what oil are you running? not that it makes any difference with cooling. but it will with pressure.
add a overflow to the drain of the radiator fill tube. make it a catch can, with the hose going to the bottom.
also if a head gasket is leaking. watch the oil level and you can also fill the rad to the top and watch for bubbles when it is running.
what do you have the timing set at? does the mech advance work? tom
