Idle is adjusted with the idle adjustment screw by the throttle cable on the front of the carb. It doesn't matter how fast you think it should be idling. It either idles or it does not idle. If it keeps stalling, then turn up the idle. I have mine set about 1400 rpm, which will stall after about 4 seconds of no throttle. You can't know your rpm without a tach. It doesn't matter if you know your rpm on a stock boat. However, the hardline tach has an hour meter too and is only $30. You will want a tach for when you upgrade to a pipe and whatnot, so putting it on now and tracking maintenance by date and engine hours is a good idea. I keep a 3-ring binder with all of my receipts, and every time I change something, break something or repair something, I write it down in the book with the date and engine hours. Not necessary, but it's a nice thing to have.
Change your spark plugs when the ski starts running foul. Some people replace them regularly, but that is not necessary. It's only $5, but if it runs good, I'd rather buy more gas.
Plug colors depend on your oil (some have darker dyes). Plug readings only count at high rpm. They are not reliable in the low to mid range. Make sure they are at least a darker tan. It you run yamalube; they will be almost black.
