Running Issue, Only in the water. '71 Johnson 50 hp.

[DooDahDave]

Hello, I Have a 15 ft Glassmaster (1969) With a 1971 Johnson 50 hp. When I put the boat in the water, and put the throttle down the engine runs great for about 300 yards. After that the rpms Drop and wont go but just above idle. (about 3 mph) No matter what I do with the throttle. I've been messing with this problem since I bought it, 2 seasons ago. I've replaced the prime hose, All the fuel lines. Rebuilt the Carbs, Changed the lower gear oil, Checked the water pump (looked new) changed the plugs, and rebuilt the fuel pump. I Also tried using non ethanol fuel. I am an Automotive mechanic so I plenty of tools and testing equipment.
I was hoping someone here may have run into this before and could point me in the right direction, or give me some advice.
Thanks in advance.

 

[interalian]

Sounds like you're running out of fuel. Try squeezing the primer bulb while running flat out?

 

[DooDahDave]

Thanks for responding. I Tried that early on. It had no effect. It holds fuel pressure great. The primer bulb stays firm for weeks running or just siting. Btw, with the elephant ears on it. Out of the water. It runs like a champ. Im wondering if losing spark to one cylinder when it warms up like car ignition coil will do at times. Any more ideas, Thanks again

 

[DooDahDave]

Thanks for responding. I Tried that early on. It had no effect. It holds fuel pressure great. The primer bulb stays firm for weeks running or just siting. Btw, with the elephant ears on it. Out of the water. It runs like a champ. Im wondering if losing spark to one cylinder when it warms up like car ignition coil will do at times. Any more ideas, Thanks again