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Backfire screen

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2004 yz, should I cut out the backfire screen in the air box? Bike runs great, started today first kick, 35 degrees out, Just wondering if there is any benefit?

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No don't cut it out!

If you want to run without you can get an aftermarket cage/filter from No-Toil or TwinAir. These kits come with special flame restant air filters designed to be run with no screen.

The problem with cutting out the stock screen is if any bits of wire come loose they will do serious damage to your engine.

You CAN use a cage from a 2 stroke YZ but you run the risk of an airbox fire if your bike backfires.

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There is some power gain, but not much on the 250. This mod makes a bigger difference on the 450.

You can cut the screen but you must remove all the wire that remains.

I've replaced two cylinders that had bad scoring from debris on customer machines and both cases the owner had 'cut' the backfire screen out (more like poked large holes in it).

TwinAir and NoToil make flame resistant ait filters. The screen serves a usefull purpose. Unlike a two stroke, a four strokes will blow flame through the carb when it backfires. The foam in an air filter will retain fuel vapor and can ignite, causing a fire (I have experienced this firsthand).

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I just had my YZ in for the carb mod and that was 1 of the things they reccomended. I just got the No-toil filter and screenless cage. Hopefully it's all good.

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Leave the screen. Unless you need that.01 HP and are such a fast pro rider that it will make the difference to pass Stewart, it is better served in place.

How perfectly jetted is your bike now? What is the A/F and how much of a variation have you found in air density to require a clip change?

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Leave the screen. Unless you need that.01 HP and are such a fast pro rider that it will make the difference to pass Stewart, it is better served in place.

How perfectly jetted is your bike now? What is the A/F and how much of a variation have you found in air density to require a clip change?

Well.....we have an enduro practice loop. With the screen in place, my best time was 6min 36 secs.

When I cut the screen, my best time was 6min 35 secs.

My butt dyno did not register any change.

Must have been all that weight I saved cutting the wire gauze out.....:smirk:

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Well.....we have an enduro practice loop. With the screen in place, my best time was 6min 36 secs.

When I cut the screen, my best time was 6min 35 secs.

My butt dyno did not register any change.

Must have been all that weight I saved cutting the wire gauze out.....:smirk:

Or you got better tractoin or (more likely) and you are finally picking up the pace. I hear that is supposed to be a 6:05 loop, at least that is what the peewees run it at. :bonk:

On the serious side, it is possible your jetting is a tad rich and an easier 'draw' leaned you out a little. You may want to move the needle 1/2 step leaner with the screen in place and see if your time is the same before and after. On the 250's, the airflow just is not that restricted compared to the screen size. It can be an issue for a worked 450 though.

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