merc 20, hi speed problem

Starts and idles perfectly, accelerates like a rocket, goes like a bullet for about 2 minutes or maybe a mile then dies out. Tried swapping fuel line and tank. Then, I soaked, blew out and rebuilt the B-Series MerCarb and integral fuel pump. Later, I removed fuel strainer (on pump): no improvement. I then found and replaced leaking crankcase bleed line: No improvement. Tried all combinations of pump check valves in carb kit. Was not happy to see new style hollow plastic float with no drop adj.(motor had foam float with brass height and drop tangs) , but installed it anyway, and yet again at higher end of spec (higher bowl level, which did slightly prolong good running time). Does not flood. Starts instantly in Michigan 40° water. Does not miss or falter one single bit upon acceleration from idle, in gear. Idles at dead slow indefinitely. Boat will plane at half-throttle and will run a long time that way. Will run forever at full throttle as long as I give primer bulb a squeeze every minute or so. While fading, engine will recover immediately when bulb is squeezed. New tank, new fittings, removed tank vent, 2 new bulbs (last one Merc branded), new Merc silver hose, all steel screw-type hose clamps. Also probed crankcase pulse hole with soft tool and shot aerosol throttle body cleaner through it. New carb base gasket. New plugs (desperation), L78V.

 

It certainly sounds like a fuel supply problem; but you seem to have that covered - unless the pump is just too tired. Have you checked all the electrical components? Sometimes coils will act up when they get hot - and your coils are mounted right on the block. Maybe WOT locally raises the temperature just enough.

 

merc 20 hi-speed

tired pump? What is that? This pump is aluminum plate about 1½"×2" with new rubber daiphragms, check valves and gaskets througout. If electrical, motor would not run indefintely while pumping primer bulb. It also recovers immediately when bulb is squeezed.

 

- "While fading, engine will recover immediately when bulb is squeezed."

Maybe I should have read the entire thread before responding .
I don't know what tired pump means exactly; but I heard the term when it won't respond to a rebuild kit. I assume it means something warped and you're not able to get a good seal.
My brother had a fuel issue with his Merc 50 hp several years ago. One of the fixes was rebuilding the fuel pump; but it still didn't run right. After a lot of hair pulling and a little advice, he had success by replacing the pump.

 

Tired Pump

I had never heard that term, but don't doubt you at all. Yesterday I removed complete fuel line, bulb, connector and carb as a unit. Squeezed the bulb HARD and heard a slight hissing and sizzling at both ends of the inner carb fuel line! As soon as gas flowed into the leak, it QUIT! I could NOT make that thing leak a single drop. The hose used was rather rigid and a bit too large for the firrings, so the hose clamps did not draw up properly. I bought correct fuel line and clamped securely, and boat improved but still not 100%. I suspect another leak at the strainer base gasket where I had to use an old one that dried out shrunk. Will let you know here later.