Hello franken, your fuel air mixture burns at a constant time whether your engine is running at 1000 rpm or 6000 rpm. Thats why you advance the timing to start the burn earlier at high rpm so the F/A mixture hits optimum burn around TDC. If you start the burn to early the mixture has already achieved optimum burn befor TDC and acts lean on the cylinder. Richening up the mixture overich conceals the problem but you're not making optimum power just burning more gas. Class dismissed.