Bogs a little after riding it hard

Hey everyone,
I just joined this site because just reading the forums is helping me out a lot. So yeah. I have a 81 kz550 and although I am not familiar with bikes I am a fast learner. I took the bike apart put it back together and it runs pretty well. I changed spark plugs, adjusted the valve clearance, cleaned the carbs. I am going to sync the carbs soon and maybe replace the air filter although it still looks good. Like I said, it runs great, most of the time. When I ride it hard, it starts to bog down really bad and it backfires a lot. All the while it still idles fine and it hasn't died on me once. I ride it cool and it goes back to normal. Also when it bogs down, it doesn't do it for long. Maybe a second, than it takes off. Tere was once when it did it for about 15 minutes will I was riding. I was far away from my house so I was forced to ride it back. But it doesn't do it that bad anymore.
Any help as to whatis causing it will be really helpful.

Check the gas cap vent. If after riding, if you pop cap do you hear air rushing in.

Also see about your petcock, could be it doesn't flow fast enough to keep up with the carbs during "hard riding" and your petcock is acting as the "main jet" effectively boiling the gas before it gets to your carb bowls. Do you notice that if you take it easy for a minute you can thrash it again until it starts bogging on you? Your gas cap also, like the prev. poster, could not be venting properly.

Yeah I checked the petcock and it is working great. I know because when it was running I accidentally knocked off the fuel line muscling the tank back on I started the bike and fuel went all over the transmission case. Now as far as the gas cap, I didn't hear much of anything. So I guess that is ok. I synced the carbs last night and now it is running great. They were pretty far off, like 10 units off when they should be within two. But yeah, thanks for the help. Proper gas tank venting would make sense if there was a fuel delivery problem.