1974 20HP Merc Cutting Off

coastie33usus · May 4, 2005

Ok so I got a 1974 Mercury 20hp that was giving to me by my grandpa. The engine sat up for about 25 years. I just fianlly got the engine back to working. I had to replace my trigger. SO my question/problem is this. When I start to open the throttle works fine but when I hit full the engine completely dies after a cppl of minutes. But it will imediatlly refire and go for a bit and happen again. I am thinking the problem lies in the fuel line. It is the same one that has been on the engine. Does this sound like it would cause this problem and If not anyone got any clues on where I can start. I am pretty sure it is somekind of fuel problem. Any help would be greatly appreciated. Thanks Kris

Motor Boater Bill · May 4, 2005

Re: 1974 20HP Merc Cutting Off

Is the tank vent open? Have you tried pumping the bulb when it starts to die? Could help isolate the problem if it is fuel or not. Could be bad fuel pump or sticking carb float.

Nos4r2 · May 4, 2005

Re: 1974 20HP Merc Cutting Off

sounds like the crank seals need replacing-25 years standing and they'll be really badly brittle and perished. Do you find it runs for longer if you put the choke on?

coastie33usus · May 5, 2005

Re: 1974 20HP Merc Cutting Off

Crank Seals?

Nos4r2 · May 5, 2005

Re: 1974 20HP Merc Cutting Off

Along your crank there's rubber/plastic seals that stop air getting into the crankcase and keep the crankcase compression where it should be. These perish with age and lack of use and you lose fuel/air mixture down into the leg. As the engine warms up they become more flexible and if they are leaking they lose more compression through them. Ask Rodbolt or Clams Canino if there's a way of testing them on your motor. If they are the problem it'll be smoking really heavily too and it'll keep running better with the choke on full.

Laddies · May 5, 2005

Re: 1974 20HP Merc Cutting Off

I doubt if it's crank seals, they are a rubber compostion and can't dry out. What was the matter with the trigger? Recheck all plug in's and connections, go thru the fuel system, thats what it sound like is the problem---

Nos4r2 · May 5, 2005

Re: 1974 20HP Merc Cutting Off

Fair enough. I had a '73 20hp blueband here a few months ago that had been dry stored for nearly 20 years with only a few hours use and they'd gone south-that's what made me think of it.

coastie33usus · May 5, 2005

Re: 1974 20HP Merc Cutting Off

Well I will deff check out all the suggestions you guys are throwing out. I am pretty sure it is going to be a pretty easy fix.

coastie33usus · May 5, 2005

Re: 1974 20HP Merc Cutting Off

So I was looking at some other places and I feel kinda dumb right now. My fuel tank has a releif valve on the cap. If that was closed when I was running the engine would that cause it to die like this?

atticus3600s · May 5, 2005

Re: 1974 20HP Merc Cutting Off

Open the relief valve and see if it makes a difference, if it does problem solved.

Motor Boater Bill · May 5, 2005

Re: 1974 20HP Merc Cutting Off

"My fuel tank has a releif valve on the cap. If that was closed when I was running the engine would that cause it to die like this?" That's what I meant in my previous post when I asked if the tank vent was open--sorry I wasn't more clear. I learned that one the hard way years ago myself! Knew about it, just forgot to open it. It's kinda like leaving the drain plug out when you launch--everybody has to do it at least once! Hope that clears it up for you. Let us know when you've had a chance to try it.

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