low rpm in gear

[dogdoc]

A neighbor just bought a used boat with twin 2004 Yamaha 200hp 2 strokes. I went with him for the sea trial all seemed good. A few days later he asked me to go along for the ride back to his dock after purchase and the port engine would not go over 3000-3100 rpm under load. It seemed to struggle once we hit 2700 and eventually got to 3000 . I engaged the gear shift lock out and it screamed up to 4500 instantly. Nothing could get it up to cruise rpm under load however. He called seller when we go back and was told it has occasionally happened to that motor and his mechanic said it was a sticky neutral safety switch. The boat has been used recently and seems well maintained, but safety switch that does not sit well with me. So the problem has occurred before, the problem self corrects each time, nothing has been repaired or replaced related to this problem.
Any thoughts
thanks

 

[seasick]

Sounds more like a clogged filter, perhaps the VST screen.
The fact that the engine when not in gear hit higher revs supports the starved for fuel theory since when in neutral the load and therefore demand for gas is less.
One addition fact will help; Did the motor stary at a set rev number or did it drop and surge, perhaps repeatedly?

 

[dogdoc]

there did seem to be some variation in max rpm, and it did vary how long it took to get to max rpm, but did seem to stick at max rpm when it got there. did not seem like a computer limitation to rpm to me

 

[seasick]

You can futz around for weeks trying to identify the issue but the first step that should be done is to measure fuel rail pressure while under load. That can be done on the water of course but also at a slip assuming the boat is well tied down. Fuel pressure specs are in the service manual but in general, the pressure should remain constant with very little swing.
If pressure is good, you then need to look for spark issues, also fairly easy to check with a spark tester. Check for loose/bad plug wires and of course fouled/bad plugs.
Next steps are a bit more challenging; check throttle linkage to make sure that the throttle cable, linkage and throttle plates are moving as far as they should.
Another possible issues is a bad throttle position sensor. Then the injectors should be checked at least to see if they are getting powered ( listen for the clicks)
Note that there are several filters that can be bad. Some as easy to check, others not so easy.
There is probably no substitute for a good mechanic. If it is not something simple, leave it to the pros.

 

[Uncle Joe]

I can't offer any possible solutions except to say that any "self correcting" issues I have ever had with an outboard...or car....or water heater....or whatever.....at some point will stop self correcting and at the worst possible time. I wouldn't be going anywhere until the issue is resolved. And shame on the previous owner.

 

[seasick]

As long as the seller didn't lie, let the buyer beware. A seller has no obligation to disclose problems unless asked.
This situation probably would have shown up during a sea trial.
Regardless, it's probably not a terrible problem.

 

[Crazyfish1]

Sounds more like a clogged filter, perhaps the VST screen.
The fact that the engine when not in gear hit higher revs supports the starved for fuel theory since when in neutral the load and therefore demand for gas is less.
One addition fact will help; Did the motor stary at a set rev number or did it drop and surge, perhaps repeatedly?

 

[Crazyfish1]

I would check the fuel lines sounds like a line is collapsing or the fuel line is getting hot and causing it to vapor lock

 

[Crazyfish1]

I would check the fuel lines sounds like a line is collapsing or the fuel line is getting hot and causing it to vapor lock

 

[Crazyfish1]

Let me no how he makes out Thanks