2002 40 hp Nissan 2 stoke rough idle, stalls going into gear

40hpNissan

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Sep 2, 2017
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Hey all, thank you in advance for reading and responding.

I have a 2002 40 hp Nissan two stroke that I recently purchased and am having a little bit of trouble. Whenever I start the motor it has a really hard time idling and will shut off if I try to put it into gear. In fact, for the first couple minutes it will shut off unless I leave it in neutral and give it some extra RPM's with the neutral throttle. It also has a hard time getting into forward (die out) if not allowed enough time to warm up. But the interesting part is, whenever it is on the trailer, and has muffs on, no extra Revs are needed, and it is never stalled going into first gear. Even after being cold. This is very strange to me, because when it is on the water it needs time to warm up it seems, even if I only turn it off for 30 minutes or so.

First, I tried real spraying a can of seafoam into the carbs, but not removing the carbs. No real results from that. Secondly, I adjusted the idle speed screw to 750 rpm's at idle in gear, which is what the stamp on the side of my motor calls for. This has helped some, but still had trouble.

Yesterday I pulled the spark plug wire and spark plug covers one at a time to see if there would be a significant drop in RPMs on each cylinder. The top and middle cylinder both had a significant RPM decrease, but no decrease whatsoever on the bottom spark plug/cylinder. I pulled the spark plugs, the top two were oily and black, and the bottom spark plug was relatively dry and was tan on top. I could feel the current in all three wires, including the bottom which showed no decrease in RPMs when removed.

Today I replaced all three spark plugs and nothing changed (when I pulled the spark plug wire and cover off the plug, on the top and middle, there was a significant RPM dip, and when I did the same to the third, bottom spark plug wire and cover, there was nothing)....Which makes me think the wire and coil pack on the bottom cylinder are fine.

I ran a compression test, The top cylinder was 115, the second cylinder middle cylinder was 110, and the bottom cylinder was 98.

I've heard the rule of thumb is you want them within 10 of each other, so the top cylinder and the bottom cylinder being 17 apart from each other has me a little worried.

Also worth mentioning, the boat performs well under power, but it might a have a very slight hesitation when giving it power. Hardly noticeable really, no hiccuping or miss firing. It also seems to still be getting good fuel mileage. Any help, or advice on how to fix this problem would be much appreciated.
 

ondarvr

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Apr 6, 2005
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What it does out of the water doesn't mean much, it's totally different in the water, and the 750 RPMs is in gear in the water.

If you put your hand over the bottom carb does it start to run better? (I'm assuming it has 3 carbs).
 

40hpNissan

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Sep 2, 2017
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Yes, it has three carbs. And I covered the bottom carb and it seem like it was trying to stall out. When I removed my hand it would recover
 

ondarvr

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Apr 6, 2005
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If the bottom carb was running lean then it should have picked up a little as you started to restrict the flow, it my stall if you completely blocked it though.

​If the bottom cylinder wasn't doing anything, then it would have almost no effect when you covered the carb, but you said it ran worse.

If the bottom carb is clean, and unless you've cleaned it recently you don't know, then I would expect the low speed jet is dirty.

​The other option is pulling the head off, if you're lucky the head gasket may be bad, on the other hand the cylinder may be scored and the piston and rings damaged.
 

40hpNissan

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Thanks for the input, really appreciate it. I have not cleaned the carbs. But am planning on making that the next step. What would be your best guess as to why there is no decrease in rpms when I remove the bottom cylinder spark plug wire?
 

40hpNissan

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Sep 2, 2017
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That makes sense what your saying about the bottom cylinder. If it wasn't doing anything there would have been zero change while covered and running.
 

Sea Rider

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Sep 20, 2008
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Clean all carbs properly along fuel filter, for a perfect idle speed, all 3 carbs must have air/fuel screws well adjusted, if possible adjust with an electronic vacuum gauge, it's much precise than by ear adjustment or playing with screws. Once dialed, adjust main idle screw to a point that when gearing forward OB will not die, adjust screw CC till that cond is achieved. OB must be well warmed up and adjusted on water with submerged lower leg, adjusting on muffs, barrel is not that precise.

That's a 13 year old OB, If was used on salt water check If there's white lines formation following the head gasket contour, if so, means that gasket is dried, hardened. Get a new one, take advantage to decarbonize pistons and internal head, check if cylinder walls are in good cond, clean all water paths, thermo.

Happy Boating
 

40hpNissan

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Sep 2, 2017
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Sea rider and pvanv thanks for the input guys... I will post again after I clean the carbs. I live in sw Florida and right now we are anxiously watching hurricane Irma. I am going to wait and see what this storm does before starting a projected I have never attempted. I will keep you informed.. thanks for the input
 
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