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Old 03-20-2016, 07:33 PM
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Compressor repair question.

I have an old stand up 80 gal 5hp compressor. It has lost it's umph and has gotten so that it can't keep up with event he smallest tool. It will pump up to 100 lbs or so and then when you use that air in the tank it starts pumping and simply can't keep up with a die grinder. It takes a long time to pump up to the 100 of course too.

I am thinking it simply needs a pump, everything else works fine.

It has a twin cyl side by side pump. Over at HF they have two pumps, one that looks just like the one it has that they call a "3hp" pump, no we aren't talking about the motor, we are talking about the pump.



And then the "5hp" pump which is a V twin.



And for some reason they have a few of each with different part numbers that I can't for the life of me figure out the differences.

Air Compressor Pumps at Harbor Freight Tools

Why does mine look like "three horse power" pump? I like the more output of the V twin, not sure if it will fit on mine, I didn't check that. Neither bolt on as the bolt pattern is different.

What do you think?

Brian

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Old 03-20-2016, 07:37 PM
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A true 5hp motor will turn either pump. If you can fit it, use the bigger pump. The HF pumps are not bad.
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Old 03-20-2016, 08:20 PM
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What brand is it? There are rebuild kits for those. I had one of the HF v-twin units and it did not hold up..

Sam
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Old 03-20-2016, 09:14 PM
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HF v twin.

A friend gave me a junk HF V twin pump that they had on a compressor for about 3 months before it quit. It got a lot of use when they were rebuilding farm equipment. He replaced it with a
Quincy (I THINK ) I don't remember the shop I used to deal with in San Jose for rebuild parts.
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Old 03-20-2016, 09:37 PM
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Quote:
Originally Posted by MARTINSR View Post
I have an old stand up 80 gal 5hp compressor. It has lost it's umph and has gotten so that it can't keep up with event he smallest tool. It will pump up to 100 lbs or so and then when you use that air in the tank it starts pumping and simply can't keep up with a die grinder. It takes a long time to pump up to the 100 of course too.

I am thinking it simply needs a pump, everything else works fine.

It has a twin cyl side by side pump. Over at HF they have two pumps, one that looks just like the one it has that they call a "3hp" pump, no we aren't talking about the motor, we are talking about the pump.



And then the "5hp" pump which is a V twin.



And for some reason they have a few of each with different part numbers that I can't for the life of me figure out the differences.

Air Compressor Pumps at Harbor Freight Tools

Why does mine look like "three horse power" pump? I like the more output of the V twin, not sure if it will fit on mine, I didn't check that. Neither bolt on as the bolt pattern is different.

What do you think?

Brian
Check out ebay. I put together a complete 5 horse, 2 stage about 5 years ago for less than $800. That included the motor, pressure switch, and motor starter. The only thing I had was a 60 gal. tank It is kind of noisey but it has held up pretty well for me.
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Old 03-20-2016, 10:19 PM
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as H Compton said - it is most likely in the one way "valve" which is just a piece of spring thin metal at the inlet port on top of the head - I have an old twin cyl Speedair I bought when I got out of the Navy in 1976 and just flipped that metal flap or valve over a year ago and it has been working great - I feed 2 - 30 gal tanks with the same compressor - just a thought - jaxx
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Old 03-21-2016, 07:36 AM
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I rebuilt a 2 cyl. compressor pump from an old Sears compressor, got a rebuild kit from WW Grainger, came with everything except piston rings.The kit cost about $ 90.00
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Old 03-21-2016, 01:18 PM
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most basic compressors have read valves in the head and are easy to rebuild
most of the work involves flat sanding the plate that the reed valves run on
stick some sandpaper on a flat surface to sand the plate, start with 220 and then some 400 grit

if you know the brand/model; new reed valves can be ordered and are cheap



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Old 03-21-2016, 02:40 PM
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I went with a Northern Tool 3 cyl 2 stage pump. Good reviews, quiet, low vibration, and plenty of cfm for what I do. Handles grinders, sanders, and my spray guns easily.
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Old 03-21-2016, 05:31 PM
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Some awesome info guys! I am going to look at those valves tonight, thanks so much.

Brian
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Old 03-21-2016, 08:43 PM
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When Jimmy Carter was president, he instituted the Truth in Advertising law... everything damn better be what you advertised !!! ... But under Reagan and later presidents, anything was allowed again... 2 HP air compressors started being labeled 2 1/4 HP... Govt did nothing... same for when they labeled them 2 1/2, 2 3/4, 3, 4, 5, 6, 6 1/2, 7 HP !!! Believe it or not, 2 HP air compressors actually got sold labeled as high as 7 1/2 HP !!! But, looking at the air output in SCFM (Standard Cubic Feet of air per Minute), they were only 2 HP or less... This info generally applies to home/small shop units rated up to 7 1/2 HP... A few years ago, something must have been finally been done by the Govt, because compressors started getting labeled with a big HP number, but in smaller print a "Running HP" number that was the real HP...

So, first, you need to see if your unit is actually "5 HP"... Look on the motor plate, does it say "5" under HP(good)? Or 'special' or 'spcl' (semi-bad)... means prolly about 3.2 - 3.5 real HP (hence the "3 HP pump")... or even much less...

A true 5 HP motor/compressor prolly doesn't plug in... usually hard wired because of the high electric current draw...

Any compressor with a 120 volt plug is prolly 2 - 2 1/4 HP or less... no matter what it is labeled... trying to draw more than 2 HP of electricity through a 120 volt plug/outlet melts them down...

Air compressors up to about 125 psi use one or two pistons the same size and are called single stage... most home units... Air compressors for serious shops producing 175 psi usually have a big piston that grabs lots of air and bumps it up to 100 psi or so, then a smaller piston that bumps the air further up to 175 psi... the big and small pistons take about the same power to run and thus make the compressor smooth running... these two stage compressors also usually need at least a true 5 HP or larger motor to turn them over... true 5 HP motors and larger are also built to withstand much harder usage for many more years than household units... household-use motors may overheat if asked to run more than 3 times in an hour...

X 5... I agree with the other guys, if you have a good unit, rebuild/repair it !!!
.

Last edited by BuzzLOL; 03-21-2016 at 08:48 PM.
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Old 04-11-2016, 10:44 AM
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Brian look at where the air discharge pipe goes into the tank. There may very well be a one way valve there. i so take it apart and clean and inspect it. Then give your set up a try again. Air leaking back into the compressor will screw up the compressor efficiency.

BB
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Old 04-12-2016, 08:45 PM
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Brian Martin,Freelance adviser
 
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BB which one are you talking about? The big one that comes out of the compressor to the tank, it doesn't look like any kind of valve there. Or the little one that goes to the little "control box?"

Brian
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Old 04-12-2016, 09:37 PM
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Quote:
Originally Posted by MARTINSR View Post
BB which one are you talking about? The big one that comes out of the compressor to the tank, it doesn't look like any kind of valve there. Or the little one that goes to the little "control box?"

Brian
I was referring to the big discharge pipe. On a lot of compressors there is a one way valve located at the base of the discharge that either bolts to or screws into the tank. The purpose of that valve is to keep the air in the tank and not going back into the compressor. I don't see one on your tank so it must be the same set up as Jaxx22 was referring to. You can't have air going back into the compressor.

https://www.youtube.com/watch?v=pduPuJ9k9Bw

BB
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Old 04-13-2016, 10:19 AM
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Brian Martin,Freelance adviser
 
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Thanks, I am getting some good info and I need to jump into it!

Brian
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