Reading your plug colour is the very best in my opinion to CHECK carb tune. To adjust the carbs I set my jets soully by idle speed. Take it for a ride, get it to idle relatively slowly, hook up an electronic tach ( a diagnostic one not an in dash unit) and adjust each jet for the highest idle you can get just on the jet adjustment. When you get each carb at the highest idle speed on the jet, then adjust your base idle speed either by throttle stop or slow run screw depending on which model SU you have. Do your jet adjustments slowly so the system has time to get it. If it appears to be loading up because it was overly rich then hold throttle at about 2000 rpms for 30 seconds or so to get it cleared out.