GT-40 RPM issue

[DP70]

Hey there - a camp I volunteer at has 2 bubble butt Nautiques w/GT-40 engines that are having RPM issues.
Boat 1 is a 99 and has PP from factory, but turned off.  It gets up quick and runs nice but rpms top at 3600.
Boat 2 is an 01 Air model so no pp.  It doesn't come out of the hole as well as boat 1 and it's topping out at 3100 rpms.

Boat 2 in particular, when the throttle is pushed past the point where 3100 is initially reached feels like it runs a little rougher but won't give any more than 3100 - it may even fall off a couple rpms.

My personal boat is a 94 Tige w/Mercruiser, so I'm not super familiar with the PCM, but to my recollection they ought to turn in the 4200-4400 rpm range topped out, yes?

If you have any experience with this and could point us in the right direction to rectify this issue it would be greatly appreciated.  Thank-you.

[jhughes]

Could be a faulty oil pressure switch or water temp switch, both are tied to an RPM cutout system (actually called the SLOW system, look it up if you want more info) to protect the engine. The bypass for these two switches is simply disconnecting them since an open circuit is "good/normal" for both switches. One is located above the oil filter on the port side of the engine, the other is on the water circ pump on the front center of the engine. Oil switch commonly goes out.

[LeonL]

Both boats, similar symptoms?   Check/replace your fuel filters.  Sounds like you got some dirty fuel.

[DP70]

Just now, LeonL said:

Both boats, similar symptoms?   Check your fuel filters.  Sounds like you got some dirty fuel.

That was the first thing we did - swapped out the low pressure pump and replaced the filters.  No change in symptoms.  Should have stated that in original post.

Going to check the oil pressure switch as Jhughes suggested

[LeonL]

I thought that with a low oil pressure or high temp the boat went into "limp mode" and wouldn't run above idle.

[jhughes]

GT40 Service Manual

[thager]

What props are being run?

[TomH]

Slow/limp mode cut-out is more around 2000/2500 rpms.  This sounds more like poor running.  If you don't know the last time on general maintenance items, I'd be looking at general tune-up items (plugs, wires, cap, rotor, filters, etc.).  Fuel lines could be gummed up/collapsing too if they've run ethanol over the years.  Throw a fuel pressure gauge on the rail to see if the pressure's dropping when it stumbles/limits out.  May need a high pressure fuel pump.

[buechsr]

2 hours ago, TomH said:

Slow/limp mode cut-out is more around 2000/2500 rpms.  This sounds more like poor running.  If you don't know the last time on general maintenance items, I'd be looking at general tune-up items (plugs, wires, cap, rotor, filters, etc.).  Fuel lines could be gummed up/collapsing too if they've run ethanol over the years.  Throw a fuel pressure gauge on the rail to see if the pressure's dropping when it stumbles/limits out.  May need a high pressure fuel pump.

this

Just now, buechsr said:

this

but if it has an FCC that filter needs to be done too

[DP70]

FCC filter is new for both boats
Both boats are EFI

Will check the rail pressure while underway and see what that shows

I recommended they change the FCC to the replacement model that has the built in low pressure pump for the next season, but that may need to be done sooner than later.

Boat 2 has a lot worse fuel consumption than the other boat