I've synchronized the carbs with the common synchronizer tool and keep adjusting the mixture screws but can't seem to get the two front plugs to look the same as the two rear plugs. The front plugs are a very light gray (almost white) while the rear plugs are charcoal in color. I've adjusted mixture screws a quarter turn at a time but improvements have been minimal. They both started at a base point referred by the manual. Do I keep adjusting the screws in opposite direction or is there something else I need to know about plug checks?