need help 86 b2000

Just bought a 86 b2000. It had a blowed head gasket and i got that fixed. Now it seems to be missing when i hit cruising speeds in each gear. Just wandering if someone could help. Does anyone no where i can find a vacuum hose diagram? Also saw in forum where someone fixed this problem by replacing the carburetor secondary vacuum diaphragm. I have no clue as to what or where that is on a carburetor. Please help. thanks