Hi Gary
The problem is the z heads they use rubbish guides they are to soft, also check the valve springs they were to soft on mine 90-95 pounds on seat should be around 130 good news is my problem is fixed and my mustang goes like a bat out of hell ,cost me a bit of doh though as i bought the motor from a agent there and i wasn't going to ship the engine back from here, this is a comon problem even though ford racing won't admit it, i would not recomend this motor to anyone and they have lost a few sales from here after my debacle ive done around 1600mile's since and has used no
oil so happy there aswell.
Good luck with it and keep a eye on it and don't let them feed you BS about pcv issue or improper intake gasket fitting All BS from ford racing, my expeirience soured what is a great sounding motor and i loved the way it idles but still bitter.
cheers chris.