Put the carbs I rebuilt in yesterday, and it appears the problem is fixed. Being lazy I changed too many things to say for certain what it was. I suspect one of 2 things. After opening up the carb body I noticed a blocked solenoid feed as indicated in the pic and this, if blocked would stop air being sent to the emulsion tube affecting a rich condition?

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Otherwise its as Wobb suggested ie knackered float valves letting in fuel with vibration. This would mean that BOTH sets of carbs have the same problem so I will pull apart the other set and check the solenoid feeds and replace the valves.