One header pipe HOT,,,others NOT...

OK, this isn't a Mopar problem, but the principle applies to Mopars as well.....

My son's off-brand(Ch*vy) small block develops one really hot header pipe right after start-up (250*+), whereas the other 7 are quite cool (~100*) at that time. We have backed off the exhaust rocker for that cylinder until it clacks good (thinking lifter pump-up) , but the problem still happens. I'm thinking that combustion gases are being pumped directly into the header because that exhaust valve not actually closing completely, or could we have a burnt valve?

Any input is appreciated....