wrong fuel pump? 50 hp Johnson

[o2bfishn]

I have a 1982 50 hp Johnson. I rebuilt the fuel pump and found that the fuel pump is for a 75 hp motor.
The problem I am having is that if I run it full throttle for more than a few minutes the engine surges like it is running out of fuel. I can run it all day at half throttle. I can get the boat on plane and then reduce the throttle to just keep it there.
My question is will the 75 hp fuel pump loose its ability to pump with lowered vacuum while running wide open on the 50 hp or is there something else?
All of the hoses have been replaced.

 

[jakedaawg]

Can you run it at idle under load for an extended time? Surging points to a fuel delivery or float and needle issue.

 

[F_R]

I'm not sure exactly what the difference between a 50hp pump and a 75hp pump, or how you figured that out. But suffice to say, they both should pump the same. Differences, if any, would be in the way it is mounted or possibly hose diameters.

Edit: The fuel pump is driven by pressure pulses within the crankcase. Those pulses are more or less the same regardless of engine size. And if the engine is running normally, the pulses are present. If they were not present, the engine would be extremely sick and would not run properly at any speed.

 

[flyingscott]

The 50 hp or 2 cylinder pump will have 2 nipples 1 for fuel in and 1 for fuel out and the pulse port is on the back of the pump,and is designed to be bolted to the block .The 70 hp or 3 cylinder pump has 3 ports and will have 3 nipples and 1 will be the pulse port. The confusion starts with the VRO equipped 2 cylinder models because both pumps can be used on them. The block has the mounting ports for the 2 nipple pump just need to remove the plug for the pulse port and plug the pulse line for the VRO. But since the VRO uses a pulse line you can use the 3 nipple pump by connecting to the pulse line itself and zip tying the pump to something. Put up a pic of your pump as a nipple would have needed to be threaded into the pulse port to make the 75 hp pump work on your motor..

 

[F_R]

Not to debate all that, but it wouldn't work at all if there is no pulse hose / nipple / port connection to provide crankcase pulses to the pump diaphragm. The OP seems to be saying it works at some speeds, so it obviously is getting crankcase pulses. Either that or the gas tank is being pressurized by being out in the sun.

All this could be settled by simply temporarilly T'ing a fuel pump pressure gauge into the pump outlet line. A low pressure gauge, that is---not one for automotive fuel injection systems.