I just had this with my Tecumseh HH100 on a RF 112. The PO left the spark plug out and left it out in the yard, with no hood, just a blue plastic trap that had seen better days. Anyway the intake valve was stuck in open position and exhaust worked fine. I pulled the head and breather assembly and kept soaking with PB Blaster and after several hours started tapping with a rubber mallet. It eventually freed up. I have not any father on that engine to know if I will need work done on the guides.