Bad Backfiring

wheelie_boy · October 27, 2008, 09:34:02 PM

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.

TRX450R_Racer · October 28, 2008, 02:49:01 AM

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.

FoxHondaRider · October 28, 2008, 02:56:19 AM

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

TRX450R_Racer · October 28, 2008, 03:32:44 AM

Quote from: FoxHondaRider on October 28, 2008, 02:56:19 AM
check for exhaust leaks aswell but most likely its the pilot jet.

Good call.

svtbolt04 · October 28, 2008, 04:13:09 AM

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.

kodiak-jason · October 28, 2008, 04:19:26 PM

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

wheelie_boy · October 28, 2008, 07:48:07 PM

Quote from: kodiak-jason on October 28, 2008, 04:19:26 PM
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...

svtbolt04 · October 28, 2008, 07:52:38 PM

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.

wheelie_boy · October 28, 2008, 08:07:55 PM

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..

TRX450R_Racer · October 28, 2008, 11:46:33 PM

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

CABLEGUY1 · October 28, 2008, 11:54:16 PM

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.

wheelie_boy · October 29, 2008, 01:45:32 AM

Quote from: TRX450R_Racer on October 28, 2008, 11:46:33 PM
Bring it over. I'll fix it for ya, but you have to buy lunch.

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.

Quote from: CABLEGUY1 on October 28, 2008, 11:54:16 PM
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.

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.

FoxHondaRider · October 29, 2008, 02:18:58 AM

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.

kodiak-jason · October 29, 2008, 04:15:49 AM

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

svtbolt04 · October 29, 2008, 05:31:16 AM

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

kodiak-jason · October 29, 2008, 01:06:10 PM

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

TRX450R_Racer · October 29, 2008, 05:49:06 PM

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.

quadguy33 · October 29, 2008, 07:42:02 PM

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.

wheelie_boy · October 29, 2008, 08:51:59 PM

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.

johnyfz450 · November 04, 2008, 03:47:14 AM

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

TRX450R_Racer · November 07, 2008, 12:47:08 AM

Did you get it fixed?

wheelie_boy · November 08, 2008, 03:33:08 AM

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...

B. Graves · November 08, 2008, 03:44:07 AM

you could buy a 09 CRF with fuel injection.

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.

wheelie_boy · November 08, 2008, 03:53:39 AM

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?

svtbolt04 · November 08, 2008, 05:04:12 AM

I would say yes.