motor stutters after a while full throttle

[darth vader]

hi,
I have a yamaha enduro 40 hp pull start.
it`s not a new outboard but hasn`t been used a lot.
It starts fine and accellerates fine but after a few minutes going at high rpm the engine starts running bad almost stalling. there is a light on the engine that lights up everytime it stutters.
I don`t think it`s overheating. could it be faulty spark plug cables????

[jb123]

im not familiar with that model or ever heard of a light, but it could be your warning system. is it premix?? overheat should be the only issue if it is not oil injected, try to unhook the heat sensors at the top of the cylinder bank to see if the light goes out/engine stops cutting out.

[Ken]

darth,
JB is right in that your motor is probably over-heating, and going into rpm reduction mode.
And you can TEMPORARILY disconnect the themo switch from the ignition control, but just long enough to find out if that makes a difference.
The thermo switch is designed to close at around 185 degrees to signal an overheat condition. On most Yamaha outboards, this causes the alarm to sound and the motor to go into rpm reduction mode. I have seen a faulty thermo switch (on my 2000 C115) close as soon as 130 degrees and signal a false overheat condition. You can remove the thermo switch (mounted in the head assy) and test in a pan of heated water using an ohm meter and thermometer.
If the thermo switch is indeed doing its job correctly, then your motor is overheating - could be weak water pump, stuck thermostat, or faulty pressure relief valve.
Good luck [img]smile.gif[/img] ,
Ken K