Re: Help please with 2011 Nissan 9.8 idle
Assuming your model is NSF9.8A3 (year is unimportant for servicing a Tohatsu/Nissan outboard). You may have either a slightly lean idle circuit, or an overfilled crankcase.
Verify that the oil level is about 1/2 to 3/4 of the way up the dipstick. Not even a drop over full. Use only 10w-30 or -40, non-synthetic, FC-W rated motor oil. During the winter months, -30 is preferred; -40 if you are in really hot (over 90F) conditions. Do not run synthetic crankcase oil. At about 10 hours, unless the motor was run very hot and hard, it's probably not completely broken in, and the rings are probably still giving some extra blow-by, so keeping the oil level within limits is very important. All new 4-stroke OB's (of all brands) will "make oil", so expect the oil level to rise somewhat, especially if you do a fair amount of idling, and also if you are running in cold water. After a solid break-in, the rings will be well-seated, and you will not make as much oil.
Inspect the plugs. If they are nearly white, you are probably too lean. If they are sooty, re-check that oil and level, as you may be getting extra oil in the cylinders. If they are tan, the mixture is likely OK. On a new 9.8, it might be a good idea to go with new plugs at 10 hours if they are sooting-up. You can keep the originals on hand for spares.
If everything seems to check out with the oil, and especially if the plugs are "too" clean, it's time to review the carb. There could be a bit of factory machining junk in the carb, or, even with your vigilant practice of running the carb dry at the end of each and every day (I wish everyone would do that!), you may have some fuel varnish residues in the idle passages.
If you want to work on the carb yourself, get a Factory service manual (Part # 003N210561, $40.00, available from any Tohatsu/Nissan dealer including me), and using it as a guide, pull the carb... check the float height (should be parallel to the carb casting when the carb is held inverted), and completely disassemble all bits of the carb, including the emulsion tube "nozzle" and the jets. Then give all the non-rubber parts a 4-hour room-temperature soak in real carb dip (available in 1-gallon cans at auto parts houses), followed by a Liberal spray-out using generic carb cleaner spray. Don't be afraid to use a whole can, and don't be afraid to hold the red tube of the can right up to (or into) a passage as you blow it out. Then... do a careful reassembly, paying very close attention to the float height. The plastic float is "non-adjustable", but the arm on it can be reshaped if needed by carefully heating it, and then gently bending it while it is still hot. (Or you can use a new float if the original does not sit at the correct height). When reinstalling the carb, be sure to set the throttle rod so that you get high idle when the choke is on, but there is only a very slight play in the linkage when on the idle stop (and the choke is off). That will allow the carb to come up to "high idle" when you turn the throttle to "restart". If there is too much play in the linkage, the carb will stay on the low-idle stop even when you turn to "restart", and you will not get enough air for a good warm restart.
After you get the carb back on, you will want to use a good shop tach to set the warm idle. About 900 in gear, 950 in neutral, is a decent speed. Any faster, and you will slam going into gear. If you set the warm idle below 900, it may not idle well in gear (and might stall) until it's fully warmed-up.
If these procedures seem daunting to you, or if you prefer not to do the work yourself, see your local dealer for assistance.
