Greetings,
If I might make a couple of suggestions;
1. I presume that the floats are not punctured nor out of alignment
2. the folk have checked the 'Float height' by turning the carburettor upside down and measuring the float to float bowl base distance
3. check that the breather pipe is clear and terminates above the carburettor
4. some carburettors have a tiny seal around the valve body and these fail over time, not sure if your carburettor has such or if it is simply an interference fit
5. please follow this link for an article that explains the condition better than I can here;
https://www.fix.com/blog/motorcycle-car ... djustment/In the past I once cured a similar problem by using an 'old' float valve with grinding paste to clean up the inner seat face of the valve body, of course it is essential to thoroughly clean the carburettor of the paste before using the carburettor.
Just some ideas off the top of my head but hope they help, come back to us if not.
TTFN
Hugh.