mid to late 70's mec 20hp

[michaelteel]

I have a mid to late 80's model merc. 20 hp motor. It cranks great, it idles good. On land I can hook up the water hose and it runs like a champ. My problem starts the moment I run it in the river. It will run slow speed just fine, but when I go to open it up it accelerates and then starts choking and stuttering. i try pumping the bulb but no change i have changed all plugs and wires and coils but nothing the thing is last weekend i was on a lake it ran like a champ. then this past sunday went to take it fishing and it started no prob and idled fine but when i go to full throttlle she just bogs right down nothing she will not stall at all just bogs any ideas will help much.
also i am looking for 30 to 35 hp i have an exellent running 9.8 merc and a 65 merc with all controls just needs to be wired back up and new prop.

 

[Drindin]

Re: mid to late 70's mec 20hp

when you hook up a hose, you force water through the water pump. Does it have the pee stream when your on the lake? If not, the water pump is bad, overheating the motor. That's my opinion. I could be wrong.

 

[michaelteel]

Re: mid to late 70's mec 20hp

when i hook up motors at house it pees all over solid stream revs up in gear awsome but when i take it to river it pees straight and will not rev up boggs way down will never get over like a few hundred rpms lol

 

[KIGuy52]

Re: mid to late 70's mec 20hp

Sounds like you might have some water in your fuel. Try purging all the fuel from your tank, hose and carb and fill your tank with new gas.

 

[Yepblaze]

Re: mid to late 70's mec 20hp

The one I had did all sorts of wierd stuff.

I went through the fuel system, and chased my tail for a while.

Then just bit the bullet and dropped the $250+ for a new ignition box and never looked back.

 

[Motor Boater Bill]

Re: mid to late 70's mec 20hp

My 1976 Merc 200 ran like that when I had a bad coil. Check for spark when the problem arises. They can idle with only one cylinder firing. If one cylinder has no spark, swap the coils and see if the problem moves with it. Get a spark tester for $6 at Harbor Freight, great little diagnostic tool.

 

[The-Machinist]

Re: mid to late 70's mec 20hp

I'd be willing to bet that you have fuel issues. I would check for air leaks in the fuel hose before the fuel pump. If not that, your high speed jet is probably clogged. That bein' the case the only hope is to tear the carb down and rebuild. While I'm at it, does your motor have the little inverted plastic cup on top of the carb that holds the fuel filter? If so check it carefully for cracks. The replacement is all aluminum and hasn't leaked yet in 5 years on my little 20.

 

[michaelteel]

Re: mid to late 70's mec 20hp

THANK YOU ALL FOR YOUR COMMENTS!!!!!

After trouble shooting for a little bit I went and replaced both coils and bingo that was it. Thank you all for great advice much appreciated.