Hey guys, It happend for the last couple of times i went out on the water, The boat goes well for the first few startups in the day and then as the day goes on I feel like its losing power. not like 20mph but the holshot takes a few more seconds and I loose about 8mph at WOT.

 

Any ideas? I changed the primer bulb but did'nt help.

Have you checked the compression and spark plugs lately? Could also be the carb.

You might have a partially, nearly blocked fuel vent.  I've had that happen on my riding mower.

 

Fill it with fuel and it would run well for fifteen minutes or so, then it would become starved for fuel.  Loosen the fuel cap and it would run fine again.  The vent is in the cap, so it was a simple fix to use a piece of wire to clear the vent hole.  I've needed to clear the vent a couple of times over a ten year period.

 

Your vent might let air in, but not fast enough to keep up with the drawdown of fuel when the engine is running.  When the boat sits for a period of time, enough air might seep in, which allows the motor to run fine for a while, until a partial vacuum is created in the tank.  This restriction will cause a loss of power.

How long is it sitting between runs when the power cuts out? It can be a symptom of a coil if it heats up then loses spark. If heat isnt an issue then I would look at fuel but based on your symptoms id be looking at a spark issue first.

 

What motor are we discussing here?

Try what Fishing Rhino said.

 

Check the fuel Iines from the gas tank into the motor and in the motor. Smell any gasoline? A leaky fuel line from ethanol starts slow and then it gets really bad.  I know.

 

If that fails, take boat to a manufacturer approved marine engine shop and have the motor tuned and checked for problems.

 

Yes, I know that costs money but at least you will get the motor problem eliminated and not be stranded in the lake when the motor dies.

Hard to discuss potential causes without knowing make, model, and year. This could be a two year old Honda 4 stroke, or 30 year old Black Max.