Need Help: Devilbiss Model 342 Air Compressor

[Chopt50Ford]

Hello everyone! I am new here and have enjoyed looking through the informative posts. I am in need of some help on my vintage Devilbiss.

I have a 1960's Model 342 that was working great for the past 15 years. Long story short, the electric motor was rattling, so I took it in for new bearings. When I reinstalled it, it worked great the first time, but on the second fill up, it buzzed and tripped the breaker. I pulled the three belts off and the motor goes on and off fine without a load. If I spin the pulley quickly for a second by hand, it's off and running. It seems to not be able to get the motor going on its own except in the morning when the tank has leaked down.

I have read a lot of about the unloader valve but I am not familiar with these at all. I ordered a new NOS pressure switch, thinking that might be the problem. It is still on its way from eBay. I am starting to think it is what people are saying about the unloader valve- it relieves pressure so the motor can start again?

I would love to have this broken down to very basic terminology and help so I can get back up and running. I really don't want to drop $1500+ on a new air compressor, but I need to get back to work.

Also, could my newly rebuilt electric motor no longer be strong enough to start? It takes very little spin on the big wheel to get it going...it seems to just be hung up on some sort of pressure in the pump.

 

[MacMcMacmac]

The unloader valve is on the back of the pump. It has the copper line going to it to relieve pressure upstream of the check valve which is located inside the cast metal finned object the discharge line is plumbed into. If you remove the 1/4" copper unloader line at the back of the crank, you can remove that safety wire which holds the retainer in place. (picture 1). Take the little sheet metal cover off of the back of the unloader valve. Fire up the compressor and see if the little brass arm moves. If it does, you may need to adjust the unloader valve fitting. There should be a thin brass jam nut that holds the unloader valve in place. Loosen that, and you can turn the valve so that it moves closer to the little brass arm. It will be trial and error to set the position of the valve so that it blows down at shut off but does not continuously vent all the while the compressor is running. You should only hear air escape when the compressor shuts off and the brass arm moves to the right and depresses the valve stem. This lifts a spring loaded rubber disc off of a valve seat which vents the trapped air upstream of the check valve.

With a compressor this old, I would be tempted to renew the rubber disc in the unloader valve. All you have to do is remove the 1/4" compression fitting in the end of the valve and the spring and "poppet" will fall out. The stem is connected to a brass retainer which holds the rubber disc. This is hard to describe, but think of a champagne glass with a hockey puck in it, except the top is upside down and the stem of the glass goes right through the puck and is held on by a nut. There is no base on the glass, just a stem. Clear as mud? Ha. You can make a new disc with most any flat rubber sheet with a hole punch and drill a hole through it for the threaded stem. Reassemble the poppet, spring and compression fitting and install and adjust. remember to put the retainer and safety wire back on to keep the parts from moving again, which would upset the setting of the valve.

The main reason these valves fail is due to wear on the aluminum shoes on part#15 in the diagram which ride on a steel disc part#21 inside the unloader assembly. I don't think these parts are available anymore. The brass arm can also wear on the retainer pin and get sloppy enough to lose enough travel to actuate the pin properly.

This unloader is not exactly like your but works exactly the same.

Ball bearings held in the cage (part#22) are thrown outward due to centrifugal force. This forces part # 21 to move inward due to spring pressure from part #15. Part #16 moves inward as well. Since it is pivots due to being pinned in place by part # 17, the brass arm part#12 attached to it, moves leftwards, allowing the spring loaded poppet in part# 8 to seal against a ring seat. It is also held in place due to air pressure in the unloader line.

When the compressor stops, the ball bearings in part#22 fall inwards due to loss of centrifugal force and due to a bevel machined in the back of part #21. This forces part #21 outwards against spring pressure from part # 15. Part #16 pivots due to being pinned in place by part # 17. Since the brass arm part # 12 is attached to part # 16, it consequently moves to the right, pushing the poppet in part # 8 open against spring pressure and trapped air pressure. This exposes the ring seat inside the valve and air is expelled at shutdown.

Confused yet?

Or is might be a weak capacitor or non-functioning start winding centrifugal switch inside the motor.

 

[Chopt50Ford]

The unloader valve is on the back of the pump. It has the copper line going to it to relieve pressure upstream of the check valve which is located inside the cast metal finned object the discharge line is plumbed into. If you remove the 1/4" copper unloader line at the back of the crank, you can remove that safety wire which holds the retainer in place. (picture 1). Take the little sheet metal cover off of the back of the unloader valve. Fire up the compressor and see if the little brass arm moves. If it does, you may need to adjust the unloader valve fitting. There should be a thin brass jam nut that holds the unloader valve in place. Loosen that, and you can turn the valve so that it moves closer to the little brass arm. It will be trial and error to set the position of the valve so that it blows down at shut off but does not continuously vent all the while the compressor is running. You should only hear air escape when the compressor shuts off and the brass arm moves to the right and depresses the valve stem. This lifts a spring loaded rubber disc off of a valve seat which vents the trapped air upstream of the check valve.

With a compressor this old, I would be tempted to renew the rubber disc in the unloader valve. All you have to do is remove the 1/4" compression fitting in the end of the valve and the spring and "poppet" will fall out. The stem is connected to a brass retainer which holds the rubber disc. This is hard to describe, but think of a champagne glass with a hockey puck in it, except the top is upside down and the stem of the glass goes right through the puck and is held on by a nut. There is no base on the glass, just a stem. Clear as mud? Ha. You can make a new disc with most any flat rubber sheet with a hole punch and drill a hole through it for the threaded stem. Reassemble the poppet, spring and compression fitting and install and adjust. remember to put the retainer and safety wire back on to keep the parts from moving again, which would upset the setting of the valve.

The main reason these valves fail is due to wear on the aluminum shoes on part#15 in the diagram which ride on a steel disc part#21 inside the unloader assembly. I don't think these parts are available anymore. The brass arm can also wear on the retainer pin and get sloppy enough to lose enough travel to actuate the pin properly.

This unloader is not exactly like your but works exactly the same.

Ball bearings held in the cage (part#22) are thrown outward due to centrifugal force. This forces part # 21 to move inward due to spring pressure from part #15. Part #16 moves inward as well. Since it is pivots due to being pinned in place by part # 17, the brass arm part#12 attached to it, moves leftwards, allowing the spring loaded poppet in part# 8 to seal against a ring seat. It is also held in place due to air pressure in the unloader line.

When the compressor stops, the ball bearings in part#22 fall inwards due to loss of centrifugal force and due to a bevel machined in the back of part #21. This forces part #21 outwards against spring pressure from part # 15. Part #16 pivots due to being pinned in place by part # 17. Since the brass arm part # 12 is attached to part # 16, it consequently moves to the right, pushing the poppet in part # 8 open against spring pressure and trapped air pressure. This exposes the ring seat inside the valve and air is expelled at shutdown.

Confused yet?

Or is might be a weak capacitor or non-functioning start winding centrifugal switch inside the motor.

WOW!!!!

I really can't thank you enough! I have been meaning to jump on here and reply after seeing an email with your reply. I have been so busy.

I ended up rolling the dice on a brand new electric motor from Grainger, since they promised to return it within 30 days if necessary. It IMMEDIATELY solved my problem. I called the guy who "rebuilt" my original motor and he was nice about it, but started to rattle off a list of things he "could have checked". I gave him carte blanche to fix everything and anything it needed, even if it cost nearly the same as a new motor. I had read and heard about the original Dayton motor being higher quality, so I was willing to spend roughly the same amount to restore it and make it new. I asked him to call me if he found anything it might need. He never called me and had it done in one day. Fast service, sure, but a complete waste of my time and major headaches along the way. I had to drive an hour and a half each way to drop it off and pick it up. He never did tell me if it could be weakened, etc., and no longer be strong enough...just mentioned his "I could have done this..."

Anyway, I am back in business for almost a week now and very relieved! I still have a leaky pressure switch but I found an NOS one from 1987 on ebay and I will swap it out when I am not relying on the compressor daily. Right now is my very busy shop time...

THANKS AGAIN!

 

[MacMcMacmac]

You're welcome. Glad it worked out for you.