serviced injectors and practically entire fuel system. Topped out prior at 55mph with 1/2 tank. Last time I took it out kept up at WOT with some Skaters going slow. Took it out today and I cant get passed 48mph and the last finch or 2 of throttle lever and she bogs. Cruises fine at low 30s at 2800 to 3000rpm and low 40s at 4000rpm but after around 4400rpm it starts to bog. I did get fuel at my marina the first time and did not add any octane boost. 91 octane.

Any ideas on what too look for. So far just this smooth idle and cruise.

Any insights.

That can be a classic symptom of a fuel supply issue. Clogged filter, pinched hose, etc.

Or fuel pump!
as mentionned check for basics if nothing is blocked then first thing to do is buy a fuel pressure gauge for 100$ or less and test when wot
as soon you feel it starts bogging check pressure and if low back off the stick otherwise it may cost an engine rebuild!

You are not getting fuel. Pump would be a starting point but others have had good suggestions too (lines, pick up, filters etc)

Will recheck everything. We already replaced fuel pump, filters, serviced injectors, and cleaned lines and checked fuel pressure at raiil.

On a separate note for the 454 Mag I have been using 91 fuel bit is 87 or 89 okay for these motors stock. Been reading that.

Since it started bogging at 4000, have you checked fuel pressure at rail, under load at 4000? static idle pressure can be fine then drop under a load, I think bad fuel regulators on these aside from sticky injectors ares most prevalent cause of bogging!

I haven't checked pressure a 2nd time but when we did trouble shoot initially we took it out on water and tested.

Thanks. Dont recall if we touched fuel regulators.