Compression release.
It is mounted on the camshaft and it opens the exhaust valve slightly, just before TCD on the compression stroke. It is what makes it easy to pull start the engine. After the engine starts, the CR moves out of the way, leaving the exhaust valve to operate normally.
When you adjust your valves, the piston should be slightly PAST top dead center to make sure the CR is not opening the valve.
If you adjust the exhaust valve with the piston slightly BEFORE TDC, you might be on the CR, and it will not work. That could cause a violent kickback when starting.
Exhaust set at 24 WHAT? Try looking that up again. That sound like a problem.