Makita Air Compressor Rebuild

[JTx1717]

Hi everyone, I'm new to the forum. My neighbor had put out a small Makita MAC700 Compressor (portable, oil lubed) out side for junk pick up. I thought I'd take a look at it before it got scrapped or someone else got it. Cosmetically it's in great condition. Upon initial inspection the motor turns and the pump puts out air. However, it's not holding pressure. Air was coming out of the oil filler tube/cap.

Before I start ordering parts, I wanted to get some input with trouble shooting. The unit retails for $200. I looked at some of the parts and they are pretty pricey. The unit retails for $200. Accumulating costs for individual parts may not be worth rebuilding it.

Makita does not have a factory service manual available so I don't even know what the specifics of the machine are (i.e torque specs). So I'm turning to forums to see if the collective knowledge of the members can guide me.

I've researched online and a tell tale sign for air coming out of the oil fill is the compression rings (around $30-$40 for a ring set). Before I start contemplating on making that type of investment, is there anything else I need to be looking at?

Also a question about the connecting rod. There's play at the connecting rod and crankshaft. Looking at the rod head on (see pictures) there's no play from left, right, up, down. However, I'm able to slide the rod along the shaft about 1/4 inch towards and away from me. I'm not sure if Makita designed it so the rod can "float". Also, there's no cap for the connecting rod, it's a a closed loop that slides on to the crankshaft.

Any input would be greatly appreciated. Thanks in advance!

 

[SGKent]

you might pull the head and see if the exhaust valve / reed is carboned up so it can't dump air or close properly. If the exhaust gets compromised any compressor will puke air wherever it can.

 

[The Cobbler]

do you notice this after the compressor has shut down & unloaded?
I'm wondering if the check valve at the tank is leaking back .

 

[JTx1717]

This is what they looked like when I pulled them off.

 

[The Cobbler]

if it's building pressure as it should, the reed valves are not the problem.
if it's loosing air pressure while it sits, there is a leak somewhere and if there's air leaking out of the compressor when it's shut off, the check valve is almost certainly bad

 

[JTx1717]

I noticed it when I was inspecting the machine while on. At first I thought it was coming from a leaking gasket but then realized it was coming from the the oil fill tube.

 

[The Cobbler]

it's normal to have some air movement in the crankcase.
what is the compressor doing .how much pressure does it build, how fast does it lose pressure.
if it's building air pressure but not keeping it, you have a leak somewhere, it is definitely not the reed valves .

 

[JTx1717]

It will only get to about 20PSI at most and then just plateau. It seems like the air just goes out of the oil filler.

 

[The Cobbler]

OK.
so it's possible the piston rings . but try to look for leaks before ripping in to it. try to soap up everything and see if there are any bubbles anywhere. I had some craftsman compressors I bought on clearance where the head gasket was blown and would only build about 20 psi . there could be a lot of other possibles other than rings . I had a senco where the unloader line was cracked & it would only build 20 psi. since you have the head off already, can you put your hand over the cylinder & see what sort of compression there is.

 

[JTx1717]

Thanks Cobbler for all your input. I took off the cylinder head along with the intake and exhaust valve plates. Of course the gaskets between them were damaged in the process. i ordered a new gasket kit along with new valves. Once I get it together temporarily and give it a go and let you know the results.

 

[strutaeng]

Is it possible to put some oil in the piston to see if pressure builds up? Maybe add a tablespoon on the air intake? Sort of like when doing a compression test on an auto engine?

Just a thought. I've never had this problem on compressors before, but not an expert on the subject.

My BIL bought one of these Makita compressors, but cant remember the model. I thought they were good machines.

 

[nadogail]

IMHO, anything Makita is good.

 

[JTx1717]

Makita is good, no doubt. But even the best of brands have their limitations and it's mostly user knowledge (or ignorance) that dictates how long a tool will last. I take care of my tools and know their limitations. The right tool for the right job. Being that this compressor is second hand, I don't know what it's been through. I have no idea if my neighbor had followed the right break-in procedure or how bad he beat up on it. But I can guess. He now uses an electric pancake compressor with an impact gun and that thing is constantly on cycle when he uses it outside on his trailer.

Cobbler - I did put my hand over the piston to see if I can gauge any type of compression while moving the fan/motor manually. Sucked the palm in on intake and pushed air out passed my palm on compression. It was strong and the pressure on both "seemed" consistent. Not as accurate as a leakdown test as you would do on a car but I didn't feel any pressure loss through touch alone.

strutaeng - It's easier doing compression tests on cars. I have an automotive compression tester and factory service manuals for my cars. So I can easily reference if the compression values are within acceptable range. I tried asking some specifics from Makita Support and they just told me to contact the nearest authorized service center.

 

[The Cobbler]

...

Cobbler - I did put my hand over the piston to see if I can gauge any type of compression while moving the fan/motor manually. Sucked the palm in on intake and pushed air out passed my palm on compression. It was strong and the pressure on both "seemed" consistent. Not as accurate as a leakdown test as you would do on a car but I didn't feel any pressure loss through touch alone.

......

I get you that it's not an accurate test. and at higher pressures it could be more of an issue. doubt you reached 20psi pressure with your hand covering the cylinder too.

I still suspect a leak somewhere other than the rings, but you've got it all apart, you could always look at the rings & see what the end gap is or if one is broken. is the cylinder wall smooth looking without scores?

 

[JTx1717]

Here's what the cylinder walls look like

 

[Mandres]

it's (almost) always the check valve.

 

[The Cobbler]

that cylinder looks in good condition. doubtful it's rings IMO. there's a leak somewhere else .

 

[JTx1717]

I thought the same about the cylinder walls. They're smooth to the touch and even ran my fingernail to find any nicks.

The Makita check valve is about 30-40 bucks. I looked into other brands but the Makita has a flared end for the inlet tube compared to the others I saw online . I suppose they did this so that people would only use their parts.

 

[The Cobbler]

I would not suspect the check valve if it's not building over 20 psi . if there is air flow out of the check valve after it has unloaded , then yes . but your problem seems to be not building more than 20 psi, and the tank holds that pressure without bleeding down, correct? then there is a leak somewhere on the compression side of the check valve . either in the head, pipe, pressure switch, a gauge , or somewhere

 

[markushofer27]

I have the same compressor , love the thing .quiet n mobil

 

[SGKent]

does the head have reed valves in it? what condition is the filter?

 

[JTx1717]

So I had a bit of time to get to the compressor. I took the check valve off and I was able to blow through it both ways. Inlet was easy as it should be. But I was able to get a bit of airflow going back towards the compressor side. So I guess that answers my question: Getting backflow to compressor. Unfortunately the check valve from the manufacturer is $30-40. There are check valves on Amazon for much cheaper but I would need to get a flare adapter.

 

[The Cobbler]

you can take the check valve apart and see if you can clean it. there's basically a spring and a teflon seal inside . the cap probably has a washer and is threaded onto the body

 

[JTx1717]

Cobbler - I did. I took the cap off and it was a real short spring and a seal/cap. I noticed the spring was a bit "relaxed". I carefully expanded the spring to give it more tension. Blew in it again and it didn't let any air through. How long it will last only time will tell.

I just can't justify buying a whole new assembly when all that needs to be replaced is that small spring and seal. All I need to do is clean the intake/exhaust plates and install gaskets. I'll fire it up and let you know the results.

 

[The Cobbler]

great . hope it works for you
I have o bunch of odds & ends of compressor parts, I may have a spring & seal . shoot me some pics & dimensions of you need

 

[JTx1717]

Thanks Cobbler. I appreciate the time to offer suggestions and even going so far as to offer parts. That's more than I can ask for. Just one more thing before I get back to you next time, how much you think I should torque the cylinder head and valve plates? It didn't take too much to remove. I'm thinking 5 ft/lbs? Maybe even inch/lbs?

 

[Monza Harry]

To address your concerns 1 at a time. Some (very little) air out of oil fill, normal the crankcase displaces the same as the compressor, Lots of air: broken/seized/missing rings or a cracked piston. The lack of pressure build up: something is leaking bad somewhere, head gasket, exhaust/intake reeds/sealing faces. Did you listen for blow back at the intake? The check valve wouldn't affect pressure build-up unless it has been doing the work of a weak exhaust reed, even then(?) and then it would only build a low pressure as that is a lot of volume that will un compress on the intake stroke and prevent out side air from being inhaled. Your re-arch of that spring will last about 2 minutes I'd bet, it will not need to be a super strong one, it is seated by air pressure and only needs to be strong enough to deal with the weight of the ball bearing at speed [~1725Hz] check your hardware or better yet an auto parts store for a generic spring of similar size.
As for your torque specs find a torque chart for the screw sizes used, in aluminum/diecast and use those. Oh that rod will knock and will want out. When it comes to reassembly, spray some aluminum paint on all faces where you wish to re-use the gaskets let dry (not much will stick to that paint), if nothing else it will be easier to clean the gasket off [it won't stick]. Rings, rod and some work around for the check valve and you are about 2/3 the way to a new one, I'd strip off the motor if it has a full case and the controls and toss the rest including the tank, it is of unknown age and they aren't worth the cost of a pressure test. A brand new 30# propane tank is about $55 (Canadian) which is about 10 US Gal. Just my idea's of what to look for/consider. Harry

 

[JTx1717]

Hey Monza, thanks for the tips.

I was able to install the new gasket kit and reinstalled the check valve. I fired it up and low and behold it gained pressure and shut off like it should (around 130 psi). I torqued the cylinder head screws to 5 ft/lbs to play it safe. Seems to hold up OK.

Thanks to everyone that gave input. Got a real good compressor for 20 bucks investment in parts.

 

[The Cobbler]

20 bucks and you have more knowledge now too. good stuff!