GT40 Won’t rev out

[Tpoole]

Hey Guys,
I’m hoping someone can help. I have a 98 SN with a GT40.
The mnaul says rev limited to 4800 rpm but our wont go pass 4200 and seems to surge a bit once there.
this has the boat at a gps 40 mph.
We mainly slalom but sometimes have a barefoot. Being 90kg, 40mph is only just enough.

Any ideas on why the low revs?
Also, sometimes need s a long crank when hot to start.

injector issues?

thanks

[NautiqueJeff]

What propeller is on the boat?

[Rednucleus]

When did you last do a full tune - plugs, wires, cap, rotor. My buddy's Sunsetter did this and a full tune up fixed it.

[Tpoole]

What propeller is on the boat?

Prop is 12 inch, 4 blade. I’m unsure of pitch.
its is 2.8 inch from lead to tail of a blade.
looks like LECENC and 01 stamped on the hub. Or would this be OJ Legend??

[Jonny Quest]

That setup wants an ACME 422 prop. 1" bore. 12.5" diameter X 15.5" pitch. If your engine is running properly, you should get 4,800+ RPM and 44 mph+ at sea level.

[Tpoole]

Thanks guys. So probably not an engine issue?
Does 4200 @ 40 sound right for a factory prop?

[Rednucleus]

Is this a change from how it used to run? How long have you owned this boat?

[Tpoole]

Is this a change from how it used to run? How long have you owned this boat?

Crazy to say, but not sure. Had it for a few years, but mainly towing little kids, so never really paid attention to top end. Kids getting bigger now and barefooting.
Long crank when hot is a recent development

[Rednucleus]

Have you done a tune up since you took ownership - if not, I would spend my money there 1st!
How many hours on the boat??

[SilentSeven]

Items to check for a long hot crank include:

- first, do tune up as recommended. Plugs, cap, rotor, wires (particularly if never done before, the engine is 20+ years old). When you have distributor open, look at the condition of the hall sensor and advance mechanism. See if it looks dilapidated and rusty. If yes, consider a replacement distributor as a possible future troubleshooting step.
- verify engine timing is correct with timing light.

See if things improve. If not, start to look for specific problems like.

- FCC pressure leak. Takes longer for fuel pump to get rail to full pressure.
- Injector leaks. Floods hot engine with fuel.
- IAC valve sticky. Idle air control valve sticks so air/fuel mixture is off.

A tired distributor could contribute to both high RPM and hot start problems. My GT-40 ran much better when I replaced the factory distributor with a rebuilt truck version.

https://www.planetnautique.com/vb5/f...e-replacements - in this thread note comment from FMSKI on his resolution for a high RPM run issue.

[Tpoole]

Great info guys.
I did plugs and leads but will have a look at the distributor.
Mechanic said two plugs quite badly fouled. He suspects injectors leaking fuel.
have replaced low pressure fuel pump, but need to pressure test FCC.

[wake2001210]

I had similar issues with my 2001 GT40. Completely rebuilt my engine 2 years ago and put a brand new prop on. Can't remember which one it was supposed to be 1-2 less aggressive than the 1578. Just never seemed to put all the power to the prop and the switched back to the 1578 and I get full rev out of the RPM's to where now I hit my rev limiter if not paying attention