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Normally, there is always a check valve between the compressor and the tank, however, some manufacturers use the second stage exhaust valve for this purpose to save a few bucks. This is ok as long as the seal is good, otherwise, you have high pressure air leaking into the 2nd stage cylinder which will make it very difficult to start the unit with pressure in the tank. Also, when the compressor unloads, it unloads only as far as the exhaust valve of the 2nd stage, which is not much of a head start for any unit to work with. If it were me, and I wanted to do this as cheaply and easily as possible, I would put in a check valve, and install a tee with a ball valve in the discharge line of the compressor between the pump and the check valve so I could dump air (not close airflow between the pump and tank!). When I was starting the unit, I would open this valve, start the motor and let it warm up for a bit. I would put a muffler in the valve to cut down the noise (the compressor is pumping). This supposes that the motor does not need air to control the throttle, otherwise, you will have to devise some method to keep the revs down while the motor is warming up. When the motor is up to operating temperature, I would close the valve and let the compressor cycle. If the motor cannot turn the pump with the valve open and dumping air, it is too small for this pump. If it cannot achieve cut-out pressure bfore stalling, it is too small for this pump. The best solution would be to install a centrifugal clutch type pulley on the motor, but I appreciate that it is an expensive option. However, if you install this type of pulley, and you can control the throttle, and your compressor has a centrifugal unloader, you can cycle a gas drive compressor exactly like you would an electric drive machine. 1. The motor starts, revs up, and begins to operate the compressor.
2. Compressor package achieves cut-out pressure.
3. Pilot valve trips, sending air signal to the throttle control.
4. Motor slows down. Clutch disengages.
5. Compressor stops. Centrifugal unloader blows down compressor. Better yet, centrifugal unloader blows down discharge line between compressor and check valve.
6. Motor is idling, Compressor is stopped. Better gas economy and longer life for both motor and compressor is realized. How does this compressor unload? Unloading is the process of either stopping a turning compressor from pumping air, or blowing down air pressure in a stopped pump between cycles. Does it have a contol valve that throttles down the gas engine? Does it have unloaders on the heads to lift the intake valves (square boxes on the heads with air lines going to them), or does it just discharge air to atmosphere when it reaches cut-out pressure? From the specs you have given, I believe someone has removed the original motor and replaced it with the 9.5hp. I might be wrong, but I don't think so. If it has run like this, it must be turning very slowly, and you are getting less air than the pump can deliver. Did you see it pumping air in this configuration? I don't mean to be belittling your machine, but it just seems like a pretty hard pull for a 9.5.
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