If it runs Ok with the filter off the filter could be dirty, worth a go. Other than that it could be drawing air into the manfold, blocked jet or even the idle adjustment screw backed off.
I'm having the same trouble with 'Chicko'. Ran really nice going to Tewkesbury, but stalled everytime we stopped [!]
Friend of mine knows the carbs really well and has taken 3 away, to build one good one