Merc 454 poor performance

" have a 1987 Merc. 454/340 hp

" have a 1987 Merc. 454/340 hp bluewater inboard with 400 hrs. It had had a major tune up, plugs, cap, rotor,elec. points replaced (it has thunderbolt ing.) and a new carb. all filters have been changed.
Prior to the the above,the motor would seem to hit a wall at 3,300 RPM's in the past it would run up to 4,000 RPM's the same as the starboard motor. It it is still doing the same, hitting the wall at 3,300 RPM's, after the above parts replacements. The prop. has not hit anything and both props were "tuned" last season. The starboard motor runs up to the 4,000 RPM.
The engine in question does not burn oil, or does it have any valve train or bearing noise.
The boat is run in fresh water and has closed cooling. I also replaced the exhaust risers on the fresh water side on both motors this season. The motors run at normal operating temp.
Any thoughts on what to do next?

Thanks,

Mike "

Do an easy carburater check b

Do an easy carburater check by swapping port and starboard out. At least this would eliminate a carb problem.

" Dennis,

the carb is brand

" Dennis,

the carb is brand new "

Do a compression check. also

Do a compression check. also these motors have problems with water getting into the exhaust & causing damage. It's winter now look inside the heads for rust. also any excessive oil usage for this motor?

i have a 74 chev pick-up and m

i have a 74 chev pick-up and my son-in-law removed the spark plug wire without marking the wires and now i don't know where they go can anyone help me?