Craftman 1.5hp air compressor won't start under load

I pulled the compressor head and piston runs smooth up and down. With head on I can turn unit easily. I can start unit with no or low pressure(30lbs) and unit will build tank pressure fine. If I try to start at 60# for example it starts stops starts etc....motor will not come up to speed.

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I would probably guess it is bad start compacitor? Can I check voltage level on capacitor and tell if bad? The centrifical switch behind fan is sparking on and off. Under...

Craftman 1.5hp air compressor won't start under load