There could be a couple of things going on here.

On the valves: If you are running out of adjustment, the pushrods are likely worn and need to be replaced.

Does the needle valve bottom out on the needle socket? If it does, the needle assembly is worn out and needs replacement. This is a problem we usually only see in engines with a lot of running time.

Both of these problems sound like the engine has a lot more than three gallons of fuel through it. 91 AC's haven't been made in about 3 years. Did you buy this engine new? When?

Dave Shadel