Popping back thru intake

I was cutting up some leaves and low grass when the motor started making a popping noise. I determined that it was coming from the intake. I did a compression check. The left cylinder was 180psi the right was 120psi. I removed the head. The gasket did not appear to have blown. The valves move freely. There is a little carbon build-up. I cannot determine if the valves are seating all the way. I quess I could spray some penetrating oil around them and see if it leaks thru. Any thing else I can do?