Jabsco head question - backpressure

[krivalsi]

Last year I installed a new Jabsco head manual pump (29120-3000) on my 1988 Catalina 28. Everything seems to work well until this year.

Symptom: In DRY mode, when I push the handle down..I get back pressure and the "upper intake hose" (connected to top/back of bowl) vibrates.

To me, this says that, on the downstroke when I am trying to suck in air/water from the top/back of bowl, I am actually seeing a vacuum.
Upon downstroke, if I hold the handle down, the vibrating stops (presumably when the vacuum is gone). If I let go of the handle immediately after pushing down the handle...it pops half way up.

Configuration:
- There is an anti-siphon valve installed between the tube on the top/back of the bowl and the Jabsco. I have bypassed this with 4 foot of straight tube...same problem.
- There is no anti-siphon on the seawater intake.

Other things I have tried:
- I have verified that the holding tank vent is operating.

Any ideas?

Gory details:
- WET mode - upstroke
- Lower: Waste is sucked into lower part of tube (no back pressure)
- Upper: Sea water is pushed into the top//back of the bowl
- WET mode - downstroke
- Lower: Waste is pushed into holding tank (no back pressure)
- Upper: Sea water is sucked into the upper part of the Jabsco (no back pressure)

- DRY mode - upstroke
- Lower: Waste is sucked into the lower part of the tube (no back pressure)
- Upper: Sea water is pushed into the top//back of the bowl (no back pressure)
- DRY mode - downstroke
- Lower: Waste is pushed into holding tank (no back pressure) (I think)
- Upper: BACKPRESSURE and Vibration in hose??????


Next:
- I will try disconnecting the hose from the back/top of the bowl next.

 

[Kings Gambit]

Last year I installed a new Jabsco head manual pump (29120-3000) on my 1988 Catalina 28. Everything seems to work well until this year.

Symptom: In DRY mode, when I push the handle down..I get back pressure and the "upper intake hose" (connected to top/back of bowl) vibrates.

This happens with my Jabsco when there is blockage somewhere in the discharge hose(s). Doesn't matter how new the head is; it's how long since the discharge hoses have been cleared (descaled) or replaced that matters.

 

[Ralph Johnstone]

- DRY mode - downstroke
- Lower: Waste is pushed into holding tank (no back pressure) (I think)
- Upper: BACKPRESSURE and Vibration in hose??????

This is quite likely an oversimplification, but I get similar results when the WET/DRY selector is not jammed fully to the right. It often slips. I usually hold the selector switch to the right, by hand, when pumping. The backpressure may actually be vacuum.

 

[Kings Gambit]

Yeah, I got that as well on the old pump that I just replaced on my boat a couple of months ago. It might be too soon for a new pump to start acting that way?

 

[krivalsi]

King Gambit - Thanks, but I think the discharge hoses (if blocked) would cause similar backpressure in the WET mode as well. I don't get that so I think that is not the problem

Ralph - That might actually help. I know pushing the switch to the DRY mode does 2 things:
1) close off the raw seawater intake flapper on the right of pump. (This would not cause the problem I am seeing)
2) hold open the flapper on the left side of pump that allows air/water to flow from back/top of bowl to Jabsco
The second one is promising since that would cause a vacuum and perhaps this exact symptom.

Thank all.

 

[Kings Gambit]

I could not push the pump handle all the way down with the lever incompletely switched over if I remember correctly; but, that certainly is a simple thing to investigate!

 

[Meriachee]

It does seem to be a common problem as Ralph noted. Our switch is somewhat questionable but the vast majority of the isses we've had were traced back to a slightly congested vent tube. Eeeeew.

 

[hvbaker]

It does seem to be a common problem as Ralph noted. Our switch is somewhat questionable but the vast majority of the isses we've had were traced back to a slightly congested vent tube. Eeeeew.

I agree with the vent tube hypothesis.

 

[krivalsi]

I just got back from the boat.
Ralph was right...holding the switch all the way to the right (DRY mode) solved the problem. I know when you switch to DRY mode, there is a hook which is supposed to open the flapper and the left hand side which allows the tube from Jabsco to rear/back of the bowl to provide air. I assume this flapper was not fully opening until I pushed the switch all the way to the right. Honestly it seems like a bit of a design defect that it is that sensitive. I might contact Jabsco.
I also agree the "vent hypothesis" sounds promising but if that were the problem, I would also get back pressure in WET mode but that was NOT the case.
Thanks RALPH.

 

[Ralph Johnstone]

Honestly it seems like a bit of a design defect that it is that sensitive. I might contact Jabsco.

I've thought the same thing but in the end, I'm still there ................. holding my pants up with one hand, holding the WET/DRY selector hard to starb'rd with the other hand, pumping the plunger with the other hand. Wait, that's three hands. No wonder it's so bloody difficult sometimes.

 

[Team118]

Something to consider.

I had the similar trouble at the beginning of the summer. Could pump fine while in dry mode, but not in wet.

I had sucked the rubber ball that sits on a spring in the pump into the housing. I had to cut off the handle of a sponge brush to knock ball out. I then boiled the ball to get it soft enough to mold it back into a round shape.

I have the Raritan model...