For some time now ive been using a single jet stair tool (heavy duty) with a 11006 spray tip in, i use it with my truckmount, but every so often it leaves stroke marks on some stair carpets,which cant always be brushed out, i generally turn psi down to about 300psi, is the jet too big or is it just the pesssure and heat causing these marks, or would changing the jet size help?