Mercury Idle/Stumble/Knock Problem

[Vic V]

I have a 1987 80hp 3 cyl outboard that runs great at high speed (anything above 1,000 rpm) , however when the engine (cold or hot) is at idle, it goes into an stumble/knock mode. It never stalls. When I get above 900-1,000 rpm, the problem goes away. When it is stumbling, the rpm is about 700. The fuel pump diaphram, fuel filter, fuel line to tank and spark plugs have been replaced. Also about a year ago, the stator was replaced. Sometimes when the engine is started, it backfires and shuts the engine down. Usually upon a restart, this doesn't happen again. Thanks for any help you can give me.

 

[The Marine Doctor]

Re: Mercury Idle/Stumble/Knock Problem

I believe you are having a LEAN POP...not enough fuel to idle at low speed...open the low speeds jets a little and see if the problem goes away.<br /><br />The low speed jets are the small screws on the front of the carbs.<br /><br />TMD

 

[D.W.B in Atoka]

Re: Mercury Idle/Stumble/Knock Problem

Vic,<br />I had the same problem on my 3 cyl. 50hp.After a mechanic replaced a bunch of parts and a big bill with no improvement I found that the trottle plate on the bottom carb was not completly closing with the trottle at the idle position.I adjusted it closed and it purred like a kitten.I was amazed that such a slight adjustment, I'm talking 1/16 of an inch,made such a huge improvement.Oh buy the way, it costs me about $200 to figure that out.<br />Good luck,<br />D.W.B in Atoka