engine won't rev past 3400 RPM
 

My starboard engine won't rev past 3400 RPM when in forward. In neutral it will rev freely to 5600. Secondaries open and it appears there is plenty of fuel flowing. Linkages appear to be in order and full throw is being used when in neutral on both engines. Timing has been checked and it's identical to the port engine which works just fine.

Some added into...the throttles and shifters have been reconfigured on this boat from stock Fountain configuration. the sticks are from left to right...port throttle, starboard throttle, port shifter, starboard shifter. Port shifter engages forward and reverse concisely but there is a "sponginess" in the action near the end of the lever travel. Similarily, the starboard throttle, once the engine reaches 3400 RPM gets the same spongy feeling in it with about 1/3 of the lever throw to still be used but it won't go past this. Looking under the helm, the way the cables are connected to the throttles differently, port to starboard.

There is 8 hours on these engines.

Is it possible the starboard throttle and port shifter cables are somehow being restricted causing this issue? Or is this something else? If so, what possibilities should I be looking for?
TIA
Pete

What engines do you have, and what ignition?
If the throttle plates are fully opening, then it doesn't matter if they feel a bit different for your current issue. May be something else that needs attention but not full throttle, but do make sure they are opening when on the water. A motor uses next to no gas or need no timing advance to rev high in neutral, it does need all the info to do it under laod

502's, Holley 850's, Edelbrock performer intake. Mercury Thunderbolt ignition

Is it TB-V with knock module?
If so, the module may be sensing a knock and retarding timing. If not need to find out if the plates are fully opening, if so then its all up to timing and fuel pressure