15 amps seems too lite for a 5 horse motor, the nameplate should give you a FLA or LRA rating, full load amps is probably near that 110 psi, locked rotor amps is at start-up. you could swap it over to another 2 pole breaker to test the theory. While you are in the breaker panel, tighten every screw that you can find but BE CAREFULL!
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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