1998 Merc 40H - loss of power at high speed

[brownsong]

We have a 1998 40H Mercury on our 18' Bass Buggy Pontoon. The motor runs fine the first few hours we have it out on the water. But after a few hours, when we try to go full speed we experience a loss of power. Anyone know why this is? The only maintenance we have ever done to the engine is having the water pump replaced this year.

 

[12Footer]

Re: 1998 Merc 40H - loss of power at high speed

Decarb it with spray decarb cleaner (available here or at any marina).<br />When this has been done, install new plugs.<br />If problem persists,get back to us.<br />It could be something else, but seeing how these are the original plugs, let's start here. They are prime suspect.

 

[12Footer]

Re: 1998 Merc 40H - loss of power at high speed

Also, is this pontoon trailered, or do you leave it in the water? It may be a leaking pontoon,and gaining wieght as the day goes-on.

 

[brownsong]

Re: 1998 Merc 40H - loss of power at high speed

Thanks for reply.<br /><br />The boat is trailered. When it loses power it is very sudden, not gradual. We will be going full speed and then all a sudden it will drop down.<br /><br />The plugs were changed last summer but will try the cleaner today and see if that helps.