The rocker arm is probably seized, look inside the valve clearance points and see if the rocker is stuck against the valve. If you just did a rebuild you should have taken them out and reinstalled them/bought new ones and applied white grease to the pins. If it sounds like the motor is really cranking easy that is the problem more than likely, I just had both the intake and exhaust rockers seize because I forgot to do that ****. You really should do that with a brand new high lift/high duration cam. You'll need a slide hammer to get them out, and one I found for really cheap was at mac tools' website. It was only 35 bucks and it was called the 5lb knocker set. Just install a M6 x 1.0mm x 60-70mm or longer bolt in the head and wack away. It sounds like that is your problem, it will run fine for about a half hour and then seize. PM me for further info on how to easily fix the problem without tearing the whole top end apart again. You'll have to pull the head bolt to remove the exhaust one. Another thing that you want to do is fill that 'pool' in the head with oil upon rebuild, warriors are notorious for late delivery of oil. Without that pool filled the new cam really puts some heat and abuse on the rockers. Be sure to check that there is oil pressure in the first place as well, afterall you don't want the same problem to happen again but worse requiring a rebuild of the crank.
