2009 klx 250s,i just put on a slip on pipe and the dyno jet stage 2 jet kit, and now my bike is back firing when i let off and surging or cutting out going down the road.Any ideas? new plug also.

 

Check to make sure the slide in the carburetor isn't stuck? Maybe you put the needle clip on wrong, when does it cut out? constant throttle or acceleration?

 

Backfiring, or deceleration popping on throttle release? This happens quite a bit when there's a little air leak between the header and silencer or elsewhere into the exhaust system. Have you removed the smog stuff?

 

You can also adjust your mixture screw to richen it up a bit.

 

Cuts out at constant throttle.the clip is in the 3 slot as per the dynojet instructions.smog stuff is still on the bike.

 

128 main and just put a 42 pilot (thats the only size i had) and 2.5 turns out on the mixture screw.It does not miss as much now but still backfires on deceleration bad.

 

It's not uncommon to have a little decel popping.

 

ya,its really bad tho.I took my pipe off and replaced it with the stock one,it still did it some but not as bad. But i reinstalled the slip on it was a lil better,i checked around the clamp and i have a air leak so im guessing thats my prob. I have ordered new exhaust gaskets and see if that was it.

 

Remember to use high temp silicone at the slip on / header joint and let it set for 24-hours before running. Sounds like you've got a leak.

 

You're running a 42 pilot, 2.5 turns out, with smog stuff still on? I think you're pouring lots of unburned fuel into the exhaust, and the smog gear is injecting air so the fuel is burning off resulting in backfires.
A 42 pilot jet is on the large side IMO.
If you use the high temp rtv type stuff on the exhaust, follow instructions. Seal it up, let it set for 24 hours before fully tightening the clamps.