mercury 90 6 cyl problems

[bigtrout1949]

Hello! Have a 1984 Mercury 6 cyl 90 h.p. It starts fine, after a brief runout will troll fine for an hour or so. After that period it starts dying without warning and becomes very particular about starting (thank goodness for a trolling motor). I have tried every fuel additive they make. Always clean plugs before taking it out. mix the fuel/oil precisely. It cuts out when I try to run it up to full throttle...sometimes???? Are there hotter heat range plugs (surface gap) I would appreciate any help. Thanks

 

[Dave1027]

Re: mercury 90 6 cyl problems

One thing I learned is not to be anal about clean plugs. I did like you when I began with outboards and ended up stripping out the spark plug hole threads. Leave the plugs alone!

First thing to rule out in your case is the fuel tank building up a vacuum. Next time it happens remove the fuel cap to see if that fixes it. Also you can try squeezing the primer bulb when it happens. If that helps then you may have a bad fuel pump.

 

[jimmbo]

Re: mercury 90 6 cyl problems

The fact you can troll for an hour is in its own right amazing. The engine is just loading up with fuel from extended idling. The fuel is puddling in the crankcase and transfer passages, eventually there is enough of it to be blown into the cylinder messing up the air-fuel ratio causing misfires. More frequent rev ups will alliviate the issue

 

[bigtrout1949]

Re: mercury 90 6 cyl problems

Sounds like you know this beast quite well! Ha! If I don't get this problem cured soon I am sure I will end up in divorce court. Just got my wife to go fishing with me. We have some interesting "discussions" on the boat ramp when the "beast" won't run to load it on the trailer..... When I run it out to clear the cylinders, it idles up to the ramp just fine, then won't stay running to load it. This gets very embarrasing. All the great advise from non boat owners at the ramp is comforting to say the least! Maybe I will get lucky and find a smaller Evinrude!! Thanks

 

[Georgesalmon]

Re: mercury 90 6 cyl problems

I'm thinkin that your getting to much fuel at low speeds. Maybe your float valves are worn or dirty or your floats are heavy with soaked up fuel. I think a simple carb rebuild might be in order.