You have to REALLY watch out for how an air filter of any kind effects the bowl venting of the carburetor. Not long ago I was doing some dyno testing for a customer who has a Pantera. He wanted me to test some air filters when I was done with the rest of the engine testing and it was quite interesting. What he had was one of those Holley "dog bone" style filters fitted with two square K and N elements that were about 3 inches tall. They are on their sides, one on each side of the carburetor. Anyway, in the testing on the 425 horsepower engine the elements and more importantly the things that held the elements were very restrictive.
Even more interesting and where I'm going with this was that he also had some air filter spacers from B and B. There were two of them, a one inch and a 2 inch. Testing the filter with the one inch spacer made it run WAY better, so I tried the two inch, this made it run MUCH worse! Examining the filter revealed that without a spacer the vents were very close to the top of the filter housing, With the 1 inch they were in an area that was about even with the floor of the filter housing. And finally with the 2 inch they were way down from the floor of the filter in a big hole. What I think was happening was that without the spacer the bowl vent was obstructed causing metering problems. With the 1 inch the bowl vent was in a favorable location and then with the 2 inch the bowl vents were possibly being pressurized by the air turning and flowing down toward the carburetor. This was much worse than no spacer at all.
I'm thinking that having the filter top while flowing more air might tend to cause the air to "blow" straight down toward the top of the carburetor and thus upset the fuel metering. All carburetors are subject to this sort of influence and it can be a very big deal. In my case the engine was down about 12 horsepower with the 2 inch spacer combination!
