most large motors should run at the lower frequency without burning out given a cubic power curve.
that said, most motors are very cheaply built.
an 8 pole motor is not the norm and usually those motors are properly designed.. for the simple fact that the shaft spins at a much lower velocity, and they can't throw a cheap fan on the back of the motor and pull 300 cfm through the case.
so if its 50/60 hz rated, that's fine. the problem is the higher frequency needs more voltage to push the same flux, and given the cubic power curve, you need 1.44 times as much torque to push 1.728 times as much power into the pump at 1.2 times the rpm.
so, that's why its not coming up to speed.
a short term work around is boost the voltage 20% and try again.
as long as the name plate amps are in range. (note that power dissapation is amps squared) the motor shouldn't burn out.