Air compressor builds slowly after 50 psi

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Old 02-18-16, 12:39 PM
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Air compressor builds slowly after 50 psi

My air compressor was an auction buy it ran well but seemed to build kinda slow. It's a mix of parts I tore down the pump and found it was full of carbon very full. I cleaned valves and Pistons cylinders look to be in great shape After putting back together it builds to about 50 psi faster than ever but then slows down The motor doesn't seem to be struggling or slow down any it's only 1/2 horse but seems to be working. Just fine. Just lost as to what could be causing it to slow down building pressure but not motor ???

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Old 02-18-16, 12:45 PM
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Welcome to the forums.

It's one of two problems..... either leaking rings or a cracked reed in the reed/valve plate.
 
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Old 02-18-16, 01:30 PM
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If belt driven, make sure the belt is tight and not slipping. I torn down a pump one time because the compressor wouldn't build up past 40-50 psi ..... boy did I feel dumb when I realized what the real problem was
 
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Old 02-18-16, 01:36 PM
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I rebuilt the head I'm getting excellent suction and output so I'm pretty certain it's not the valves And I really don't think it's the rings unless they aren't lined up right cause cylinders are immaculate crosshatching very clear and smooth walls. It builds to 60 psi in about 2 mins but to get to 120 takes almost 10 I do have a leak in my delivery tube from the pump that's small and I know would contribute just didn't think it would this bad. Plan to replace soon
 
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Old 02-18-16, 01:40 PM
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Belt is next on the list. This belt is ancient it's one of those really old red fabric like belts. I'm sure a new one would have to help
 
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Old 02-18-16, 02:41 PM
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The leak is probably half if not all of your problem. Picture a hole, 1/64" diameter; not very big, but, over all, perhaps similar to whatever gap you have. At 50 psi, a 1/64" orifice passes .225 cubic feet, which may not sound like a lot, but your 1/2 horsepower pump is only putting out something south of 2 cfm, so seems pretty conceivable that you're losing at least 25% of your air by the time you reach 50 psi. And it goes up from there, so it's naturally slowing down.
 
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Old 02-18-16, 03:45 PM
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Right I knew it would be an issue for sure. I didn't hear the leak until I shut everything down. (Wife wanted me inside to spend time with her. lol ). It's just a leak at a joint. I plan on running a new tube anyway because right now it comes out as half inch down to 1/4 or so then up to 3/8 before going to check valve. What a mess. I think carbon got into my check valve also it's not closing and I just put it in last year. Chasing down problems but I think it will be worth it once done. Little pride into it never hurts
 
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Old 02-18-16, 03:55 PM
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Last time I measured cfm I got around 4cfm or so I forget exact timing but from 0 to 120 psi in my 30 gallon tank took around 9 mins? I think. I know I got around 4cfm after the math. It's a single stage two piston pump But now that it's clean and back together it got up to the 50psi range in half the time as before so I'm hoping my cfm has improved. I mean it was packed full of loose carbon. Idk how it even worked before I have only found one other pump like mine it was on a compressor on cl it had a 2hp motor running it at 3500 rpm. My motor is only turning at 1725 I wonder if this would help. I couldn't find much info on the pump when I looked it up in the past about rpm rating
 
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