Ok we have an Older Air comperssor From 1961.
It has a Cast Iron Compresser.
Looks like the compressor is a DeVilbiss Model WP210B1009
And the Whole unit was built by Kargaro CO Model # MBI-KA-61-4474

When I was given to me I saw it work Saw it go all the way up to 80PSI and Shut off like any normal compresser would.
However now it will only go to 20PSI. I know it is not the guage as it will not fill a car tire.
We have Replaced the releif valve, the pumptroll, and had the Electric motor sent out the motor came back fine.
I bypassed the pumptroll and put power directly to the motor and it still shuts off at 20PSI.
When the compress reaches 17-18PSI you can hear it start to slow down.(any compressor I've ever heard runs strong till it reaches the pressure point) and when it hits 20Psi the motor can not turn the compressor over and you have to throw the switch to kill the power.
Since the motor tested fine, I think it is the compresser. but the compressor does not get hot. is it possible there is blockage in the lines? creates to much back feed to the compressor? does the compressor need rebuilt?


biran

It sounds like a broken piston ring or rings
Start it up and list to what sound the barrel is making if it a ringing sound almost certainly the rings have gone.
check also the gaskets at the top and bottom of the barrel for any leakage you'll see or feel the blow-by and heavy wheezing
Repair: Piston rings gone Maybe an Auto factor store or Motor engineer may be able to source some to fit the piston? And do drain the oil and fish the broken bits out (With a magnet) Should be in the sump or near the sump plug (be good news if it was a magnetic type plug?)
Blown Gaskets... As its not an engine you can get away with applying "Instant Gasket" (cheap in all auto stores} over the old gasket/s after cleaning the jointing surfaces and old gasket unless you can source a new one?(Still do it regardless ) Then tightening the barrel nuts/bolts down evenly if a Do it in acrossways pattern (A torque wrench would be a great help)
PS When you said the Compressor does not get hot that is almost certain Ring failure
Good news... the Company is still in existence
But I did a parts search for the Model on the site turned up nothing (not surprised)
But wait...
I found Gold! check out this PDF(Acrobat) file Its the complete parts and maintenance manual documents for your Compressor. And I wonder if the company this came from can find the parts if you can quote the part numbers from within this file? www.controlsdepot.com
Ps You need the free Acrobat program installed to view this go to www.adobe.com/products/acrobat/ and download (27 MB) If you click on the link and nothing happens
TIP Untick the Adobe Yahoo! Toolbar
and Adobe Photoshop® Album Starter Edition boxes on the page before starting the download
Unless you have a real need for them !
Cuts 7 mb off the download

Ok it Runs for aprox 1min 15sec before it starts to slow down. it is at approx 17psi at this point. then after another 15sec you have to turn it off.
I'm not an expert with compressers. My Wife says she hears a ringing. I do not feel any air where the gaskets are.
So the best way to check the rings would be to drain the oil and see if there are pieces?

Brian

Hi Brian
There might be something on the file above on the best method of getting into the sump to investigate
If you are interested I have an ancient compressor too and its noisy big end gone (bare electrical cables!) yet its good for over 30psi (2 Bar) which is ok to get car tyres to full pressure
Target for yours is perhaps double that
50 Psi minimum if it's going good
A lot of people I know use them connected to high pressure paint spraying guns

Check page 5 of the document re the exhaust and inlet valves assembly they look easy to take apart I think if either one is sticking the Pressure will not be there Check the seating and the valve face for cracks or distortion
Also the inverted valve seat spacer "caps" on each one for damage (one could have collapsed with age or weak? hence low PSI?
Check Trouble shooter page 7 section C under Warped valve disk and Dirty Valves headings
Also try a new air filter or see if running without the air filter for a short time increases the compression pressure?

Ok

Looks like the Rings are fine. I removed both exhaust valves and see no oil on the tops of the piston like the manual say you'd find if the rings were shot.
What I have found out though is this. when the tank is empty the Compressor is easy to turn over. After 1min 30sec or 20 psi the motor strugles and When I try to manualy turn the compressor over it take some doing. Like the compressor is building up pressure. Maybe a valve is not opening?

Brian

Brian
Check the crankcase breather pipe/tube/filter for blockages/obstruction thats the cause of the issue.
I had a pinched breather pipe on a Motorbike engine and it would push an oil seal out when the pressure built up under the pistons Slow the engine and would rev easy or stall depending if the seal was spat out

Ps There may also be a small pinhole in the oil filler cap? Check for this if you can't find the pipe or filter. Use a needle to clear this

take all the lines loose and check for pluged lines.
there is something that is blocking the air from compresor to tank .
do this disconect the line from compreesor to tank and run the compreesor if it keeps running then its pluged from that point on to the tank. be carfull as these lines when charged with air can give you a face full.

edit there could be a check valve there somewhere that is bad. its a valve that will allow air to flow one way only thats probaly your culprit. be carefull