Originally Posted by 
Aaron IIA
				
			 
			I would continue to run it on 91 and retard the ignition just enough to stop it pinging on heavy load. Retarding the ignition will actually make it run cooler. If you retard it too much, the exhaust manifold will run hot though. When advanced, the fire burns early, so there is more time for the heat to be absorbed into the water jacket, so will run hot. When retarded, the fire burns late, so it is still burning while the gasses enter the exhaust manifold. This holds true for petrol and diesel engines.
You may find that you may need to time it with 0deg advance. I would not bother with the timing marks too much, as unless they are the flywheel mounted types, I have found them to be mounted wrong.
Aaron.