I have a lawn mower with an 18.5 thatstarts but blows gas up through the carb. It runs but not good.

I think you may have an intake valve that's not totally closing. Can you do a compression testr and come back with some numbers?

Valves out of adjustment, or carb needs cleaning or adjusting.

Had same problem install in line fuel cut off close valve run carb dry,, then open valve and restart, close valve when not in use

I am going with the intake valve open, it is stuck open or it has burned itself or the head