305 mpfi engine miss

"I own a 2004 Stingray 240CS 3

"I own a 2004 Stingray 240CS 305mpfi 260 hp. Late last season I noticed some performance losses (50+ mph @ 5000 rpm to about 45 mph @ 4500rpm). I then developed an engine miss right at 4400 rpm and continues to miss at WOT. Being late in the season I winterized the boat, as with every year using Stabil in the fuel system.

This season along with normal servicing I have replaced the dist cap and rotor because of corrosion, the spark plugs (they looked perfect), compression check all cycl were mid to high 170's, and fuel additives out the "ying yang.

Upon the request of our local Mercrusier dealer I replaced the water seperator(again) and dumped a bottle of "ValveTech" fuel restorer in the fues system. I ran the boat 2 weeks ago and at first it came up on step fine and ran up to 4700 rpm for about 2 or 3 minutes and all of a sudden the miss came back. All the local Marina's have stated fuel problems have pleged everyone this year. I am a hobbyist and have work on Mercrusiers for many, many years. Any advice before I take it to the local dealer.
Thanks"

Have you checked compression?

Have you checked compression?
Any stored codes?
Could be a hung valve.
It will be hard to isolate which cylinder it is while operating due to the fact it only happens at higher rpms.
Does it run properly at low and mid range rpms?
High exhaust back pressure maybe due to half plugged exhaust manifolds or risers?
Is the spark arrestor clean?
Has it been over-revved or over-heated at all?

Rod

Pour 8 oz. of SeaFoam in the o

Pour 8 oz. of SeaFoam in the oil and run the boat for a few hours; repeat w/the balance of the can. Change the oil and filter. SF will clean varnish or crud causing valves to stick along w/all the oil passages.

Rod Stewart:

Welcome back!

Rod Stewart:

Welcome back! Did you retire again?

"Yes performed compression tes

"Yes performed compression test, mid to upper 170's.
The engine runs fine at idle and comes up on step fine (concidering its a mid cabin crusier). No engine miss fire below 4400 rpm.
Never over reved.
Never over heated, eng runs constant 180-190.
Not sure about the codes, do not have the previsions to read them. the dealer will be my last resort to read the codes.
Some people are talking elect fuel pump. With only 170 hours would a fuel pump start to fail so soon."