So I own an 03 kawisaki ninja 500. Today I checked the valve clearance (it was right on nominal for the given tolerance), and cleaned the carbs. Followed a service manual and made sure everything was removed and cleaned properly. Set the pilot screw to the reccommended starting point, checked that there was spark. Well, put the carbs back on the bike and it just turns over, with the occasional backfire. There is definitely fuel getting to the carbs and the cylinder, and there is compression.

Just can't figure out what the issue is at all, and I am really hoping someone can give me some pointers. I need to replace brakes and chain and sprockets tomorrow morning!