Rochester carbs, as mentioned, have a small filter that'll do exactly that, let you start, idle, run, but it collapses if dirty and eventually won't even allow you to run fast at all. I always run Yamaha's Ringfree into my carbs when ever I let them sit. Most additives are isopropyl alcohol, which is great in the spring and such, but Ringfree has zyline(sp?) in it which cleans up your carb while it sits.