The '85 or so 20hp Mercury motor on my little boat runs great at times and then for no apparent reason loses most of its power and sputters. It starts easy and idles fine, maybe a little rough but doesn't go dead or anything. Sometimes it starts out running great then goes to running bad, others it's just the opposite or never runs good at all on that outing. I've had the carbs rebuilt and was told they looked great to begin with. I know it's not the gas because it is fresh. The plugs are new as well. Any ideas?

Thanks,

Kevin

Start with the cheapest repairs first. Fuel filter, fuel lines, fuel pump. Those are my top suspects. Clean the fuel filter. If there is anything in it that looks like it could be the inner lining of a fuel hose than replace the fuel hoses. The fuel pump on my 60 HP Johnson took me about $15 and 1.5 hours. These steps ended up fixing my similar problem when I thought I needed a carb clean.

I have also heard of trash in the fuel tank being picked up and clogging the pickup line.  Sometimes when you stop the motor it falls back into the tank.  Then your motor runs fine for a while until it gets picked up again.  That sounds like it is worth a check too.

make sure the vent is open on your gas tank

And sometimes the vent will tight up starving the engine.

Garnet

Thanks yall, I'll try those. I've made sure the vent was open and I've tried another fuel tank (out of my dad's boat) as well. Still does it. I'll try the fuel filter next and see how that does.

Thanks

Kevin