Hello all. I have an unusual problem with my Victa Mustang mark 3 mower. It will run fine flat out but if you try to run it at 1/2 throttle it will start backfiring & die after 30 seconds or so. It will also start backfiring if you try & open the throttle from 1/2 & either die or very slowly pick up revs. Any help would be greatly appreciated. Thanks all.