Please help

[geo123]

got a mercury 500 thunderbolt, the problem is that the motor runs fine out of the water. Idles and revs, but as soon as you put it in hte water it does not want to pick up revs and just dies???
I've cleaned the carbs & replaced the spark plugs.
Can anyone help????

 

[WillyBWright]

Re: Please help

See if all cylinders are sparking. You can do that with a timing light while it's running. If it flashes, it's sparking. Try each plug wire in turn.

Check the deposits on your sparkplugs after some runtime. Clean sparkplugs would mean a problem. Ideal deposits are dark brown and slightly damp-looking.

If you haven't rebuilt the fuel pump yet, that would be a good idea. The diaphragm gets hard over time and can fail when brought back into service. That would allow excess gas to enter one or both cylinders that drive it.

You should also rebuild the water pump. The rubber impeller also gets hard and can fail within minutes of return to service. That would bake the cylinders. Not good!

Adjust the idle needles about 1/4 turn richer than smoothest idle. That would be about one turn out. You need that extra for acceleration. If you adjust for smoothest idle, it'll bog and die on acceleration.

 

[tmcalavy]

Re: Please help

What year Merc 500? Regardless, if you pulled the carbs to clean them and then re-installed them you may need to do a link-n-synch...getting the timing advance synched with the carb butterflies opening up. If they aren't synched, it'll idle great and then fall on its face when you give it throttle on the water. Last time I had one done on my 73 Merc 500 it cost about $50...but a manual can tell/show you how to do it yourself.

 

[Clams Canino]

Re: Please help

What's the prop pitch? An overpropped motor won't spool up properly.

-W