1.) Nor correctly set ignition can act as wrong synched carbs. Check that F aligns with case mark at around 1000-1200 rpm, full advance when it should, something between 2500 - 3000 rpm.
2.) Verify the jetting before synch. Not too rich pilot, needles, airscrews/fuelscrews. All carbs must be correctly jetted, not dirty inside. Ignition wires have all good connections, fastened.