Re: Nissan 6hp running problems
Hmm... Seems you seem to have two different problems.
The bubbles in the fuel line could indeed be from a leaky o-ring at the 2-prong fuel connector at the motor. When you changed the primer bulb, etc., did you do the whole hose, complete with the female end at the motor? If not, you can certainly get a new one from any Tohatsu/Nissan dealer. The male connector, which fastens to the motor lower cowling is also available, but is seldom a culprit in a vacuum leak that allows air into the fuel stream. However... the motor uses very little fuel, and a few bubbles on the suction side should not typically lower the bowl level, especially with a strong fuel pump. Still a good thing to address, as the problem will only get worse over time, until is does cause a too-lean condition.
One thought is that everything here may still be fuel-related, since you have good spark when the motor won't run. I'm leaning away from the CD box, as they are very reliable, and it seems you don't run long enough for it to overheat. In the shop, we would rule out the CD box by swapping in a "known good" spare to test.
The plug condition would indicate rich running. While this may be from choking and a lot of choked starting, it's still very possible that there is a carb problem. These carbs have very small, very high-velocity passages compared with carbs of yesteryear, to satisfy the EPA emission requirements. I have had a few of these (very VERY close tolerance) 4/5/6 carbs that looked OK, yet just would not behave perfectly, even after extensive and proper cleaning. By "proper" cleaning, I mean a _complete_ disassembly, including the jet, "nozzle" (or emulsion tube in old fogey talk), idle mixture screw, (the one buried beneath the EPA-required brass plug), and a long soaking in "real" submersion carb cleaner, such as Tyme, followed by a blow-out with compressed air. This thorough cleaning usually fixes these hard-to-diagnose mixture issues.
For a point of reference, I'd like to relate a similar problem that occurred in our shop a few years ago. We had a 4/5/6 carb that was almost perfect, but would occasionally deliver mixtures that were way off, as evidenced by the plug coloration, and poor running. We couldn't find a concrete fault with the original carb. A replacement carb fixed the issue, but the mechanic wasn't satisfied that he couldn't fix the almost-new carb. So, on a slow day, and after many hours of very careful disassembly and reassembly over clean blotter paper, an almost obscene amount of bright lighting, a good magnifying lens, and very close inspection, the mechanic did eventually find an almost microscopic fragment of metal in the air bleed circuit. Possibly left over from the manufacturing process. Obviously, no shop that expects productivity is going to spend an entire day on a 6 hp carb. Our guy did it on an off day, to satisfy his curiosity.
My thinking at this point is that you could go through the carb one more time, being super diligent and very thorough, and see if that helps. If you're still stuck, see if you can get the motor to a dealer, and see what they say. As for warranty or not, explain the whole situation to them, let them diagnose the issue, and if the cost seems unreasonable, plead your case of being only a week out of warranty. But your problem seems to be getting very difficult to diagnose in the field, especially without the resources of a dealer shop at hand.
