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Motor won't run at full throttle - Boggs down when full throttle

cbb174

New member
I have a 1990 75hp Mercury outboard engine on a 15.5 foot bumblebee bass boat.

When I first launch the motor always seems to run best. When I give it gas at full throttle it picks right up and goes full speed and I always slow it down to about 4500 rpm driving down the river. After stopping a time or two the motor will not get up and go when given full throttle. It gets to about 3500 rpm which isn't enough to level the boat out. It is trimmed down properly and is still sending full throttle fuel. I have tried slowing down and trying again and no luck. Sometimes it will eventually pick up speed but it takes a long time. Yesterday when it was bogging down at full throttle I squeezed fuel with the rubber pump in the line and it picked up and ran.

I bought the boat a few months ago and had the carb cleaned soon after. I keep additive in the fuel as well.

Anyone have any thoughts?
Carb already dirty again?
Coil Pack?
Spark plugs (I haven't pulled all 3 off yet, but i checked one and it was fine and they all look new)?
Spark plug wires?

It would be great if someone else has had this bogging down at full throttle issue and fixed it somehow.
 
If pumping the gas line gives it back the "missing power" that indicates that your fuel pump is not doing it's job properly.

Is the gas tank built into the boat?

If so it could have a venting problem. If not enough air is getting in the fuel pump may be working too hard to try and suck fuel through the line. So if it is built in I would first try hooking up to a portable tank. If that solves your issue you would know that the issue is somewhere in the tank itself.

If the issue remained on the portable tank then it's time to service the fuel pump, but since your problem seems to take a bit of time to develop each outing, I suspect it's in the tank (vent, anti-siphon valve etc)
 
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