I think I posted this on another airbox thread. A guy on Thumpertalk with a KLX450F motocross model had some dyno printouts of his bike with and without the backfire screen. It showed a max increase without the screen of about .04 of a horsepower if I remember correctly. Everything was allegedly identical in the tests except for the screen.

Now...something to consider. A full blown racing bike like the KLX450 may have a different/better design for the airbox. There may be intake size issues that make a difference too. It may be an apples-to-oranges comparison with a KLX250/300. I don't know. I do know that my '01 KLR650 needed a full step up on the main jet when I removed the screen in that bike. I had noticeable detonation at wide-open-throttle after removing the screen. That's not a dyno reading of course, but it seems pretty indicative of real change in air volume. Now...that was on a KLR650, so this probably varies from bike to bike. Only a dyno would give the real bottom line. I pulled the screen out of mine. Just one less thing to get in the way of any performance tweaks to my bike. This along with jetting, pipe, and open topped airbox was done at the same time, so I didn't get to test the screen separately like I did on my 650.

 

And I'm thinking I may have been a jet size or two rich at the time. Once the screen was removed, the jetting was right, and the gains were from not only less restriction, but from proper jetting.
I'm not kidding. The gain was very noticeable, no butt dyno BS.

 

So really you could have achieved the same results by rejetting.

 

Lost in the edit.
From proper jetting as well , and it's only a guess that the jetting contributed, because the difference was so significant.

 

i could see it helping a tad. quick question. are you saying the screen part of the yz will fit, or are you talking about the air filter? im guessing your trying to say the air filter, but a "backfire filter" might be a name for a backfire screen. also, if you can it back to back screen/no screen with the same air filter, did you notice a power increase at all? just to say if its worth it or not. i might/might not do it either way cus ive had my bike backfire a few times lately. motors cold most the time, i just start the bike up and go a lot of the time. had one that could have made people duck, sounded like a shot gun when off.

 

You're talking about 2 different things here.
Backfiring, and backfiring through the carb are 2 completely different things.

 

I have been following this thread and what the poster is talking about is a Yamaha air filter than will fit into the KLX and the brand of the air filter is Twin Air who have managed to incorporate a "backfire screen foam" into this filter. Twin Air does also make a direct KLX air filter but that particular model does not have the backfire foam incorporated into it.

 

This is no flame whatsoever...no pun intended...but does anyone have a first hand experience with a backfire through the carb that has resulted in an acutal fire in the filter or airbox? I worked part time at a motorcycle shop for about 15 years, and much of that was during the period when bikes had points and adjustable timing. Even in the case of some real POS bikes where everything was screwed up and there had been some severe backfiring through the carb(s), the worst I ever saw was some darkening of the filter material on the inside.

Now I know there have to be some cases of the "perfect storm" where enough fuel, bad timing, bad carb, and everything else resulted in a fire. But I think a more modern design like the KLX is less apt to provide these elements with its ignition system and more sophisticated fuel system. I'd think a mechanic or owner would have to have the cams out of time and probably a couple of other issues all working in concert to have a real fire in the airbox. Of course this is what liability lawyers bring to the plate, resulting in backfire screens and other aggressive safety measures. Nothing wrong with safety, but I really think the backfire screen in the KLX and many other similar bikes is probably extreme overkill. I think there's much more hazard involved in removing the kickstand switch, starter/neutral switch, and some other similar features, but we do it all the time. A likelihood of an airbox fire is very remote.

 

I have never heard of it happening.

 

TNC, I tend to agree with your comments. I have already ordered a new air filter from UNI and I also do plan to remove the screen as I have no doubt it creates a flow restriction and I am not too concerned about a backfire causing a fire in my airbox. Worst case scenario I drop the bike, and call my insurance agent.