Unloader partially bypassing

oneness

oneness

Member
I'm stumped!!

About two weeks ago I started losing pressure, though the motor and pump are running just fine. Checked my water supply, and it's fine...force fed the pump with a sufficient supply and still had the problem. It was an intermittent problem, but I finally figured out the unloader (K7) was dumping about 2gpm or so back into the tank with the trigger pulled.

I changed the unloader with a spare that came with the trailer. Worked fine for a while, then same problem. Ordered another, installed it yesterday, and guess what? Worked fine for a short time, then same problem.

What in the world could be causing all three unloaders to partially bypass?
 
A K7 should bypass 5% when it is set properly. Of course though, Mike that is nowhere near 2 gpm. Anything else change recently? What is the machine's output and what size K7? Hot water? Possible scale in the coils? Gun not opening properly? Take the first unloader apart. See if there is a groove in the ball at the end of the piston. Check the mating surface for debris. I have seen chunks of plunger from a previously broken pump get stuck in the outlet orifice. Here's a link to an exploded view if it helps.
http://www.generalpump.com/PDFs/ZK7-Unloader.pdf
 
Russ Johnson said:
A K7 should bypass 5% when it is set properly. Of course though, Mike that is nowhere near 2 gpm. Anything else change recently? What is the machine's output and what size K7? Hot water? Possible scale in the coils? Gun not opening properly? Take the first unloader apart. See if there is a groove in the ball at the end of the piston. Check the mating surface for debris. I have seen chunks of plunger from a previously broken pump get stuck in the outlet orifice. Here's a link to an exploded view if it helps.
http://www.generalpump.com/PDFs/ZK7-Unloader.pdf
Click to expand...

Havent had time to tear the unleader apart, but I just can't see how it would be the unloader alone causing this, since I'm getting the exact same results from three unloaders, one of them being brand new.

I tried force feeding the pump, using a short hose between a spigot with more than sufficient flow hooked directly to the pump inlet. No leaks when under pressure, and did just fine for about 2 minutes when I fired it up. Shut it down to check the oil (I had just changed the oil), topped it off, fired it up again, and had lousy pressure. This has been how it has been acting...sometimes it has great pressure, sometimes it has lousy pressure.

Guage on the pump head shows a consistent 3000 psi, even when the pressure at the tip is lousy. When I have good pressure, there's nothing coming out of the bypass hose.

The motor runs fine either way, no change in speed or sound, and the fact that I'm still putting out 3000psi at the pump head tells me the motor and pump are doing their part.

I haven't checked the orifice in the unloader, but I did order the proper part for my machine (don't recall the range, but mine fell within it.).

My next step tomorrow is to pull all the HP hoses off the machine and run a new hose from the pump to the unloader, and hook a 10' hose to the oulet of the unloader and see what I'm getting then. Somewhere between the pump head and the nozzle I'm losing flow (and thus pressure). It is clearly the unloader going into bypass, but the question is, what is causing it? I've tried various guns, and get the same problem, even with an open hose. This is all with nothing between the unloader and the gun except a 10' whip hose that normally runs to the hose reel.

I'm just stumped.
 
Russ Johnson said:
You're reading 3,000 psi at the pump head with an OPEN hose?? Now I'm stumped. Let me know what you find on further inspection.
Click to expand...

No, sorry. I wasn't clear on that. 3000psi with the proper tip in the gun to attain 3000 psi...I have 3kpsi at the pump, but much much less than that coming out of the gun, because the unloader is bypassing.

I bypassed the coil, and everything seems to be working fine. I ran it for a good while under full pressure and it never bypassed.

Could there be a problem with the flow switch that would cause this?
 
That's the MV60 (Meccanica Veneta). Real similar to the ST5 by Suttner, but some are spring loaded so they can be mounted horizontally. Loosen the screw on the side and slide the reed switch out of the way. That switch will come apart in 2 pieces. Hold one end with a wrench while turning the other end. The piston with the magnet in it should slide right out to you. Check this area and see if your clog is there.
 
Ok...next issue. The unloader is now constantly cycling when the trigger isn't pulled. I did some searching and found that the bypass orifice is likely the culprit (though not sure why it is now a problem, since nothing is different on the machine except bypassing the coil/flow switch). I have jury-rigged a solution for now, but I need to get a smaller orifice (mine is a #2, I assume the next size down is a #1?). Does anyone know where I can order a couple of these? The two places I usually order parts don't carry them, and I need one rather quickly.
 
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