You didn' clean the head and crap that clogged the cooling fins on the head. That is why the exhaust valve overheated and unseated. The valve seat may be loose, too. Any kind of oil leak on the head will accumulate and stop the necessary cooling needed. I use Brakekleen® to get rid of the crap building up and replace gaskets as needed. Setting the valves clearances, I use 6mm exhaust and 4mm intake clearances for wear on older brass gauges that flex. Tight with newer stainless gauges, so you have to allow for what type you use and the age. I use brass gauges because they are more flexible in tight spaces.
This is old, but I just faced the exact same issue on a friend's Craftsman mower with the 20hp Courage engine. Just quit running on her. The problem was overheating due to 20 years of the entire cylinder head being caked with mud and grass debris. Cooked in hard and carbonized! Cleaned it out and suspected ACR, but found loose exhaust rocker. Engine would not crank at all due to valve not opening properly. Got stuck on compression stroke. Cranked fine with plug out. Exhaust guide out 1/2" as in video. Placed piston at TDC so valve would not fall into cylinder and removed spring/keeper. Used a socket that fit over valve stem and contacted guide and drove it back in until seated. Peened aluminum head around it to keep it in place. Pushrod was not bent, so reinstalled spring/keeper, adjusted valves and it fired right up. To make it easier use a foot of rope to fill the cylinder to keep the valve closed. I had none with me, and it was hard to get the valve spring back on. She will make sure her cylinder head is clean from now on. She was paying a company to service it annually, but they failed to do that simple service. Four bolts to get the cover off. It runs like a champ now!
You didn' clean the head and crap that clogged the cooling fins on the head. That is why the exhaust valve overheated and unseated. The valve seat may be loose, too. Any kind of oil leak on the head will accumulate and stop the necessary cooling needed. I use Brakekleen® to get rid of the crap building up and replace gaskets as needed. Setting the valves clearances, I use 6mm exhaust and 4mm intake clearances for wear on older brass gauges that flex. Tight with newer stainless gauges, so you have to allow for what type you use and the age. I use brass gauges because they are more flexible in tight spaces.
This is old, but I just faced the exact same issue on a friend's Craftsman mower with the 20hp Courage engine. Just quit running on her. The problem was overheating due to 20 years of the entire cylinder head being caked with mud and grass debris. Cooked in hard and carbonized! Cleaned it out and suspected ACR, but found loose exhaust rocker. Engine would not crank at all due to valve not opening properly. Got stuck on compression stroke. Cranked fine with plug out. Exhaust guide out 1/2" as in video. Placed piston at TDC so valve would not fall into cylinder and removed spring/keeper. Used a socket that fit over valve stem and contacted guide and drove it back in until seated. Peened aluminum head around it to keep it in place. Pushrod was not bent, so reinstalled spring/keeper, adjusted valves and it fired right up. To make it easier use a foot of rope to fill the cylinder to keep the valve closed. I had none with me, and it was hard to get the valve spring back on. She will make sure her cylinder head is clean from now on. She was paying a company to service it annually, but they failed to do that simple service. Four bolts to get the cover off. It runs like a champ now!
Sounds just like mine!
you wouldn't have good compression if a valve was stuck open
I can’t see anything
I can't see anything!!
was recorded by a potato with a camera
Bad camera work. Sorry