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Thread: Air compressor not pumping properly.

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    Default Air compressor not pumping properly.

    This is not quite automotive, regards a workshop compressor. I have a 8cfm compressor , is now a fung she, but when I bought mine was made in Taiwon. Its preformance dropped significantly, I thought after 36 years i t was just worn out. I bought rings and valves and did a rebuild. It was actually in surprisingly good order, the cylinder had almost no wear and honed up very quickly. In short the performance is no better. Seems to have little suction, yes I did remove the air cleaners. The compressor will pump to 120psi but pumps very little air. There is nothing obviously wrong, can anyone give me any tips?



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    8CFM isn't really a whole lot. There's plenty of reasonably priced compressors on the market these days that will pump at 100+CFM.

    If you really want to get that one going again and have done all that work, the only thing I can really suggest is to double check and reseal all connections in the air path and make sure there's no blockages on the intake side between the filter and head. If it's pumping to 120PSI, it won't be in the block.

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    did you check the condition of the reeds?
    Last edited by Rick; 03-03-17 at 08:37 PM.

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    Ive repaired many compressors through the years.

    You mention it pumps to 120psi.

    Is the issue a lack of flow at 120psi which points to a restriction in the piping?

    Or does it take forever to get to 120pSI which points a valve not sealing or a blown head gasket? (if the cylinders are fine)

    Does it also struggle on restart which points to a leaking one way valve? (which in turn is fighting the compression its building)

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    I did a couple of quick tests on Sunday on one cylinder. I pumped it up to 120psi and stopped the motor (petrol). the air held indicating that the exhaust valves are good. I feed air back into the in take, it pushed the piston to BDC., I kept air pressure on, no air was coming out the sump, indicating that the rings are sealing. I pushed down on the intake valve, the valve was easy to push down indicating that the valve was not sticking. I removed the air cleaner assembly no restriction. No leaks on gaskets. One of these problems, there is nothing wrong, but it doesn't work properly so there must be something wrong. I originally condemed the compressor on the basis of it not pumping past 50psi, but it was the guage which was faulty. I don't think I have made the compressor any better by fitting new rings and valves. I tried to use a die grinder before i fitted the rings and it would not supply enough air, it still won't. The grinder is supposed to require about 3-4cfm, about half the nominal capacity of the compressor. It seems not to have good suction. Even if the exhaust pipes were blocked it still should have good suction. It does seem to take a long time to pump up. When you use the die grinder the air pressure drops quickly to 40psi.???????????

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    8cfm is nowhere enough to run a die grinder, hence the quick drop in psi, A minimum of 17cfm is required to run air tools adequately

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    Where the feed pipe enters the tank there should be a brass fitting.

    This is a one way check valve.

    This is what stops the tanks pressure from feeding back to the exhaust valve.

    If you dont hear any leaking back from the exhaust valve, its because of the check valve, not the exhaust valve.

    My last compressor repair was a few months ago, it was a unit that couldn't keep up with my die grinder, ended up being that it blew the metal head gaskets.

    I ended up removing both the head gaskets (twin cylinder), i polished the head and valve block mating surface with the large knife sharpening stone till they were perfectly flat, sprayed some copper spray onto the mating surfaces and bingo, no more headgaskets to blow and no leaks.

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    Thanks Godzilla, I will check this one way check valve. I have great hopes of finding a problem there, as I have said before there has been really no change sine beofre and after the ring and valve change, so here goes. I will let you know what I find, but probably won't be for a few weeks.

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    Well its certainly been a few weeks, but to my dismay the antidrainback valve is in good order. Leaves me to check the exhaust valves.

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