Mercury 50HP 4-stroke power loss

[Steve6964]

I have a 1999 Mercury 50hp Bigfoot 4-stroke outboard(carborated). The motor is on a Myacht 35ft houseboat. While running the motor at WOT(3600rpms), the power dropped instantly to about half. Could only get about 2000rpms at WOT. Mercury mechanic checked the following and found no problems: ingnition timing, spark plug wires, replaced spark plugs, electronic ignition and checked compression. Also, I connected a fresh tank of gas to rule out the possibility of water in the built in gas tank. I also squeezed the primer bulb while under load & had no change in power. The engine idles and revs up okay while in neutral, but when under a load it has no power and misses alot. Could this be an electrical problem of some sort? What is the “switch box” I hear everyone talking about? Could the motor be going into some electrical failure only when it is engaged into gear? Any solutions would be appreciated.

 

[Steve6964]

Re: Mercury 50HP 4-stroke power loss

Also, I've cleaned the carbs with spray cleaner & changed the inline fuel filter.

 

[jim dozier]

Re: Mercury 50HP 4-stroke power loss

When it is on the water and under load start unplugging spark plug wires one at a time and verify if you are running on all cylinders. It sounds from your description that something is wrong with your ignition and 1 or more of your cylinders is not firing consistently.<br /><br />While you are out there try choking it when it is acting up. Or pull off the cover and any carb airboxes and squirt some gas into each carb one at a time and note the effects, if it picks up any that cylinder is lean. Someone on this board also suggested you can put 2 fingers in the throat (partly blocking the airflow) which will richen the mixture in that carb selectively.<br /><br />Also you can't clean the carbs with spray cleaner unless you removed them, disassemble them and sprayed them with cleaner and compressed air. But I bet they look nice.<br /><br />Also, despite the fact that the mechanic was said to have checked the ignition timing I would verify that the ignition is advancing with the throttle under load.