I am seeing many pics of planes, and none of them have an air filter attached on the carb?

There's not much dirt up there... Unless you live in LA

Hey, Minn.. how can you trust air you can't see... Hmmm?

The only time you are at significant risk of sucking a lot of dirt into the engine is on the ground. If you fly from asphault or concrete, there is no need for a filter. Most grass runways are also typically clean enough that a filter isn't needed. I have heard of some that use filters becasue the site conditions warrant such, but in general, the vast majority of sites don't require a filter. One might think "why not just use a filter anyway." I have used filters before and they will rob power. I dropped 150 RPM when I used a filter on a YS140. 150RPM may not sound like a lot, but it equated to 0.15HP on that setup, or a 5% drop in power, just from the filter.

I use filters all the time because we fly from dirt runways. If they were grass or paved I wouldn't bother with a filter.

I am required to use a air filter at all times because I use a airplane engine to power my truck and I like a drivin' in the dirt. [>:] But with my airplanes, I don't bother because where I fly, we take off from a 400' asphalt runway and there is no dust/dirt often (but there is fine dirt off to the sides of the runway)

dick,
it's the air that you CAN see that you shouldn't trust