the 350 in my dad's '72 C10 is not running right, it started acting up when they were coming back from our other house which is 2.5 hrs away using the thruway. wouldn't idle, runs like crap, sounds like it's missing, smooths out when revved. and sounds like a pulsating hiss coming from the passanger side of the intake manifold, it used to constantly hiss with no pulsation. we ran a compression test on 6 of the 8 cyls, it was getting dark, so we gave up for the night but will get the other 2 tomorrow probably. all but the center 2 on the passanger side are 120, the center 2 on the passanger side are 110. any idea's? thanks. My dad thinks a couple valves burned up