Loss of RPM on Mercury 50 hp

[scott0530]

I have a 2005 Mercury 50hp on a pontoon. Aside from another problem with picking up trash, I also have a RPM problem. In idle and take off, the motor will go up to 5000 RPMs but after about 40-50' the RPMs drop to 3200 and will be consistant after that. Any ideas? I have replaced the water pump, thermostat, rebuilt carbs, replaced spark plugs, and new lines from gas tank to motor.

 

[CaptnKingfisher]

I'm still a novice myself but if youre confident in your carb rebuild and having cleaned out the jets, the next thing I would do it run test it and check to see if all the cylinders are still firing after the RPM drop. The safest way to do this would be if you can recreate the condition on a fast idle in neutral.

>Use a heavily insulated pair of pliers. pull the spark plug wires independently while motor is running (with RPM loss). If pulling a plug on one of your cylinders does not result in the engine shutting down or losing even more power, in otherwords - you pull a spark plug wire and engine remains running at same RPM, then that cylinder is not firing.

Like I said, I'm no expert mechanic, but a steady and predictable drop in RPMs like that makes me think a cylinder is losing spark. Then youd want to inspect your plugs, coils, powerpack, etc.

 

[scott0530]

Thanks for the info. I will try that

 

[Silvertip]

Your statement that AT IDLE and take-off RPM goes to 5000 rpm. They better not be 5000 rpm at idle. In fact you should see a gradual increase to 5000 rpm as you push the throttle forward. If rpm jumps to 5000 rpm the minute you move the throttle then there is an electrical issue with either the tach or the tach sense output from the engine. You need to be a little more clear about this.