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oups.com...

It sounds to me to be brushes, or a worn commutator, or both. Check
the windings and make sure they are still in good shape, insulated and
clean. Try blowing out the motor assembly and motor with another
compressor.

If it is brushes, you can fix it easily, if it is the commutator you
are toast. Available, but $$$. A bad capacitor would certainly keep
it from starting, but generally speaking if that was it, it wouldn't
start again later as if nothing was wrong. But a bad capacitor
wouldn't account for the sparks.

But for $150, you shouldn't have it fixed at all.

Mixing motors, I fear. If it's a universal motor, it might have
brushes/commutator. They'll spark as part of the nature of the beast, just
like your router or other things with universal motors.

If an induction motor, it might have a capacitor, but it wouldn't spark long
before igniting, sparks not being part of the nature of the beast.

Brushes can easily be replaced. Not worth rebuilding a universal
commutator, that's for sure.