How can this be down to the presence or not of an unloader valve?
'Presence or not' or not working properly?
The very simple check/test of seeing if it works from zero pressure, immediately after showing signs of stalling, will indicate whether the motor is actually restarting correctly after a high pressure restart.
Very simple, easy to do and informative. The job needs analysing as to whether the problem is due to the motor not fully speeding or the compressor tightening up.
The motor is well oversized for the job. My 10cfm, @10 Bar, compressor has a 2HP motor, but only 1425rpm (so better starting characteristics)
Guessing is no good at all. Actual checking , to isolate the problem, is the first priority!.
Edited By not done it yet on 27/08/2016 07:21:05