Merc 120 fails at speed-help

As last season came to an end, the engine began to fail at speed. I mean like fall flat on its face. Started easily, idled perfectly, accelerated well and then cruising at 3m rpm to 4m rpm it would act as though it ran out of gas. Ease back to idle, hold it there for a few minutes, and go again...sometimes yes sometimes no. Then repeat the above. Need your input on other potential problems causing this issue.

Thinking maybe the fuel filters and/or fuel pump. After it warms up (weather) plan to put a pressure guage in the fuel line after the pump and see what I get. I suspect the diaphragm in the pump could be leaking (it's only38 years old). Found a diaphragm kit online for the Carter pump, but have never repaired a pump and am curious about the complexity of the job. The kit costs just under $40 and maybe not worth trying to save $40 to $80 and just get a rebuilt/new pump. Your info and thoughts on this will be much appreciated.

Need some more ideas on what could be wrong. Carb rebuilt 2 years ago, new points, plugs, cap and rotor last spring and ran like new most of the season.

Thanks for your input,

swabbie

Re: Merc 120 fails at speed-help

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Re: Merc 120 fails at speed-help

Sounds to me like you found the problem. I'd go with a new/rebuilt one and not try to rebuild your own.

Re: Merc 120 fails at speed-help

By the way, Welcome to the forum.

Re: Merc 120 fails at speed-help

Thanks for the input. Will change the filter in the pump and the carb and then check the anti-siphon valve.

Is the Sierra pump ok @ $100.00 or do I need the Merc pump @ $266.00 if it's a pump failure?

Thanks,

Swabbie

Re: Merc 120 fails at speed-help

Not to sound like an ***** but it must be a marine pump. It will have the extra fittings for the overflow hose.

Re: Merc 120 fails at speed-help

Understood Windy, most suppliers offer the Sierra at verying prices as a replacement for the oem.

Thanx