ATV Florida Forum

The 05 kx 250f that I just traded for my kfx (which is sick btw),  Backfires really bad when you get off the throttle. The bike is basically new with very low hours.  The owner said let it sit for a while and the carb gummed up.  So he dropped the carb and cleaned it all out, and he thinks it might be the spark plug causing it to backfire.  Does this sound right?  I know backfiring is often jetting, however the bike is totally stock.?.  Thanks in Advance for any help. 

If it's poping on decel the pilot circuit is lean. You need to clean the pilot jet better and/or back out the air/fuel screw more.

check for exhaust leaks aswell but most likely its the pilot jet.

Good call.

Rejet the whole bike. It is running lean. Has nothing to do with the spark plug. Look on Thumpertalk.com and find out what jetting is best used for sea level applications. I know you're probably not RIGHT at sealevel, but it usually goes 0-1000ft elevation.

Main, Pilot, air/fuel adjustment.

You should'nt have to rejet if it's all stock. If it has a pipe & filter on it then yes you need to rejet. It is possible that he did'nt clean the carb good enough.
  -Jason

Thanks for all of the help guys.  ^ And thats what I was thinking kodiak, the bike is stock everything.  When changing the carb could the previous owner possibly have messed up the jetting somehow?  I really dont know anything about carbs...

No, the bikes come from the factory running LEAN as hell. ALLL bikes do. The bike is backfireing because of the lean carb.

Sources? I go to MMI.

But this backfiring is pretty bad, and the owner said it didnt backfire like that before he cleaned the carb.?. Could it be a combo of lean jetting and not clean enough?  I guess either way it is not something simple and I will have to work in the carb either way..

Bring it over. I'll fix it for ya, but you have to buy lunch.

I took my carb off twice recently and cleaned it thouroughly.At least I thought I did. When I first fired it up it ran great and the throttle response was perfect.Came out the next day and it was back firing again. I finally took it to Steve " aka Lugnuts" and he soaked it and cleaned it again and now it runs perfect. He is very knowledgable and reasonable. What I thought was clean was actually very gunked up.

Thanks a lot for the offer Frank, I think I am going to give it a shot with my dad and then if I have any problems I may get back to ya.


Do you know what he soaked it in? Do you guys have any recommendations on specifics ways to clean the carb? 

Thanks a lot for all of the help.

carb cleaner works well take it all apart and spray carb cleaner in the jets and all.  Seafoam in the gas should run it through and clean it all too.

svt, no disrespect here, but they come a little lean from the factory for emission reasons for importing. If they came over that lean then they would be lined up at the dealer for burnt pistons & valves under warranty.

 sources? over 25 years of working on them.

-Jason

Yeah man, but you have to know that a bike that is back firing is running lean. The possible things it could be would be listed as a fuel delivery.

The air is changing, and maybe the last time the guy rode was when it was in the summer. The factory jetting could be too lean for this time of year.


Change the spark plug, ride it for 20-30minutes, let it cool down and pull it and let us know what color the tip is.

White = lean
Black/Gunked up = Rich
Brown/Golden crispy = Target area

I agree w/you on the possible causes for backfiring. But remember one thing when working on anything 99% of the time the root cause of the problem is something the owner\previous mechanic did incorrectly.
 As for the air changing I would beleive that if we lived up north where it gets cold and stays cold,and the unit in question was a 2-stroke but its not. In all my years of wrenching I only came across one bike "2-stroke" that was tempermental to temperature change.
 
 It's all good, if he takes his time and cleans the carb real good the problem should be fixed. Then again neither one of us can hear it or see to properly diagnose it.

   -Jason   

Get some Deep Creep. It is a carb cleaner that is made by Seafoam. It is the best stuff you can buy. I know Bennett Auto Supply carries it. Also just don't spray it out. Get some really small drill bits or some wire to put in the holes of the pilot jet. You can also get a can of Seafoam and pull the pilot jet and let it soak in that for a day or 2. If you don't ride that offend I would put a little Seafoam in the tank every time you put in gas and it will keep that from happening.
Good Luck.

The seafoam seems to work. also when you take the carb apart don't spray the o-rings/ gasgets with carb cleaner seems to stretch them. my auto parts store sells a bucket of parts cleaner with soaking basket reasonably cheap you might try that.

Thanks so much for all of the help guys, when I go up to my dads in 2 weekends i am going to try to clean it and Ill let you all know how it goes.
And Svt thanks for that link to thumpertalk, there is a whole thread with pics on how to get into the carb and what everything is.

start small after cleaning maybe bump the needle one or two before moving to a smaller jet

Did you get it fixed?

spent about 5 hours on it tonight.  Did the whole seafoam thing and sprayed the jets with a jet spray and its still backfiring. I didnt really get it warmed up to good, so tommorrow morning i am going to let it warm up good and see if it still backfiring...

you could buy a 09 CRF with fuel injection.  :P

no, but seriously, it is because its lean. just listen to everyone and clean the carb real good. maybe buy a few larger jets to bump it up a tad since they come lean. i made the mistake of letting a XR run lean too long and it cost me a rebuild. i did the work myself and came out to around 150. if you cant do it yourself, lot more.

i cleaned it real good tonight... We will see how it runs tommorrow.  Is it ok to ride it for a day with it backfiring bad?

I would say yes.

i just read through this thread, and Fred, i see how your thinking but your thinking of it wrong, the bike is giving a false lean condition from the jets being clogged, that doesn't mean its just way to lean. clean it first.

if it were my bike id clean it good, and maybe fatten it up just a little bit, but you don't have to. and backfiring wont hurt anything, asuming that its not so bad you cant even ride the bike.

But he HAS cleaned the carb. A clogged jet will do just as much as a jet made to squirt less fuel. If he has cleaned them all with the same problem still persisting, then he is running lean. You said the bike sat for a while? Take the intake boots off and check to see if they have started to dry rot. 

His definition of bad backfire might be a few decel pops, or sounding like a fire fight in afghanistan.

The hot start screw is stripped and not seated to good, so i think that may be causing the backfire. took it for about an hour ride today and it seemed to backfire less as i rode it more?.? i could have just gotten used to it though.

o, sorry, didnt realize he had already cleaned it. although its jetted lean from the factory, its not jetted that lean. so the only remaining cause of a false lean condition would be anything on the carb body, or intake that is leaking. iv never delt with a hotstart, its just a choke right? if it is directly connected to airflow through the carb and its behind the middle where the fuel delivery has taken place then if its striped or leaking that could very well be your problem. a riped vacuum line, or ripped intake boot also would do this.

if you suspect an area that you think is letting air into the carb (like your hotstart) take a propane torch (don't actually light it) and aim it towards the suspected leek as its running, if the bikes idle picks up then its leaking becasue it will burn the propane as it enters the leek, which will raise the idle speed.

if i were you, id take the whole intake off up to the motor. make sure everything is sealed up good. i screwed up a gasket on my old XR and had the worst backfiring you could imagine and the bike wouldnt even run right. it was cause there was air getting in after the carb. inspect all the places where it seals up at. its very easy to get one of those on just a tad wrong and cause this too.