Steve:

Valves? On a four stroke sticking valves will affect the high speed run, improper valve lash will show up more at low speeds. An exhaust leak wont make any great difference, unless the muffler pressure ends up too low for good fuel feed.

On a two stroke the exhaust leak will have a greater effect, again, through lowered fuel pressure.

In either case, the use of a non metallic gasket usually results in the leak coming back in just a few minutes of engine operation. Flatten the mating faces as well as you can, then use a soft metal gasket with RTV on both sides, or preferably RTV alone.

Bill.