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I just rebuilt a 318 for my 73 dart swinger. I did the port work on the stock heads and sent them out to a machine shop for new steel guides and smaller 11/32 stem valves (increased valves size to 1.905/1.6). They also put in pc valve seals, comp cams springs and retainers. After about 50 miles, the #4 exhaust valve stuck open and bent the push rod. Took the head off and back to the machine shop. They said the stem had galled itself to the guide, probably from oil starvation. So, they put a new valve in as well as a bronze guide to fix it. When I reassembled the motor, I used an old valve cover with the top cut off so I could observe the oil flow. Oil was flowing abundantly through the stock rocker shafts and running out to the tip of the valves. Seemed good to me. Drove it for 5 miles and "bink", it siezed again, this time #2 exhaust.
Called the machine shop and they were baffled, so they said they would try to open up the stem to guide clearance from 1 thou to 1.4 thou. They installed another new valve and bronze guide. I thought my stock rockers may not be pushing enough oil to the stem tips so I replaced all of the rockers with new stock ones, observed the oil again, all seemed good. Drove it for 5 miles, and "bink", #2 stuck again.

My question is do you think that the positive type valve seals are starving the valve stems? should I go back to umbrella seals? This is very frustrating. Please help.



Are the PC valve seals the white Teflon seals ?
If so there is a big part of your problem. You need to pull both heads , take them back to the machine shop , probably expect to replace all the exh valves and switch to a viton seal or an umbrella seal.