stick a wideband A/F meter on it, and see what your air fuel ratio is. Lean wont help none. I've experienced pinging at the same revs yours begins at too on old motors of mine. A trick you can do is lock your dizzy, and see how much total spark it can stand. Then pop rivit a clear plastic 360' school protractor to an old rotor button. Then hold the bottom of the dizzy shaft in a vise. twist the rotor and see how far it moves. remember that 1' at the dizzy is 2' at the crank pulley. Unlock your dizzy, and build up the slots with weld untill it's only able to mech advance to the max spark it could stand when your dizzy was locked. That way your engine isn't having to contend with too much advance in the lower rev range.