2006 15 hp Merc 4 stroke runs erratically

[EdE2]

My 15 hp Mercury will start easily but it runs very roughly most of the time and with considerable power loss. Occasionally, it will kick in and run like a champ with normal power. It never stalls. I had the thermostat and fuel pump replaced last year. Any suggestions on what could be wrong and how to fix this? My apologies if this question has been answered on any previous posts.

 

[guyjg]

Start w/new spark plugs and verify that each plug is getting good spark capable of jumping 7/16" gap. Properly burning plugs should be tan/brown not black(rich mixture) or near white (lean mixture). Carb may need cleaning. If the plug change doesn't improve the running try this: Disconnect the fuel line and let the engine run the gas out of the carb and stall. Disconnect the fuel line from the tank and drain the line. Connect the line to the engine and stick the loose end into a can of Seafoam and pump the primer bulb until it fills up the carb. Reconnect the fuel line to the tank. Let the engine sit overnight or 24 hrs. Start the engine and let it run while it burns off the Seafoam...it will smoke intensley but won't hurt it. The idle and throttle response should improve greatly. If not the carb may need rebuilt. Dump the rest of the Seafoam in the fuel tank per instructions on the can.

 

[EdE2]

Start w/new spark plugs and verify that each plug is getting good spark capable of jumping 7/16" gap. Properly burning plugs should be tan/brown not black(rich mixture) or near white (lean mixture). Carb may need cleaning. If the plug change doesn't improve the running try this: Disconnect the fuel line and let the engine run the gas out of the carb and stall. Disconnect the fuel line from the tank and drain the line. Connect the line to the engine and stick the loose end into a can of Seafoam and pump the primer bulb until it fills up the carb. Reconnect the fuel line to the tank. Let the engine sit overnight or 24 hrs. Start the engine and let it run while it burns off the Seafoam...it will smoke intensley but won't hurt it. The idle and throttle response should improve greatly. If not the carb may need rebuilt. Dump the rest of the Seafoam in the fuel tank per instructions on the can.

Thanks, I will try your suggestions and let you know how they worked.

 

[racerone]

If there is a sudden power loss, and if it " kicks in " to restore full power it is likely an ignition issue.--------Check all electrical connections and plug in connecters.-----------If it happens often , then running with a timing light hooked up may show which cylinder is affected.--Carefully look at sparkplug wires and boots.--------How many hrs on the motor ? Salt water operation ?

 

[EdE2]

If there is a sudden power loss, and if it " kicks in " to restore full power it is likely an ignition issue.--------Check all electrical connections and plug in connecters.-----------If it happens often , then running with a timing light hooked up may show which cylinder is affected.--Carefully look at sparkplug wires and boots.--------How many hrs on the motor ? Salt water operation ?

I would estimate that it has about 250 hrs. on it and I do run it in salt water. It sits on a mooring from May until October. It generally does not loose power once it has gained it but if the motor is run at low speed it will start running rough again and may not regain power.