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Motor loses power at full throttle after 10 minutes of use

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Fishing Mike

New member
I have a 2010 Mercury 25hp 4 stroke that I have been having problems with. At full throttle it runs fine for about 2 miles then it loses power while the throttle is at full speed. After a minute of idling, it regains power for about a half of mile. If I prime the motor while it is idling, it regains power for a short period of time. I've replaced the fuel line and connector. Does anyone know what the problem is? Any help is greatly appreciated.
 
Try a different fuel tank and hose. I've seen where the check valve in the bulb can lock up and stave the motor.

Jeff
 
Fishing Mike said:
I have a 2010 Mercury 25hp 4 stroke that I have been having problems with. At full throttle it runs fine for about 2 miles then it loses power while the throttle is at full speed. After a minute of idling, it regains power for about a half of mile. If I prime the motor while it is idling, it regains power for a short period of time. I've replaced the fuel line and connector. Does anyone know what the problem is? Any help is greatly appreciated.
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*Update*

I had a professional dealership have a look at my motor. They determined that the thermostat was stuck open. While the boat was there, I had them service the motor. I haven't had a chance to take the boat out and see how it runs, but I hope they fixed the problem.
 
It sounds like you needed the t/stat whatever happens but that wouldn't make the engine bog down like your describing. We will hope for the best for you.
 
This is July....... Stuck closed thermostat...... yes a problem. Stuck open and that causing any kind of a problem???????? Engines ran for decades without stats, just free flow cooling water. Agree with Swagonmaster on tank not venting properly.

Engine fuel pump suction is only a slight vacuum. It can only lift so much fuel, so fast. If the tank isn't vented once that small vacuum migrates to the entire fuel source (the tank volume), fuel can no longer flow to the engine. Your stopping and restarting comment suggests that the vent, and for that engine you're probably running on a portable tank, which would be in the lid of the tank, probably partially closed/obstructed...it happens. Your squeezing the bulb and getting positive results supports this assumption. Removing the cap and running would answer the question immediately. Compressed air and a small piece of wire should open it back up.
 
Texasmark said:
This is July....... Stuck closed thermostat...... yes a problem. Stuck open and that causing any kind of a problem???????? Engines ran for decades without stats, just free flow cooling water. Agree with Swagonmaster on tank not venting properly.

Engine fuel pump suction is only a slight vacuum. It can only lift so much fuel, so fast. If the tank isn't vented once that small vacuum migrates to the entire fuel source (the tank volume), fuel can no longer flow to the engine. Your stopping and restarting comment suggests that the vent, and for that engine you're probably running on a portable tank, which would be in the lid of the tank, probably partially closed/obstructed...it happens. Your squeezing the bulb and getting positive results supports this assumption. Removing the cap and running would answer the question immediately. Compressed air and a small piece of wire should open it back up.
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You guys were right. I took the boat to the river today and had the same problems. I replaced the fuel tank, new fuel line, connectors, and primer bulb. The motor runs fine. The only problem is it loses power after 5 or 6 miles of riding. Whatever it is, it isn't major. Just aggravating to find.
 
Fishing Mike said:
You guys were right. I took the boat to the river today and had the same problems. I replaced the fuel tank, new fuel line, connectors, and primer bulb. The motor runs fine. The only problem is it loses power after 5 or 6 miles of riding. Whatever it is, it isn't major. Just aggravating to find.
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Squeeze the bulb when it starts slowing down....what happens? Is the bulb soft.....obvious there is fuel in it or collapsed...flat when you squeeze it?
 
Texasmark said:
Squeeze the bulb when it starts slowing down....what happens? Is the bulb soft.....obvious there is fuel in it or collapsed...flat when you squeeze it?
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When I squeeze the bulb, it is soft. After idling for a minute, it regains power. I'm lost. Plus it is getting expensive to find the problem. I need help!
 
Fishing Mike said:
When I squeeze the bulb, it is soft. After idling for a minute, it regains power. I'm lost. Plus it is getting expensive to find the problem. I need help!
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You squeezed the bulb....when, idling? " After idling for a minute it regains powe"....what idling, or you were back up and running? Did you pump the bulb while up and running while it acted up....in short, you used the bulb as a fuel pump bypassing the action of the engine's fuel pump with squirts of fuel filling the carb. bowls so that the engine could run off it and if so, did it pick up or no change?
 
Texasmark said:
You squeezed the bulb....when, idling? " After idling for a minute it regains powe"....what idling, or you were back up and running? Did you pump the bulb while up and running while it acted up....in short, you used the bulb as a fuel pump bypassing the action of the engine's fuel pump with squirts of fuel filling the carb. bowls so that the engine could run off it and if so, did it pick up or no change?
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The motor runs good for 15 minutes. I have to leave it idling for a few minutes before it regains power.
I recently had the fuel filter cleaned and it ran better. But the problem still exist. I honestly believe it is something wrong with the fuel intake, but the Mercury authorized mechanics can not find the problem. It is aggravating and expensive trying to find the problem. I am considering selling the boat and rebuilding if I can not find the problem.
 
If you had power in the first place, heat could be affecting an electronic component or ignition coil in conventional ignitions, in a trigger circuit....not familiar with your engine, but in general. It has happened to me. Test for that is quick. Running with the cowl off, a piece of wire and a 13/16 (probably) socket wrench handy, when the engine acts up, stop immediately, pull both plugs. Take the wire and ground the case of the plugs to the engine block. Pull the starter. If you have spark on both I wouldn't expect "fire" to be the problem...assuming that your plugs are new.

If that test passes, a second quick check is taking a can of aerosol Sea Foam, (why and not starting fluid.....SF contains lubricating oil needed in 2 stroke fuel), plugs back in, engine running but still loosing power, squirt into the venturi of the carburetor, giving a few seconds for a result to occur......if you get a reaction your problem is inadequate fuel which has already been addressed as to troubleshooting techniques. Since you solved the question of no/low supply, the result must be in the carb...maybe a piece of degraded fuel line floating around in the bowl getting lodged where it doesn't belong.
 
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