I bought a new 250ex for my wife.I had the Honda dealer install a FMF Megamax slip-on,K&N air filter(stock air box) and a Dynojet jet kit.The bike runs well and has no flat spots or skips when accelerating.However when I decellerate it pops and cackles out the exhaust.Is this typical for an aftermarket exhaust or does it need a bigger jet?

 

It's the engine braking.

 

is a blaster faster than a 250R atc

 

n2atv It sounds like your carb is jetted to lean. Try a bigger pilot jet and possibly raise the needle.

 

its just the engine braking. i think its a honda thing to do that. mine does it to.

 

If it runs well, with no flat spots or skips leave the jetting alone.

With the new louder muffler, you just may be hearing things the older Honda muffler suppressed.

If you decide to mess with the jetting, record your current jettings exactly so you can always come back. Also, only change one thing at a time.

 

It is jeted to lean. Take it back and tell them to do it again.

 

if it runs good on acceleration then i would leave the jetting alone. if it is very constant popping on the deceleration then it is a little lean, but on a 4-stroke i wouldn't really worry to much

 

It's too lean but only on the bottom end. Since it runs and accelerates all right you don't need to change the needle or main jet. What most likely needs to be done is to richen up the pilot jet. It's one of the screws on the bottom or side of the carb (I'm not familiar with the 250ex carb). If you richen it up 1/2-5/8 of a turn I think you'll find that the crackleing will go away. The different jets control fuel flow in different parts of the throttle range. The pilot jet is from idle to 1/4 throttle. The needle is from 1/4-3/4 throttle and the main is from 3/4- wide open. Since it's crackleing when you let off the throttle that would be in the idle- 1/4 throttle range that would mean the pilot jet needs adjusting. If you had a flat spot or sputter mid throttle while accelerating I'd look at adjusting the needle. If it was running bad while wide open then you'd go with the main. I know I repeated myself there but I hope this helps you out. There's always exceptions to every rule but this should be a good guideline to start buy if you have any other problems down the road.

 

I want to thank everyone for the help.Here is what I did yesterday.I went up one size on my main jet.That made absolutely no noticable difference.Then I removed the quiet core insert from the FMF Megamax muffler.That helped it a little but is still does it.I havent tried adusting the pilot screw yet.I will try that and see what happens.
Just for comparison,I have a 1987 250X that I also have the same exhaust and filter mods on and it doesnt backfire at all on decelleration.The only changes I made to the carb on it was 2 sizes up on the main jet and I raised the needle(lowered the clip)to the fourth notch.
That is exactly how the carb is set up on the 2003 250ex now so I should be getting close.