If the bike was in my shop I'd do a quick compression test to make sure it's not a burnt intake valve, first. Then I'd check fuel mixture (excessively lean can cause backfire through the intake). Then I'd check valve clearances.
Has the bike been chipped since it received its new exhaust system, or does it still have the factory chip? A full system plus a stock chip equals a high probability of backfires. I just rode a customer's S4RS with the same symptoms your bike is displaying and it was all due to an extremely lean condition brought on by an arrow full system and a factory chip. Maybe an adjustment to the fuel trim (which only affects the fuel settings at or near idle) would help some.
Poor TB balance can cause a myriad of poor performance issues at or near idle, but I've never seen it cause a proper backfire. That said, I definitely haven't seen everything there is to see.
Check the simple stuff first. 99 times out of 100 it's something simple.
Good luck.
thank you !! from what you said i would def. bring the bike to you so u check it but i guess your far away from where i am ...
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and yes, whatever you said is right.. i dont think the ppl who had the bike before did smth with the stock chip bcoz it backfires a lot from the exhaust.
i just did a valve adjustment .. everything seemed good. (i ll upload a pic of the valves) check it , see if u can find smth wrong there..
i was playing with the idling scews today .. and it seemed like it went away for a while when i opened up the air a little more.. but then again came back..
i dont have any experience on these things .. but when i look at it .. smth tells me that the gas gets collected there and then it lights up creating the backfire on the intake.. but at the same time i was playing with the other heads idling screw and there was no backfire there... idk im confused .. i know the deal will rip me off!! cant trust these guys here ...