Be sure to clean up the valve seat (the area I've highlighted in red), and if you have the time, the rest of the combustion chamber also (the concave part). Some B12 carb cleaner and a wire brush will do the trick. You'll want to use a brush soft enough that you can bend the bristles with your fingers fairly easily.
I would do it to all the chambers if you have the time. Any carbon buildup thats left in the chamber will promote the buildup of more carbon later on. Anything that you clean and isn't going right back together any time very soon, either oil it or WD-40 it to keep it from rusting (anything thats not aluminum that is).
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