I would say check for air leaks in the case and the base gasket and in the intake manifold and the the exhaust gasket. Those screws should not require that much thought to get the right setting. A small air leak wont really be noticed at high speed but at low speed makes a difference. I mean can you imagine this? Your friend or little brother comes and turns your screws. "whats this do?" Your like ARGGHH!!! It took my threee weeks to get them set right!!! No, they should be obvious as to the correct enough setting. But then again, I have spent hours and hours and hours tweeking similar screw settings or timing settings myself to try and achieve the perfect whateverness only to find later on that it was something dumb like that. Dont forget to check crank seals. They matter alot at idle and low rpm. Blow smoke in the intake at top dead center to find air leaks.