Has the primer face got the right needle in it.
Variations in manufacutring tollerences mean theres a few different length needles to get the fuel height right.
With the primer face in your hand assempled tip it so that the needle closes and check that the float is sitting right, generally the relief in the float should be concentric to the outer circle in the face for the post the main jet screws into. If its too long the lower edge of the float will barely be in past the outer edge of the face. If its too short the float will come in as far as the hole for the jet.
Dont roll the o-ring on. Pinch it with your thumb at 6 oclock and fore finger at 1 oclock and then stretch it on the rest of the way around.
The head could be warped. Try it on the block with no gasket and see if it sits flat, if its warped it will rock.
I assume the motor is not over revving? Are the o-rings on the inlet manifold OK? I have seen split mainfolds as well.
