Honda-SOHC

Ive recently bought a 1975 CB750F1 as a bit of a project bike. Its a runner so i put it through an MOT so i could run it a bit to see what needs doing. Starts no problem and generally runs fine. However as soon as its warm i get a low knocking sound!!! It does seem to smoothe out if i rev it but as soon as it idles its back. Leave the bike to cool and its fine again. First thoughts were carb balance which has been done, with no effect. Any other advice before the inevitable winter strip down


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