Another Backfire Question.

I have read and read the posts here.. 2007 KX250F, it backfires moderately on deceleration only. Idles and revs fine, runs great, I have checked Timing, Valves, Filter, etc..

The jetting was at 40 Pilot and 182 Main.

I switched to 42 Pilot and 180 Main... fuel screw is 2.5 turns out, I tried 2 turns out and set on 2.5

Still absolutely no change in the backfire behavior on deceleration.. so if I accelerate hard in 2nd and then let off abruptly it will back fire. Only does it in the upper rpm's.

I have not checked the needle yet, but I assume its stock setting. Bike Runs great, I just assumed the backfiring could be jetted out of it.

I have also put a new header gasket in, and checked the seal there & checked for any air leaks with wd40.

Only thing I have not done is re-pack silencer, although this bike has a newer Leo Vince Exhaust.

Which way should I go from here ?? Maybe I should of went a step richer on Main, Instead of Leaner.

Everyone says these bikes are lean on pilot and too rich on main when stock ?? Backfire isn't hurting anything, but I would like to get rid of it.

my 07 always popped on decel, as did my 04 CRF250

Try a 45 pilot. Thats whats in my buddys 07 @ 2-1/2 or 2-3/4 turns cant remember which right now. We tried a 42 and it was of no help.

Well just to close the thread out, I went to a 45 Pilot (yes I read it here). and the backfiring finally improved, although it still backfires if I accelerate really hard and then chop the throttle, although it has improved. I just can't believe it needs a 45 pilot. This has also created a slight bog, but you really have twist it fast while sitting on the stand to hear it. I don't notice it while riding. The o-ring mod, might fix that, but its barely detectable. It's in the 70's here, so I will have to go back to a 42 probably when it warms up.

Usually backfire on decel only means exhaust side of engine has air leak - header pipe gasket at head- gasket at mid pipe - or exhaust valve hanging slightly open

BTW I run 45 pilot and 175 main, I have the adjustable accel pump and not sure where my air screw is…I have same bike…

BTW I run 45 pilot and 175 main, I have the adjustable accel pump and not sure where my air screw is…I have same bike…

So do you have any backfire on decel ?

No especially since I replaced the seal at header pipe to head (crush gasket) and I also put a New seal in at The mid pipe to muffler section

Well just to close the thread out, I went to a 45 Pilot (yes I read it here). and the backfiring finally improved, although it still backfires if I accelerate really hard and then chop the throttle, although it has improved. I just can't believe it needs a 45 pilot. This has also created a slight bog, but you really have twist it fast while sitting on the stand to hear it. I don't notice it while riding. The o-ring mod, might fix that, but its barely detectable. It's in the 70's here, so I will have to go back to a 42 probably when it warms up.

The bog you describe has to do with the AP circuit. It has nothing to do with the pilot, you just never noticed it before. Running it on the stand tells you nothing, ride it in the trails to test, that is the only true way to know whats going on with the AP. The stock leak jet is a 55, TG or PC (can't remember which) suggest a 50. I have had luck using a 40. The more you lug it, but expect to be able to crack the throttle WFO the richer it will need to be (smaller jet size is richer).

Also what is your fuel screw set at?

Edited by Joshx221

well, but bike absolutely had no bog with the 40 pilot jet, going to a 45 reduced the backfiring, but did create or at least make the bog much more noticeable. I understand it's related or caused by other things besides the pilot.

Almost all bikes do this. Theyre designed to accelerate hard and then shift up. When you bring up the rpms and then drop it off you arent putting as much fuel into the engine so it tends to do that a little.

Jsmith, when you have a little down time, if you haven't done so already, that o-ring mod you mention made a noticeable difference on our 06. I thought it would be a little gimmicky. Besides, if it worked so well, why don't they make them that way to begin with? Well, it worked great and took very little time. It came with our JD jet kit, and we ignored it at first. Guess those guys know what they are doing.

Of course there are other improvements that can be made to the AP circuit, like shaving the pin on the diaphragm to get more volume of gas under it, but we stopped at the o-ring. It felt like a new bike.

We also invested in the remote fuel screw, which makes adjusting the thing a breeze. We tried the extended ones, but they were very disappointing. R&D Flex Jet for $33 is priceless IMHO.

Would it help backfire to turn it out? Maybe, since it would allow a little more fuel to flow at closed throttle, and it costs nothing to try with the Flex Jet. You can adjust right on the track. We found that we adjusted the fuel screw a lot more often (every ride vs. never) when we installed the remote adjuster, and it did make a difference.

Sounds like you covered all the other bases above, except maybe repack that sweet LV when you can.

Edited by Dragon67

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