back in the 70s the zenith carb was junk and still is.you have a bad carb #1,Do you have a --t-- jet on the bottom of the fuel bowl? of the carb. the float should be brass, float it and see if it sinks and remember that gas is a thinner base than water.It could float in water and not gas,very decieving test with water.If the engine cranks and has a non constant crank then do the compression test and if the head gasket is blown,it wii be reght between the 2 cyls where the distributor goes in the head.Do you have water in the oil?If the compression test proves good then throw away the gas carb and look into a propane conversion.Check to see if any intake nuts are loose.the intake and exhaust are bolted on the same side and have nuts on top where you can see them and in the middle between the 2 manifolds.remember it takes three things to make an engine run,spark,fuel,compression-later