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'74-'79 Mariner55hp fuel issues

belmont6

New member
I have Mariner(Yamaha built) 55hp 2stroke 2cyl. ( Model 55EL 663EL-232059) Idle is very rough and it stalls. Especially when I go into gear it stalls.Hard to start again if stalled. Engine will run/start at "fast idle". When I put engine under load it cuts out. The problem seems to come from the lower cylinder as when I unplug the upper cylinder with running engine it stalls but when I unplug only the lower cylinder it doesn't stall. Am a complete novice here but it appears both cylinders are properly pressured and both plugs are sparking brightly. Prior to this malfunction I did notice the bulb would harden after a few pumps but will not now. I can get the bulb to harden if I disconnect fuel line from engine entry point and clamp end of pipe. Any ideas? Fuel pump replacement ?
 
You might want to pull the fuel pump apart and check for a torn diaphragm.-------------Would be easy / cheap to replace just the diaphragm.----Not sure why folks want to run out and buy new parts before determining what the exact problem is.--------Might also be a leaking float valve in a carburetor that is flooding the engine.-------Compression test does not hurt either.
 
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