Tell you what:
Wire an external switch in series with the cap/switch combination for testing.
Let the motor start and then turn off the added switch. If it behaves, you may be set.
Also if you can bring out two wires across the switch and monitor the voltage across it. Initially it will be zero and then probably near line voltage. This should, at least give you an idea that things are working.
If they are, then, I think you have the basis to to try a permanent fix.
You don't want the contacts to stick and that's what they are doing. Fixing the contacts would be difficult, if not impossible.
Move the voltmeter to the winding. Note the back EMF. Your going to probably want, to know this value and maybe subtract 20%.
The centrfugal switch for a dust collector, I guess has it's uses primarily because the dust collector is constantly turning on and off and you don't want the starting winding to be engaging while the motor is running.
Usually you can hear the centrfugal switch cut in as the motor slows down. It's difficult to hear when starting up.
What you want to do is replace the centrifugal switch with something like this:
http://www.magnecraft.com/library/se...831_Series.pdf
in 120 V.
http://www.alliedelec.com/Search/Pro...DESC=831VS-120
You'll need a piece of DIN rail to mount it. Another $10.
Monitor the voltage across the start winding and the contacts of the relay would go where the centrifugal switch is. See:
http://www.achrnews.com/Articles/Fea...00f932a8c0____
This is a 5 terminal version. Contacts, sensing and power, rather than two.
This would gives years of trouble-free service. You'll just have to get the adjustments right. Measuring motor current could help. If it's engaged too long, then the current would be higher.
Then you should also be able to remove the centrifugal guts as well.
References:
Reversing and Repair of Electric Motors
Potential Starting Relay Voltage Specs - Editorial Archives - Air Conditioning, Heating & Refrigeration NEWS
Kickstart®Hard Start Devices - Contractor FAQ's
What you're seeing in the cap value is the tolerance, which is/can be very sloppy for this application.