Install a pcv valve system and your problem will evaporate in a trip or two. For now, leave the fill caps and breathers off during the week layup to let evaporation work its magic inside the engine. Also, if you made any idle adjustments to the carbs (lowered it) turn it back up to make sure this is not reversion. I would guess that is the cause myself, but pcv should do the erradicate the issue. At least it has for me in the past.