C3 mis-fire

Can anyone help ? I have a 1979 z650 c3 , us import , it has had replacement Uk carbs fitted ,cleaned and set up , new points ,condensers,coils . My issue is that it struggles to get past 85 mph , it runs really sweet up to there but starts holding back at about 75-80mph . I’ve had around 6 previous z650,s but never experienced this issue , has anyone else had similar issues or any ideas what the problem might be ? Grateful for any help

Could use some more info. You running stock exhaust and air box? What type of UK carbs are you running, vm24ss or something else? What size main jet are you running?

Hi thanks for your reply , I’m running standard pipes and air box , new filter , not sure what jets are in tho , kept the ones which came with the carbs ?? Standard Mikuni carbs from what I can tell . Came off a B1 possibly ??

Could the ignition advancer be stuck?