Yes, true. I was referring to the spark arrestor, though if they pull you up for a bit of a chat (take care, trees down, bridge/trail washed out ... that kind of thing) and you give them a whole bunch of lip then you never know just where they'll search.

 

I also could tell a difference in power when I cut out the screen as it was the last mod I did on my bike it makes more of a difference than pulling the airbox lid which also adds a slight boost on mine. I usually just run with the snorkle pulled and the lid on.

 

Right, I'm pretty sure the OP is talking about the backfire screen in the airbox, but it sound like others in this thread are thinking he means the spark arrestor in the muffler.

I cut my backfire screen out but didnt think it made much difference in power.
Dan

 

I had the same question but because the OP discusses the screen in the same sentence as air box & carb mods, I assumed s/he was NOT asking about the exhaust spark arrester.

If indeed the OP was referencing the spark arrester, I'd still encourage them to consider safety first, especially if they are out riding trails in the forest. It's one of those situations with low risk, low pay-off, and significant consequence if things go wrong. A forest fire risks others' lives and gives ammunition to those who want to ban us from such areas.

 

I agree with gc about the spark arrester. The first thing I ordered for my 300 pipe was a tip that comes with a new screen. No need to give the folks a reason to stop us from riding in "our" backyards.
The backfire screen I see as an engines last defense(albeit a poor one). At least it keeps a mouse nest from becoming a mouse condo, if you were to store your bike long enough to attract their attention.

 

As far as muffler spark arrestors, I had a chance to run mufflers on my enduro race bikes back in the day ranging from 175cc to 390cc without spark arrestors on some occasions, and frankly I didn't notice any difference in power. However, I never owned a full-blown MX racing bike. Even though a couple of my enduro models were closely based on the same platform in MX and trail/off road, I think the power characteristics out of anything designed to be a real trail bike as opposed to a pure track racer probably won't show a highly noticeable power difference with or without a spark arrestor. Of course I'd always encourage people to run an appropriate spark arrestor in their muffler, because I don't think you get any benefit it trying to run without it...legal and common sense issues aside.

 

Thank you TNC. The back fire screen in the airbox causes so much restriction ( among other problems like pressure wave reversions ) I had to re-jet when I removed mine. A very nice improvement in the upper rpm range. It's a cheap part to replace if you hate the results. Do it.
Also... how would you feel if you found out that a serious fire had started in the area you had been riding earlier that day? Exhaust spark arrestors are one of those things that let you sleep better.

 

The only problem I have with that screen is that I get grease on it from the air filter lip when I try install that dange huge OEM air filter into that tiny air box. The backfire screen is extremely large compared to the carb throat so I doubt it is a significant restriction if it is clean.

That tiny OEM snorkle inlet is worse.

 

I read an article in a recent dirt bike rag. They routinely pull the air box screens on test bikes. Dyno runs before and after rarely (if ever) indicated any power gain whatsoever. But they do claim seat-of-the-pants improved throttle response.

 

I am no aerodynamic engineer, but I have a hard time disagreeing with HB & ID on this. I have not removed the screen on my KLX but I have tried removing it on other bikes I have owned and I've seen no discernable improvement in power myself.