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I have had these problem on two sets of carbs. One was back firing through the air box and the other just would not stop overflowing.If you take the float valves out and inspect with a magnifying glass, you should find that the brass is pitted. A new set of these worked both times >>> https://www.ebay.co.uk/itm/Honda-CB-550-Four-F-F2-K3-Vergaser-Schwimmerkammer-Ventile-Carburetor-Float-Kit/401500262334?hash=item5d7b47cfbe:g:atwAAOSwwKhcq3pI
Surprised to hear you say that Trig, most people who use keyster kits say the jets are okay but the float valves give problems so they buy genuine ones.Sent from my X6pro using Tapatalk