Trying to tune my 78 GS550. Have cleaned the carbs, air filter, etc. It starts fine and pulls well up to around 6k where it starts to stutter. I can push it pass 6k but it is not very happy about it. The carbs were running too rich and with several adjustment to the pilot screw I have cylinders 1, 3 and 4 running OK (according to plug color). Any ideas on why the 6K rough running? Thanks