My take on this:
How big a can of worms do you want to get into? If there is carbon on the back of the intake valve, then it must be sucking oil up through the valve guide. Does the motor smoke?
Maybe the valve stem is worn, in which case a new valve (with an unworn stem) may fix this. Ideally you could cut a new seat at the same time, but you could probably get away with just lapping the new valve in.
Maybe the intaket guide is worn, so now you need a guide (they're cheap), and you definately need to cut a new seat, and possibly a valve as well..
And if youre doing the intake you might as well do the exhaust as well (which are more prone to giving problems anyway....
Unless you have a seat cutter (expensive), or know someone who will come to you to cut the seats, You'll have to pull the engine and take it to a machine shop. And then there's always the temptation to say "well, since I've gone this far, maybe I should..." and before you know it you've bored it, new piston etc etc and a $700 bill.
If the motor was running OK before (except for your carb issues), I would leave it alone. If you really want to get rid of the carbon, you could turn the engine until its at TDC on compression (both valves are firmly closed at this position) and scrape out whatever you can from the port. You'll need something like a little screw driver to "chip" off the carbon. Then you could blow out all the crap with compressed air. None of this will end up in the engine because the valves are closed.
I've heard about the water thing, but dont think I'd ever do it. The version I heard though, was to spray it in in a mist, like from an old Windex bottle.
Let us know what you decide!
BTW Micah, I read your website with interest. When I move to Ohio I want a place big enough for a reasonable size garden, so I'm trying to learn the ropes now.