Tohatsu/Nissan 6hp revving and lurching over half throttle

[gggg]

I picked up a 6hp Nissan 4 stroke that looked to be in great shape. I had an issue with idling, but a carb cleanup had it in business.

Then I got on the water and found I have no top end power/speed and anything over half throttle causes some weird revving and lurching of the motor.

I hoped it was a spin prop, but no such luck. Any idea what could cause these symptoms? Lower unit gear issue maybe? Or a fuel issue?

I’m handy with a wrench, but pretty new to outboards

 

[pvanv]

What is your actual model? What method did you use to clean the carb? How did you rule out a prop issue?

 

[km1125]

If the "revving and lurching" is really surging (speed up/slow down) then a frequent cause is lack of fuel flow. Same with "no top end/power". How's the fuel filter (both in-tank and in-line) and hose connections? Any chance you have something clogged or letting some air in the fuel line?

Also, when you had the carb apart, are you sure the float was moving freely and allowing the needle valve to open all the way? Did you remove the main jet and emulsifier tube when you cleaned the carb?

Also, what kind of vessel is this on? Is it possible you're overpropped?

 

[gggg]

The model is NSF6BS. I have it on a 10ft inflatable zodiac.


I disassembled the carb and sprayed everything out. I'm pretty sure I got to everything. I did spray out the inline fuel filter, but not the tank filter. I assumed I wouldn't get such a good idle if it was a fuel clog or air leak (but I am new to this stuff). I sprayed around the needle valve, but I didn't take it apart. I didn't take the fuel pump apart either.

I tested the prop by buying a new prop, I was overly optimistic on that route I guess!

It's hard to say what exactly is happening with the surging. I want to say it's more of a loss of power, and then a sudden lurch forward.

 

[km1125]

Are you sure you're not just ventilating the prop? If you're using the stock pitch prop that could easily be the case. I'd definitely recommend the 'mouse ears' high thrust prop for your application. I was using the same setup and was very happy when I went to that prop, especially when the boat was loaded down.

If the engine RPMs surge but the boat actually slows down, then the engine RPMS drop and the boat speeds up, that's a classic symptom of ventilation.

 

[Sea Rider]

With the motor revving evenly and strong throughout the whole throttle range and before playing with props, check at which lower leg height is the water flow passing by. Ideal should pass right under the upper plate as in pic, that's with motor trimed to 90 deg and deck load evenly distributed. With how many souls do you usually go boating with ?

A zodiac is a very light vessel, with the current factory delivered prop should perform fine provided it's not heavily overloaded and the lower leg has made a spot on motor/transom match for top prop thrust...

Happy Boating

 

[pvanv]

The model is NSF6BS. I have it on a 10ft inflatable zodiac.


I disassembled the carb and sprayed everything out. I'm pretty sure I got to everything. I did spray out the inline fuel filter, but not the tank filter. I assumed I wouldn't get such a good idle if it was a fuel clog or air leak (but I am new to this stuff). I sprayed around the needle valve, but I didn't take it apart. I didn't take the fuel pump apart either.

I tested the prop by buying a new prop, I was overly optimistic on that route I guess!

It's hard to say what exactly is happening with the surging. I want to say it's more of a loss of power, and then a sudden lurch forward.

You did not clean the carb properly.
Put a tach on it and see what rpms really are, and if you are revving up as the boat slows as KM suggests.