A MVVS can take a while to run in. It is also likely to be a combat version if it was sold in the US and will have a BIG hole in the carb. Having said this run it in [ I would use castor min 20% ] Then establish a good full throttle setting on the needle. Set it to just below half throttle and apply the pinch test [pinch the fuel line and the rpm should rise a tiny amount before dropping ] adjust the slow running till this happens. If it is going to throttle it should do so now. Both the MVVS I owned had good throttles.