I,m new here so I got to get my 2 cents in sometime to. nice to meet so many people that have real experience working on mopars. you said you used a ld4b intake with 360 j heads, what intake gaskets are you using? the smaller intake ports on the ld4b can leak on some combinations ,are you using good aftermarket gaskets, fel/pro ect. not the metal ones. also I agree with the leak down should be done. the only thing cyl.4/8 have in common is the pos on the dist cap. and dist cam that opens the points, also is this miss only at idle or high speed. does the engine shake at idle ,does it smooth out when a cylinder is canceled,?
