Apé cuts out on overrun.

Hi...I've just fitted a new Deloitte carb to my Ape 2008...it's got a larger jet ...I thinks it's a 63....it runs like a sewing machine on the flat and uphill but tends to cut out going downhill on the over run. Could it be a float problem..?..or maybe a fuel tap issue. It has a brand new inline fuel filter.
any help gratefully received.
John