Please Help - Nissan Power Loss

[Adamcox26]

I have a nissan 40 hp outboard. NS40C 1994 I think.
I have some power issues. At first it was a cracked fuel fitting on the carb that was causing it to starve. I replaced all the fuel lines and also rebuilt the fuel pump. New plugs, wires and coil pack. I cleaned the carb a while back and apears goog on inspection. Now that I have the motor starting and running I am noticing power loss. This last weekend i took it out. The motor started and idled great. I was able to ride a good distance to my spot about 1.5 miles away. The boat (Jon 16.5 wide side console) got on plane ride fine. I switched to electric trolling for the rest until I left. In the way back I got into a no wake zone, throttled down to idle. After that I couldn't get on plane and the motor sounded bogged down and working hard. I think it dropped a cylinder. I'm not sure how to tell which one. If anyone can point me in the next direction i would really apreaciate it.

 

[Sea Rider]

Welcome to Iboats,

Start engine while at idle pull upper plug cap check if engine misses, dies, put back do same with lower one. Was old coil pack working properly, when replaced ? is so, put it back on and start engine, check if all ok. Sometimes new plugs could fail, install new ones and check same. Report back.

Happy Boating

 

[Adamcox26]

The plugs were replaced and gaped to specs. I noticed they did look a bit dirty after the one use. I'll try a new set this weekend. As far as the coil pack, it was replaced before I got the boat. I know the previous owner personally and i have checked the wiring and connections. It was spliced in using wire nuts and dielectric grease. I can't replicate the problem out of the water. And both plugs get good spark.

 

[Sea Rider]

Being a 94 year oldie OB, could have already internal head gasket failures as corrotion on middle gasket metal rings, small water droplets gets into combustion chamber in any of both cylinders and OB loses performance at higher rpm. Test with new plugs, see what happens. What works good on muffs won't necessarily work same while OB is running on water at load. On 2 strokes OB's It's normal for plugs to be found bit oiled/fouled after a short term use. Use 50:1 fuel/2 stroke oil mixture. Run carb dry after outing if OB won't be used for days, will keep internal carb clean.

Happy Boating

 

[Adamcox26]

I'll give it a try. When I said the plugs looked wet I didn't mean with water. Just oil and carbon. The motor only smokes for a second when I first start it. I'm going to do a compression test this weekend. Thank you for your help.

 

[Sea Rider]

Micro tiny water droplets going inside combustion chamber won't make plugs look wet with water when removed, for that will need to have already a blown head gasket. On 2 strokes OB's it's usual to find wet plugs compared to 4 strokes OB's, due to 2 strokes oil unburned leftovers around plug tip, one will be found more wet than the other, don't remember if upper or lower one. Making compression tests will not show faulty head gasket issues unless completely shot, blown. If OB has poor performance or compression due to bad rings, deformed cylinder shape it's a different story.

Happy Boating