i have the same issue as you. 4 into 1 exaust, bike sounds like it bogs like yours. Take your air filter out and run it and watch how much difference that makes, youll have a whole new bike. Temp fix is to buy a K&N foam air filter to replace the paper filter, lean out your mixture by turning each screw on your carb counterclock wise 1/2 turn. Both of these things will lean out your mix by allowing more air.Your bike will run better, but still not like it does without the air filter. Perm fix will be to take the carbs off and adjust the needle position on your carbs, or something like that. If that doesnt work, then rejet.
