the cure to the common bog...

clean your air filter.

i usually take my air filter off and clean the sponge out with carb cleaner whenever i notice gas and oil dripping down the carb. (this is probably the most common reason for all the "help, my carb is leaking!" threads, too.) i normally do this every week, but i haven't in a while, and my bike started four-stroking.

the "element" (sponge) clogs up with oil and prevents the air from sucking in. i've noticed that after cleaning it out, my bike runs smoother and doesn't bog at all.

i think the air filter is more for show than anything else. since the carb's facing backwards, the odds of it sucking anything into it are pretty slim, except maybe for water if you splash through puddles.

the only reason i haven't taken it off completely, is i'm afraid someone will jam a dime or a rock in my carb when i'm getting a burrito...

i know this won't solve everyone's problem, but the next time your bike starts bogging suddenly, take your air filter off and ride it around the block. if it goes away, you're welcome.