It sounds like you have both too-lean and too-rich conditions, depending on throttle. At 6200 feet, you should be running high-altitude jetting, to avoid drowning the plugs in fuel. That contributes to the premature sooting, but the high-altitude jetting will be too lean at sea level, so you would need to change back to standard jetting for use at low altitudes. If your dealer is unfamiliar with those jets, they can call Alan or Tommy at Tohatsu America for details. If your dealer gives you a glazed-eye look when you mention high altitude jets, drop me an email:
[email protected], and I'll be happy to help you.
If you are having problems tipping-in (coming off idle to power), the accelerator pump is probably not delivering enough extra fuel to avoid the stumble, and the slow or midrange circuits may not be clean enough. When going through these carbs, it's important to remove the tiny screw that retains the accelerator pump plunger and spring, and make sure those passages are clean as well. Then test the plunger. It should operate freely, with little or no resistance. If the pump bore in the carb bowl has Any oxidation, change out the bowl. The best way to clean these carbs is a careful and complete disassembly, a 4-hour bath in real carb dip, a blow-out with generic carb spray, and then a careful reassembly and adjustment, paying particular attention to float height.
Yes, it is critical to run the carb dry at the end of each and every day, unless you can guarantee that you will be running again tomorrow. Today's US fuels are not nearly as clean and stable as they were years ago, and they leave varnish deposits as they evaporate out of the carb. This is especially problematic in the newer carbs of the 4-stroke motors, which have longer, narrower passages (which are necessary in order to produce very emulsified mixtures of air and fuel to meet EPA emissions guidelines). I would also recommend a water-separating fuel filter, to help pre-filtering the fuel, and keep water contamination to a minimum. Likewise, if your fuel gets to be over 30 days old, dump it in the car's tank to get rid of it, and run fresh fuel.
In order to check the ABS solenoid, you remove the single retaining screw with a #2 phillips driver, and then pull the solenoid up out of the carb. The solenoid has 2 wires (polarity does not matter) that operate a heater; that in turn causes expansion of a chamber, which causes the brass pin to move. They can fail, but rarely.
BTW, when servicing the water pump, always install a complete kit (includes the wear plate and cup as well as the impeller) to get more life from the pump. Same labor, a few more dollars, and a lot longer pump life.
If you are having problems locating a servicing dealer, you can look them up at
http://www.nissanmarine.com/search/index.html. The 9.9/15/18 units of your vintage are the same motor in Tohatsu, Nissan, and Mercury , so even a Merc dealer should be able to help you. OTOH, if you want to tackle the work yourself, start with a Factory service manual (forget the aftermarket manuals), and review it before beginning work.
