Mercury 125hp....no top end

[tattoo]

Hey guys my friend has an older merc motor and he says it is 125 hp...he ran it on the muffs and all is good but when he puts the boat in the water the motor wont throttle up.Im guessing its either electrical or fuel problem when a load is being placed on the motor.....it speeds up on teh muffs but once in teh water it does not....thxs

 

[CharlieB]

Re: Mercury 125hp....no top end

You haven't determined yet if it is a weak fuel supply or weak ignition at RPM.

Both problems will display your symptoms, yet allow no load RPM.

If the boat is still in the water, try squeezing the primer bulb when it won't speed up, if that improves then you may have a restricted fuel supply, weak fuel pump, or air leaking into the fuel lilne before the pump.

Also try momentarily 'bumping' the choke/fuel enrichener to provide a heat beats of additional fuel to see if the motor is running slightly lean from dirty carbs.

If NO improvement, then you need to test the high speed windings of the stator to ensure adequate voltage at RPM under load. No load voltage output from the low speed windings is enough to rev the motor, but the additional air drawn into the cylinders under load acts as a larger insulator on the spark plug, requiring more voltage to arc across the same gap.

Download the free CDI Ignition Troubleshooting Guide found thru this link

www.rapair.com

Look up your model and follow the detailed testing to determine of indeed you have a weak ignition.

You may need a DVA Meter to test stator voltages once disconnected from the ignition module.