'91 F150 Will NOT Start When Humid

Since I've had this truck everytime it rains or snows it won't start. I talked to a guy and he said the butterfly might be stuck. Where do I find the butterfly on this engine. It just keeps trying to turn over but never starts. It eventually starts to flood. Any help would be WONDERFUL!:banghead:

There is probabley a crack in the distributor cap letting moisture in. Comon problem, since ford put the caps at the front of the motor. Next time it does it pull the dist. cap and see if there is moisture in it, if so wipe it out with paper towels, spary WD40 in the cap and then wipe it out, it will still coat the inside of the cap. WD40 displaces water.