I had a similar problem in one of my 350 MPI's. I see you already changed plugs begining of the season but to me seems like a spark issue. Do you have a bad plug wire, cap or rotor? IF you do you could have easily fouled a plug, Are all the plug wires on the right plug? I would start there, your about due for a total ignition tune up anyways and it would eliminate many possible problems. In my case I had the same problem, I changed plugs wires and it took care of the problem. Good luck!

Another thing to mention, you are way overpropped if you are only hitting 4100 rpms at WOT. You are lugging the engines and putting too heavy of a load on them.

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Originally Posted by Griff (Post 2980446)

Another thing to mention, you are way overpropped if you are only hitting 4100 rpms at WOT. You are lugging the engines and putting too heavy of a load on them.

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I agree. To turn higher rpm's with only 300HP/ 350 MPI and 26P props , the drive should have 2.43 D/R . He probably has the 2.00 D/R hence the low 4100 with the duo prop setup. With the 2.43 ratio holeshot would be better too on a heavy cruiser .

I figured 4100-200 is low for my boat/engine. Not sure what D/R i have. For 10-11K lbs, I top out at 48-50mph which is the specs for my boat.

As far as the impellers, they have about 100 hrs so it is time for replacements.

As far as the engine and tuning, it does not knock or mis-fire, so not so sure about that. Again this happened one afternoon after we had been around the lake, sat for a bit, then on the way back to the docks, it would not go past 3200rpm. I will check everything this week.

theres two fuel pumps one down low they get weak, and that will cause the same problem. low fuel pressure