Depends on how you did the plug chop. If you were running at WOT and then killed the engine, you are looking at a main jet issue. That is how the plug chop should be done. You also have to make sure that you start with a set of fresh plugs. When tuning carbs you should always start with tuning the main jet and then work your way down. Of course you need to make sure that everything is up to spec first (valve clearance, compression, ignition timing, etc).