Every KLR600/650 and XR250 that I've owned, and this current KLX does this to a small degree...and that only after an aftermarket exhaust, airbox mods, and a rejet were performed. Even with optimum jetting it seems to occur to some degree in many of these instances. I think the popping is occurring in the exhaust pipe and in most cases not detrimental to the exhaust valves. I'm no expert at this, but this is just my observation.

 

That's what I figured too as it only started happening after fitting the 2Bros Muffler. It is my understanding that if it is caused by the exhaust its ok. It's when the backfire is happening up at the valves that it can be harmful. Me no expert either.

 

Well, what do you know! I Googled "deceleration backfire" and got tons of hits. Were they all in agreement?...hardly. I read a large number of the responses, and some things seemed fairly consistent. The backfire seems to occur in the exhaust pipe. The backfire can be caused by a rich or lean condition, as there is almost always unspent fuel in the exhaust gases. The backfire has to get air to ignite, and this air can come from a leaky exhaust seal...at the head or even in the length of the pipe. An aftermarket exhaust is more likely to cause this type of backfire because it can allow more outside air...even all the way back from the end of the pipe to a hot spot in the pipe...to be available for the combustion of this unspent fuel.

The classic "air, fuel, and spark" are still at work here. Even though a spark in the sense of an actual spark plug isn't present, a location in the pipe is hot enough to ignite the right combination of air and fuel. This is why it can be a rich or lean condition that can result in a deceleration backfire. The three elements of air, fuel, and spark(heat) just have to be aligned to have combustion. The ignition can also be caused by a trailing element of flame left over from the last combustion cycle which ignites in the pipe.

There wasn't an ironclad cause and effect for all engines and conditions for the obvious reason of variables between different bikes.

 

Nice desription, TNC....i just want add a bit....my friends experience that already change their exhaust with after market exhaust got same problem, backfire and the exhaust sound not smooth when we pull the throtle. Actualy, the after market exhaust is free flow model, we can knew it from the sound, the sound of after market exhaust is louder than fabrication standart.In this case, i think because the lean condition ( we talk about klx with EFI version). Remember this klx version, EFI mapped from fabrication, look like limiter.
So, if want change the exhaust, must remap the EFI or add additional part, like Power Commander 3 (PC3) or other brand that same usage.
It will reduce the backfire on exhaust. it works and my friends already proved it.

i'm not the expert, but i just want sharing..
thank's
unnoku

 

Yes,thanks for all of that, but I guess the question is, is it normal to experience back firing during decel with an after market muffler and is it going to damage anything????????????

 

you can mess with that and with a carb as opposed to fuel injected you just have to put up with different issues (performance at different elevations) or you will be forever adjusting the thing and not riding.

 

After reading all those many articles and posts about decel backfiring, the possibility of damage to the engine was almost never mentioned. The possibility of damage to a fragile exhaust system seemed to be the only concern. Even from just an annoyance factor, some found it to be a pleasant sound in certain situations...obviously a preference type of thing for most.

I'm sure that there are some situations with certain types of bikes and exhaust systems where the possibility of damage can exist, but in a more simple single cylinder design like the KLX and other 4-stroke dirt bikes, it's probably all occuring in the exhaust pipe and doing no harm to the engine. The bikes I've owned personally that had this characteristic showed no signs of damage to the head, valves, or anything else. I'm talking about single cylinder dirt bike 4-strokes. I've had some of them apart after many miles of use and even competition and never observed an internal engine problem. Dirt bike exhaust systems are pretty tough too, and this probably helps prevent any exhaust damage. I'd suggest just to get optimum jetting for performance and clean running, and not worry too much about it unless the sound is much too harsh and frequent. Some decel backfiring may just be part of the nature of many modded bikes.

 

Thanks Candyass, that last paragraph of yours has answered everything. Think i'll just jump on it and give it a dam good walloping. Cheers.

 

Some off throttle deceleration popping can be normal, but if your getting big bangs (backfires) you are probably fuel loading the pipe (rich) on whatever carb circuit you were running on before you chopped the throttle.

 

does this happen with the fmf pipe too? i have the two bros, not installed yet thought. would the different types of tips for the twobros cause or effect this problem to?