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Hi: Make sure the compressor is unloading after it shuts down. Check the following: #1 There should be a bleed line from your check valve to a needle valve on your pressure switch that blows down the pump and discharge line to the check valve. You can tell if it is working by the slight hiss from the valve after it shuts down. This has to stop after less than a minute or your check valve is leaking and you are starting against full tank pressure. #2 You may have a centrifugal unloader that runs off the back of the crank that does the same thing. A copper line usually runs from the high pressure cylinder dischare area to the unloader at the back of the crank. This is an internal flyweight governor type of needle valve which bleeds the air into the crankcase and is usually difficult to diagnose by listening. If your flywheel bounces form side to side after the pump comes to a stop, it is a good indication that your pump is not blowing down. The bleed line may be fouled with carbon (especially if you are using motor oil in the pump), or the stem of the needle valve may be worn too short to actuate it. You can turn this valve in or out to set the preload on the stem. There may be another copper line in this area going to the low pressure cylinder. It is a crankcase scavenge line that takes in oil vapor from the crankcase to lubricate the valve plates. #3 You have pneumatic unloaders on the intake valves. In this setup, air pressure from the tank runs through an oil pressure operated valve on the back of the pump to unloader pistons on each intake valve (Westinghouse 5yc and 3yc pumps have integral plunger type oil pumps running off a special throw at the end of the crankshaft). When the compressor stops or loses oil pressure for any reason, air pressure from the tank will go through this valve, through these lines and activate a plunger in each head which pushes unloader claws down onto each intake valve plate, holding them open and preventing the pump from pumping air. Air goes in, and comes right back out again. The pump is freewheeling after it first starts, giving the motor time to spool up to full rpm, before oil pressure builds, air pressure is cut off, and the unloaders release, permitting the pump to compress air again. O rings on these plungers may have failed and the pump is pumping against full tank pressure on startup. Other problems: Low voltage, loose wires, failed motor capacitors, fouled discharge lines, failing starters, overly long power cords, broken valve plates (If your interstage valve was blowing, this is the reason. Never plug an intertage safety valve port thinking the valve is defective. It is blowing for a reason). 99% of the time, copmressors are starting poorly because of faulty unloading after shutdown. Westinghouse compressors are massively overbuilt and will last forever if you take care of them.
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