low revs on full throttle

[simonc]

Hi,

I have a perfectly maintained Mariner 40HP 2 stroke outboard, and unfortunately last week I was out with the boat ( Petecraft 14 open). When I opened up the throttle to plane the boat it was impossible so I Increased the throttle until I noticed I was on full throttle however the motor was only giveing 3000 revs and 10mph. I just could not get the boat to plain. The motor sounded very smooth but low in RPM. Does anyone have an idea how much a motor like this should rev at max throttle. It was very annoying and I ended up heading back to port.

If any one has any ideas please let me know

Thanks all.

[benri]

It could be some sort of fuel restriction - Maybe dirt in the carburetor. Did you start the motor during the winter months? Could be gummed fuel too.

[Granitu]

its the carb provided you had no missing spark which hint a spark plug

[skip]

As Granitu said my money would be on the Carburettor (but do check the condition of the plugs), when you go past a certain position it probably moves from the low speed jets to the high speed jets or if it's a twin choke carb it will use a different set of jets as well for sure. Either they are blocked or not set properly.....

You can have a look and make sure the butterflies are opening completely.....remove engine cover, get to carb (which probably has an airfilter on it) and with the throttle at idle the butterflies are closed, when you go to full throttle they should be vertical at 90 degrees to the closed position.

When you laid the boat up for winter, did you remember to run the engine dry of fuel (eg. remove the fuel line) and let the engine run till all the fuel in the carb float is used up. This helps prevent gumming and varnish accumulating...it's also a good idea to run an injector cleaner through the mix at the end of the season, followed by a fuel stabliser/conditioner for storage after fogging.

[lazyfisherman]

You would have to look at the specifications for rpm at maximum throttle but for outboards of this type it should probably be somewhere around 5500 rpm with wide open throttle.

[simonc]

Thanks guys,

Just to let you know this is the first time I am using this motor and boat. The previous owner always dried out the carbs by disconnecting the pipe and letting the motor run untill it stops out of fuel after every use. The boat was used in winter too weather permitting so it was never really winterized. The motor is only about 3 years old and in mint condition throughout. I was also advised that it could be the fuel line seeps air and does not allow the right amount of fuel to be delivered to the motor.

I will have to check it on the water again after changing the sparking plugs, and fuel line,connections and bulb.

Thanks again.

[benri]

You might also want to check for a clogged fuel filter.

[fisheye]

outboards have a safety feature  that is when something is wrong like overheating they will drop the revs (usually an alarm bleep goes on as well), will still work smooth but will not let you over rev the engine just to give you enough power to reach land without doing further damage to the engine so something might have triggered this feature. Usually when there are problems with spark or fuel flow the engine will not run smooth at all.

[caldaland]

while youre at it check the air and fuel filter.

[simonc]

Hi Fisheye,

Thanks for the info regarding the overheating alarm. However when triggered, does it remain on constantly even after the motor has cooled. The low revs occured immediately, in fact I had just left port so I could not have been using the motor for more then 5 to 10 minutes. Also I never heard any beeps and before I used the boat I replaced the fuel filter, the water seperator filter and the gearbox oil. The only item I did not replace was the spark plugs, which I intend to replace now.
What I did note before I left was that the fuel bulb was not becoming very stiff before I started the motor, but as it started normally I thought nothing of it, could this point to anything.

Thanks guys again

[skip]

You've got some good pointers to try out including the possibility of air in the fuel line, but based on what you are saying, I reckon the high speed jets are blocked. I have a portable generator that has the same issue, runs fine at low rpm as soon as I flick the switch for full revs it stalls. When I took it in they told me blocked high speed jets which were replaced, so could be the same for you.

Out of curiosity, can you read off the markings on the prop for us, should be something like 10.5x12P or something similar