What causes Decel popping, backfiring??

Hey guys,

Well one more question OK? I have my bike with me at work today and am going to work on it tonight at the shop. I am going to pull the carb probably clean it again, check float height, change oil, new plug, air filter cleaning etc. I rode yesterday evening after installing yet another plug and after motor was hot it was popping and backfiring quite a bit under hard "deceleration"(spelled that wrong). Just wondering what you think is the cause behind that. I know when I first got it it would do that but last night it was doing it more so than ever. I mean loud pops and backfires when I engine break into a turn or down a hill etc. Too much gas or too little. I am going to get the BK mod dialed back in tonight too. Just wondering what the 426 pros think. Let me know guys, I really appreciate it fellas. I had a blast riding last night runs good midrange and top but bottom is still sluggish. Help just wondering what to look for tonight while I got it all apart. Thanks in advance!! Frank :)

Typically that is caused by a lean condition when you chop the throttle. Most of the recommendations are to increase the pilot jet size, if you cannot remove the decel pop with the air screw. Do a search on that I know you will come up with more threads and posts than you care to read.


My 00 was popping and pooping on decel. The cure

Ear Plugs :)


I placed the 45 pilot in the beast and walla no mo poopDepop

Before I would go changing jets & such, I would try adjusting your fuel/air or pilot screw either richer or leaner to get rid of the popping. This has been my cure for this symptom. For my altitude in Ga. stock jetting is dead on. Some of us raise the clip 1 notch & that is it, even with BK mod.

Fastfrank, that condition could just go away as the temparatures heat up. What part of the country are you in? Here in Vegas, a bike that is lean in the winter will run perfect in the summer and vice-versa. Cold air is denser so it creates a lean condition. How hot will it get where you are at?

Hey guys I am in VA. It was hot yesterday, bike ran great all winter. I have been fudging with the jets and of course I have probably screwed something up. Jetting is back to stock now, the BK spray is off though cuz I messed with that. Will correct that tonight. It was hot yesterday evening more so than usual so I am sure that had something to do with it. I do have a 45 pilot and a 165 and 168 main but I have tried all those jets and it still runs better even with the decel pop with the stock jetting. Midrange and topend are good (not as good as could be) but bottom end is where the prob is. Still got the nice bog if you stab the throttle off of idle. I will check everything tonight and hopefully I can get the beast back to no bog and runnin like a scared rabbit. what duration of spray time wise should I set the BK mod at (.1 to .2 secs something like that right) let me know. Thanks as usual guys for the info. I am determined to get this thing back to the way it was. Frank :)

also with stock jetting, and bk mod timed right where would you suggest the air screw? 1.5, 1.75, 2.0 turns out? thanks, frank

It's actually the pilot screw, not the air screw. You would just need to experiment with that on your own. If you go more than 2 turns out on the screw than go 1 size up on the pilot jet. Good luck.


It is normal for a bike to have decelleration popping, especially with the throttle closed. This popping is almost a blubbering, soft, muted, sounds healthy, like a muted harley on decel. This is NOT what you are describing.

What you are describing is an ignition problem. On decel, the ignition is not firing, unburned air/fuel is building up in the exhaust (after missing several strokes) and when it does fire, you get a bang out of the exhaust. Sometimes it will sound like a wet burp burning in the exhaust, at other times it will seem like your header will explode.

At high RPMs, this will be more of a sharp crackling noise and then cutting out, to the point of dying at very high RPMs until the RPMs come back down.

The ignition problem you have can be several things, bad stator, bad CDI, bad coil, bad plug wire, bad plug, fouled plug, etc. The most likely is the fouled plug. You can test this. If when you replace a plug, it runs fine for a few minutes, then starts this symptom, the problem is your plug. Verify it by putting in a good plug when the bike is already hot and see if it the new plug still helps for a little while. If new plugs don't offer temporary help, go through the ignition troubleshooting checklist in the Electrical section of the manual. Swap parts with a buddy if you can.

If it is fouled, there are several types.

1. Carbon fouled, from running too rich or too cold. Replace plug and fix jetting.

2. Oil fouled. Clean plug gap with file, regap plug, and replace rings or valve seals.

3. Wet fouled. Dry plug, fix jetting or accelerator pump.

4. Additive glazing. Clean plug gap with file, regap plug, and change brands of gas.

By far, the most common fouling is carbon. You have a rich pilot and are going richer. Try leaner, like a 38, 1 1/2 turns out. Drop your needle two clips from stock (raise clip 2 pos). Main is a very small factor, leave it for now. Run a new plug like this and never quite wide open. Does it still foul? If it does, and the plug is black and sooty, you are still rich. If the plug is helped by drying it or cleaning the gap, it is wet fouled, oil fouled or glazed. If after cleaning the ceramic around the center electrode, the plug starts and idles fine but has these symptoms on decel and high RPMs, it is carbon fouled.

Look man, you've had this problem a while. Everyone on this post has assumed the backfiring you described was the normal lean popping on decel and are urging you richer. That is NOT it. You have ignition problems, most likely a carbon fouled plug. To clear up the carbon fouling, you would go leaner, exactly the opposite of how you would clean up lean popping.

I feel for you. I know we could lick this in an afternoon. I recently fouled a brand new plug in two laps, found my main jet rattling around in bottom of bowl. I you are thinking, it can't foul that fast, you're wrong. Don't troubleshoot with just 1 or 2 new plugs. You may need a box of them to get to the bottom of this. When I was fighting a plugged pilot, I went through 5 in one day troubleshooting. You can NOT clean them, the problem is NOT in the gap of the plug, it is the ceramic.

Good luck,


I READ this, I dont know if its true: Why do you get backfire/popping on decel with a lean pilot circuit? The lean condition prevents the flame front from spreading to all parts of the cylinder (like a trench that firefighters dig to stop a forest fire from starting). There are some unburned pockets fuel that make it to the header where they ignite.

I dont get it either. I fixed it, but I dont know why what I did changed the circumstances under which the popping occured. Solving this requires a new catalog, and a fresh roll of TP!

Shawn has it right made the same exact jetting changes on me 00 and whammo

I imagin longitude, lattitude and attitude make a diff :)


Y'all are all giving the right advise for the wrong symptoms. Listen to what FastF says,

I mean loud pops and backfires

Does this sound like the muted pops of deceleration? Listen to the man.

Sorry but I've been there too often. I know this one. ARrrggghhh, the frustration!!!


I had the same deal, with the poping on decel, I fixed it with the air screw. If you have the 45 pilot in there, go to 2 full turns out on the screw. This should clear it up. I did the BK mod, went with a 45 pilot and needle in stock position, and a 168 main, and the thing rocks. If your near sea level, this jetting should work for ya. If your still getting the popping, Id make sure the headpipe isnt leaking at the cylider head, then go after the ignition. But it seems to me, if it was an Ign. problem, the bike wouldnt run all that great on the top end like you said it did.

Well guys,

Didn't mean to start a debate. LOL I do appreciate everyone's input though. I got my mechanic to pull the carb last night. He completely disassembled it and cleaned it thoroughly. We put it back on. I changed the oil and installed yet another fresh new plug. He reset the BK mod to .1 to .2 seconds and then I started it up. Still had the bog if you stabbed the throttle. Ok then we fine tuned the BK screw and that helped a little. Then we started fine tuning the pump timing screw. Well we got it where it would only bog after you stabbed the throttle off idle after about the third or fourth stabbing. Best I can expect under these conditions just sitting in the shop. My mechanic is going to come to the track this weekend and help dial it in a little more with the air fuel screw whatever you call it on the bottom of the carb. Wish me luck I am gonna get it out tomorrow and see what happens. Thanks again for all your help. I can't imagine where I would be without the help of all you knowledgable guys!! Frank


Just think of those load pops and backfires as Gas me, Gas me, GAAAASSSSS ME I wannna run dont shut off

Just my $4.25

my 00 even after the mods "BK & Jets" , spits, pops and bogs when it is cold or not at prem temp range.

I have also noted when riding even at optimimum temp, that depending on if I am rolling the throttle on and stabbing it depends on the gear I am in. If I am say 1st or second and just bang it, I feel a slight bog, if I am in 3rd and up rolling on the beast excelerates no worries.

I dont know maybe its just the way the yz is wired,

I agree with you, I just feel you pay 6k for something and you want it to be perfect, as hard as we bash these dirtbikes it is hard to keep them perfect (running condition). So many different variables come in to play. (Temp, jets, elevation, "RIDER!!!!", etc. etc.) Even running bad this thing still will overshoot the 80 ft table or 70 ft. double at my track. In 5 years from now I bet the two-smokes of yesteryear will be gone and the 4 stroke supreme bikes will be the only way to go. Face it motocross and supercross world the 2-smoke is slowly fading away and the 4 stoke is in!! (hope i don't offend anyone by saying this) Just my $4.26 cents worth!! Frank :)


Ya I will be posting Vintage pics of those smokers then :D

Our kids will ask

Dad Billy Bob called me a stinky 2stroker

Whats that mean? :D

Plus Look at what we pay for the wifes, man talk about upkeep :D:)

[ May 31, 2002: Message edited by: EgoAhole ]

I am trying to picture this in my head but clockwise would be turning the screw out there bud. It is going to be 93 degrees here tomorrow so I am gonna get the ole 426 out early in the am and turn some laps. Hopefully she will be running better than ever!! wish me luck. Everyone be safe this weekend riding and such!! Frank

Unless it is reverse threaded than the screw will come out when turned counter-clockwise.

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