[elhigh]

I once read somewhere that a farmer could lose as much as 1% of his above-ground storage tank's capacity on a hot day.

Rather than plumb that vent through the air filter, shouldn't it go to the evaporative canister, the one that catches vapors from the crankcase? Volatile vapors that get cooked out of the crankcase oil get caught there and burned; seems to me that if that's the job it's for, that's a perfectly reasonable place for the fuel tank vent to go, too.

If it isn't plumbed for that already, you may need a larger evap can to deal with the greater volume.