I have one that simply lost its screen a couple of years ago, I didn't take it out...can't tell any difference in how it runs.  I doubt it impedes the air flow to any significance.  I consider it sorta neutral--doesn't hurt anything; doesn't really help anything either.  Most engines don't have a venturi screen, and I haven't heard that it's a problem with them.  But I guess I'd just leave it alone.

--Ray

John, i fly daily on grass and dirt and the cox engines i have without screens need to be taken apart regularly and dirt cleaned out of the reed and tank. I wouldn't think of taking the screens out of my norvels. If you fly in any dirt i would highly recommend keeping the screens in, as far as casto clogs go, i feel sure over time one small blast of engine cleaner would clear the screen and evaporate .