Do something to measure the psi's and you have a working figure to deal with, perhaps adjusting the size of the exaust to a smaller/nozzle and/or rigging it up to move a servo and/or seperate prop can do the trick as well if you cannof find sufficient thrust. If I get the gis of the dialog right, it sounds like you have a very light airframe, so even a minute thrust may do the job, be it not entirely responsive in a practical sense if it was heavier.