Can't quite get the jetting right on the XR600

I have made a few mods to my 98' Xr600, ie FMF Powercore 4 pipe, snorkel removed etc and I've tried various jet setups and at the moment the 158/68 seems to work pretty well but when your let off the gas it hesitates and continues revving for a second before dropping down.

Does this mean the mj might be a bit too big?