1966 Merc 500 , I installed a new fuel pump diaphragm kit and it is still not working,fires up and runs at idle but dies at full throttle.I disconnected line going to carbs only slight dribble when cranking over.
Check the fuel flow coming from your fuel tank.
Look for hose that is collapsed inside, prime bulb failure, vent on the fuel tank.
Did you replace the (2) valves in the fuel pump? One valve should suck, but not blow, and the other should blow, but not suck.
If you detect any leaking by, replace the valves.
If the Merc starts and idles okay, but stalls as you attempt to speed up, try adjusting the idle jets on both carburetors. Open them both 1/8th of a turn and try to accelerate, if she still stalls, repeat the adjustment an 1/8th turn more, etc.
Counter-clockwise opens these jets.
This is a user boat and motor ran good last year,and have been using it for at least 10 summers. I am wondering if there is not a weird internal problem with vacuum before the fuel pump.I have gone through and Checked all these things,taken the fuel pump apart several times . New fuel line and primer bulb. I would think fuel should flow out strong from the fuel pump when you crank the motor over ,? little to nothing does at this time, the fuel line is not clogged because when I prime the bulb there is good flow. Thanks for your help.
I think I finally fixed the fuel problem, thought I had it fixed a few weeks ago ran great,but then failed again, I took fuel line off between fuel pump and carbs,and there was a non visible obstruction in it.replace black rubber fuel line and runs good again.frustrating because I can see no problem with the old Line except that it is 54 yours old.maybe some kinda internal collapse. Thanks again for your help.I am learning all the time to keep these old motors running!