As Don said above, pull your powerhead again, and look for the tiny bleed hole under it.
I had the exact same problem a few years ago on a motor I was restoring. I followed Dons advice and that was the solution.
Get a sharp object, I used a dental pick. Flip your head upside down, and pry all around. My bleed hole was invisible from years of carbon buildup from the old 30 weight oil/ gas mixture.
Bob D
