2000 7.4 Mercruiser MPI won't reach full RPM

[B BARKER]

My good friend has a 2000 240 sun deck w/bravo 3 drive 7.4 mpi that won't reach but 4,000 rpm.Any one have any ideas ?

 

[Espos4]

Lots of ideas, but More information would be really helpful.

How long has he had the boat?
Is this a new condition?
Has anything been done so far to correct the condition?
Do you have any maintenance history On the motor?

What is the condition of the props and hull? Any damage or excessive growth?
Does it have the proper pitch props on there?

 

[B BARKER]

He has had the boat for about three years,middle of last summer he changed the gimble ring and short shifter cable. The hull is in good shape fresh water boat prop is good no dings or breaks it would run 5200 before he changed the gimble ring and now it won't go over 4000.

 

[Espos4]

4000 rpm could be from 1 cylinder misfiring. Maybe the plug wires were disturbed or damaged while accessing the lower shift cable.

A good visual inspection is needed there. It may just need a good tune up, plugs, wires, cap and rotor if they haven’t been replaced recently. I assume that year 7.4 has a distributor??

A fuel pressure test (while running under load) would be a good diagnostic test.

Getting more in-depth, next would be a compression and leak down test if the previous attempts haven’t helped. He could even do this while the plugs are out for the tuneup.

 

[Skuza]

Cap, rotor and plugs did the trick for us when it wasn't getting up there. Also try running the tank down as low as possible, change fuel/water separating filter and a fresh tank of non ethanol gas from a busy station. These are simple/ cheap things to try first.

 

[Lazy Daze]

it would run 5200 before he changed the gimble ring and now it won't go over 4000.

All good advice above. The statement quoted above jumps out at me as it sounds like the problem popped up right after replacement. Like mentioned by the other guys, first thing to do is go back and retrace everything he did. Remove plug wires/inspect wires & plug tip/reseat wire - making sure they are on the proper plug. 1,3,5,7 are stbd side - many caps have numbers printed on them (or google for cap firing order).

FYI... just being a fresh water boat doesn't eliminate growth. Can you clarify if the hull is perfectly clean?

Edit: Oh, if all of this is good, the next thing I would do is look at fuel quality/water sep filter... and check the fuel pressue.