well an air leak would definitly do that. Check for an air leak where the carb mounts to the intake manifold.
I doubt that the airfilter is interferring with the slide because it would have to be half way into the carb for it to interfere.
if you remove the new airfilter, does the engine run like it's supposed to?
back your idle adjustment screw almost all the way out and try re-adjusting it.
sometimes the scew can get hung up on the slide causing the slide to be open too far.
did you take the top of the carb off and pull out the slide before or after you installed the airfilter?
it's quite possible that you re-installed the slide wrong and it could be binding up inside the carb.
check your throttle cable and make sure that it has a little slack...not too much tho.
A lean mixture will cause a high idle, but it would have to be EXTREMELY lean for it to rev like it's at full throttle.(you did mix 2 cycle oil in the gas right?)
If the float level in the bowl is too low, the engine would rev up and then die because there wouldn't be enough gas in the bowl.
When an engine runs out of gas, it creates a lean condition causing it to rev up, and then die right away.
It sounds like yours doesn't die...it's just revving like mad all the time.
Adding an airfilter(even a high flow one) will make the mixture slightly richer than with no airfilter at all, so i don't think the filter has anything to do with the problem. remove the filter and see if the engine still revs real high, and that will eliminate the filter.
I'm thinking you have a problem with either the carb slide or you have a serious air leak.
