Yamaha 40hp Big Miss at idle *Vids

williamz

Recruit
Joined
Aug 26, 2014
Messages
5
I have an issue with my outboard. It has a big miss at idle and will shutdown when a big miss occurs. I completely redid my pontoon boat this year. New gas tank, new line.. The miss did go away for a couple hours and purred like a kitten but came back. Brand new NGK plugs, and I already tried swapping out the top coil. Issue remains. For the video I did a high idle and would unplug each spark plug wire to hear or feel a difference. When all 3 wires are connected, the miss is there. If I remove the top plug wire, you hear spark but the big miss is still there and nothing changes. If you just disconnect either the middle or bottom spark plug wire the engine rpm drops like it should and the big miss is gone. Put the wire back on so all 3 are hooked up the miss comes back. So to me, there is something going on with that top cylinder. Any ideas people?
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
compression ,spark, timing and the proper fuel/air mixture is needed to fire a cylinder properly.
Seems like you tested spark but that is all so far.

My guess if a lean sneeze is what you are talking about, and carbs need attention
but the other things are too easy to test to start out with and then you know they are good
 
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