Hi guys, i'm new to this forum. I have had my KLX250S for a year now and had no problems...until i replaced the exhaust a couple of weeks ago. I fitted a Leo Vince 3X pipe, which is sweet. But, it's backfiring constantly when decelerating hard. It sounds great and pulls well but the explosions are a bit much!
I just fitted a 2bros juice box hoping it would fix it nice and easy but still got same problem. I'm not very tech savvy so would appreciate a few easy tips. Thanks

 

The biggest thing is air entering the pipe. If its getting in through the front it WILL backfire on decel. It could also be getting from the back which you can do nothing about. Adjustments to the carb such as a small change in idle speed or mixture could help as long as you are sure there is no fresh air entering the front of the pipe. You may have to live with it.

Cover the exhaust in the back with a rag and your hand. The bike should stall.
While its dying, listen for air leaking out at the connection and where the head pipe mounts to the block. If you get any leaks there,fix them first.

You could also try blocking the aspirator (smog) thing with a vacuum plug.

David

 

+1 on the air leakage. I had a problem with air getting around the slots on the far end of the head pipe where the silencer slips in. I used a lot of silicone, and put the clamp over the base of the slots. The silicone gets roasted and crusty the first time out, but it will still hold the seal.

 

When I added the slip-on pipe (FMF Powercore 4) before the rejet I got alot more popin on decel. Once the Dyno-jet kit was installed and the pilot air screw was adjusted it's almost all gone. But I also have no exhaust leaks either.

 

Usually the popping is caused by an overly lean condition, created by the butterfly valve closing when you close the throttle. The inertia of the bike rolling along keeps turning the motor over which keeps sucking mixture on the intake stroke. There is an enriching circuit in the carb that is sucked open by the high vacuum created which allows fuel to flow through the pilot circuit. By turning your pilot jet screw out you will be able to lessen the popping. I was able to fix mine somewhat by turning out nearly three turns. When I went up one size on the pilot, the popping disappeared altogether.
Give that a go after making sure that your exhaust flange is correctly mated and tight.

 

Much obliged guys. I checked the exhaust and yes it doesn't look as if it's flush so i'll sort that out as soon as possible. I had the exhaust taken off a few weeks ago to clear of water after almost submerging the bike in a flood in Jakarta, where i live. Clearly the guy didn't fit it correctly (and i didn't check as i was pissed wet through and just wanted to get home).
I need to get the bike running smoothly as i'm riding to Bali in a couple of weeks: a full 2 days riding.
Cheers again.
Yusaf

 

After some thought I bet this is an efi bike and if is working correctly shouldn't be lean so most likely is an air leak somewhere.

 

On my german bike EFI.

Off the throttle at over 1800 rpm it will shut the fuel off as in no squirts of the injectors. 0 ms. then it can't backfire in the exhaust. If I hold the throttle a little open, it will crackle like crazy. This is only since I took the cat out. Before it was always quiet on decel. The cat burned anything that would backfire before it got to the muffler. So they do have a purpose...

David