Good question. He blamed the fire on removing the screen. I can see how the screen would reduce the flame front from progressing through...maybe. Or maybe he just soaked his air filter in gasoline to clean it up and didn't want to mention that part. Admittedly anecdotal - take it for what its worth.

Could see this discussion on another forum
http://www.thumpertalk.com/forum/arc.../t-361802.html

 

I assume the screen hole size is smaller than the "quenching distance", which is the smallest opening size that a flame can propagate through. For gasoline, it is ... wait ... 0.2mm, or about 75mils. Ahem, I doubt the screen has mesh holes of only 75mils???

 

Correction: quenching distance is 2mm (about 80mils). Could be about the size of the mesh openings...

 

Check his small parts to see if they are singed.l

 

OK, this whole deal got me to thinking. Why don't all manner of 4-stroke engines with paper element filters without backfire screens catch fire with more frequency? I'm old so I remember the days of my hot rodding youth when all engines had adjustable timing, ignition points, and nasty big carbs...a perfect recipe for fire...and we had some. But unless you let the fire just flat get out of control on top of the manifold, I can't recall ever seeing the paper filter elements being the real source of promoting the fire.

I've had numerous hot rods over the years ranging from a '32 Chevy 5-window coupe, a couple of different '57 Chevy's, and many others. The worst offending carb fire monger I had was my first car, a '57 Chevy that I installed a 409 dual 4-bbl engine in. I put a big cam in it along with a dual point distributor. I rebuilt both Carter 4-bbl carbs, but it was still an occasional challenge to get the dual point distributor (a Mallory), carbs, and timing all in line for a smooth run. I saw flame shoot out of those trick Stellings & Hellings dual air cleaners (w/paper elements) all too frequently. I was always amazed that the filters didn't even appear singed. The fuel vapor would ignite, but there was never really any sustained fire unless you got some fuel on the carb body or intake manifold. This was the same thing I observed on many other engines that would have a serious carb backfire. Yeah, if you let the exposed fuel on the carb body or manifold sit there and burn, I'm sure it might continue to build into a fire.

So, could the perfect storm of a totally flooded motorcycle engine, mistimed cams, and an out-of-whack ignition timing come together to start a real fire? Surely it's possible. With today's relatively fixed ignition systems, it's going to be harder to do. A stuck float can allow fuel to back into the airbox if the bike is sitting just right, but it usually goes out the bowl overflow as designed. Mistimed cams could also add to the possibility of the engine not starting but allow flooding and backfiring.

I've had many XR's and KLR's and now a KLX with the screen removed and haven't had so much as a hiccup out of the engine that might lead to a fire. I'm not concerned. But for those who are, leave the screen alone. My experience with my last KLR650 that required a step up on the main jet when the screen was removed tells me that the screen removal "can" allow more air flow at higher rpm's. I don't think all airboxes are created equal, so some may benefit and some may not. Engine size may also contribute to this issue. You'd have to test every engine on a dyno to truly quality all this, but with the screen removed on my KLX I don't have to worry about it.

 

But seriously, I'm always learning something new on here. Quenching distance, I did not know that.
Dan

 

Don't worry, dan. The next time your KLX is flying upside down as in your race vid...the fuel in the carb siphons down into the airbox...the motor stalls causing a backfire...and the spectators on the course are treated to a huge, glowing fireball of green and red. Then...we'll know the true story about those screens and fire potential.

I hope the video is still running then.

I'm surprised you didn't know about quenching dynamics. I'm constantly applying it on my dirt motor trips. In fact here's a pic of quenching in actual action.

 

Oh that Quenching. I'll quench to that

 

I don't know if the screen purpose is to block backfires or not. When I first saw it during the first maintenances, I thought it was to block debris in case the air filter disintegrates with time. Later, I recall reading somewhere that a spark arrester on the intake side is required on a dirt bike to be legal in many places, same thing as an exhaust spark arrester. Not sure if it's true, but would make sense in light of current environmental paranoia, be it justified or not

 

Alright dan...I stole this off of one of West Oz KLX's pics on another post, but I thought it would be appropriate to the subject here.