howdy folks - just got a 2003 scrambler 500, wondering about carb jetting etc. i think the stock jets are a 152.5 and 40, but this machine has an aftermarket muffler, white bros i think it is... and i'm planning to put in a better filter also. what kind of jetting should i start with? i'm not sure whats in there, but it seems to backfire a bit, so i'm going to go through the carb and just get it set so i know where i'm starting at.... and what about the needle, is it adjustable on these or should i shim it? i guess my question is with a muffler and filter where about should i be with main, pilot and needle jets and where should i set air screw? thanks a lot to anybody with a little guidance on this and cheers to all.