Open air cleaners?

[my 85 gmc]

Last summer I added an open air cleaner with a K&N 4 inch air filter to my 85 GMC pickup which has a 305 LE9 engine.
The question I have regarding this modification is it seems to make the truck sluggish in town when the weather is hot has anyone else noticed this(or is it a figment of my imagination)?
To help cure this problem I reinstalled the factory air cleaner which also has a K&N air filter, removed the door from the snorkel in the air cleaner assembly and opened up the air inlet snorkel in the rad support.

Thanks Rob

I guess this is a poor man's attempt at ram air!