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Mysteries of the SU pump

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roscoe

roscoe

Jedi Knight
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So after a Sunday drive I parked the car and later in the evening drove it to the barn to put it away. It stalled with a tad of sputtering before I got there. Knowing this wasn't a one time issue I let it sit and it started, ran fine but sputtered and stalled again. OK probably fuel pump. Not such a wild guess as the points clicking seemed to be a bit "different". The pump didn't seem to run for as long before stopping because pressure had built up. I pulled the pump out today and sure enough when I ran it on the bench with the inlet and outlet opened it would suddenly stop unless I tapped it. I cleaned the points and the screen; it still did it a couple of times. I restarted it again and from that point on (pun intended) it seemed to run fine and I couldn't get it to fail. My question to those wiser than me is, should I trust it. I don't mind banging on the wall behind my seat if it fails, but there are plenty of inconvenient times that I wouldn't want to be burdened with doing that. Plus my wife will make fun of me (perhaps rightfully). It is about 6 or 7 years old, has the latest diode installed and the points actually looked OK after I touched them up. So, any suggestions other than dirty points as to what can cause this? I'm pretty sure it won't act up if I try to make it but....do I trust it. I will probably buy a new solid state unit as I've been meaning to have a spare. Glad I sold my airplane. The repercussions of fuel system issues can be relatively unpleasant.
 
Jon Said:
"It is about 6 or 7 years old,"

"buy a new solid state unit"---:congratulatory:

I Agree-
encouragement.png
 
roscoe said:
... I will probably buy a new solid state unit as I've been meaning to have a spare. Glad I sold my airplane. The repercussions of fuel system issues can be relatively unpleasant.
Click to expand...

Buy a new SS and install it, and send your current one to Dave DuBois for conversion to SS as a spare. Last I tried, I could not source good points.
 
If you put a diode across the points, they don't spark and they last indefinitely. However SU pumps are the world's major cause of mental breakdowns and suicides, they have a variety of failure modes all confusing and definitely inconvenient.

i speak as someone who remembers his father clouting them with a spanner in the nineteen forties!

Buy a new one if it doubt and as others have advised, get the old one fixed as a spare. Also take vallium till you know the car is fixed.
 
You could install a back-up fuel pump with a switch where you can select either pump (or both off). Having had fuel pump problems in the past, one time being a real aggravation, I decided to fix that issue by installing a generic electric pump from the local parts store as a back-up to the SU. I chose to install it in series rather than parallel because it was a simpler modification plumbing-wise. After 14 years and about 70K miles so far, the system is working well on the same pumps. I don't have to worry that a failed pump has to be dealt with on the side of the road, in the rain, and at night.

Pumps can sometimes stumble or temporarily stop, for whatever reason. I have felt that sudden loss of power on I-20 in Atlanta in the middle of afternoon traffic. A flip of the switch to the other pump kept me going. The SU worked fine when I switched it back on later. Similar situations have occurred at long intervals while on the road or cranking but failing to start in the morning at some hotel. It's a comfort to know that the back-up pump is there if needed.
 
Solid state units are the way to go. Mine has run fine with no problems for ~20 years. Agree to send the old one to Dave DuBois for conversion to SS as a spare.
 
Hi Jon,

In 1989 when completing a rebuild of my 1964 BJ8 P1 after a 15 year storage, I was given an opportunity to acquire and install a new replacement SU points fuel pump. Since then I installed a roll-over cut-off switch that interrupts the pump’s electrical feed on impact or roll-over and a Transient-Voltage Suppression (TVS) to eliminate the development of coil-induced high voltage to 21V (and shunts to ground voltage over 27V) . This voltage range provides sufficient power to clean oxidation from the tungsten points during operation and eliminate point flash burn. As a result, and with proper diaphragm/points adjustment, I expect I will experience reliable SU fuel pump operation, without a maintenance intervention, well into the future.

You mentioned the installation of the latest diode. Since the later SU points fuel pumps came with a few diode versions, which one is installed on your pump? If you have not installed a TVS (less then $1.50), I suggest you replace your diode with one. When cleaning the points, did you also readjust the points/diaphragm (instructions on Moss site)? If not, your lower points/diaphragm can be operating close to its trigger margin.

I recommend you carefully consider keeping your present SU fuel pump as you would not be able to resurrect its electronic version.

All the best,
Ray(64BJ8P1)
 
Thank you all for your input. I know the solid state pump is a good investment and my birthday is coming up so perhaps I'll get one. Ray, thanks for the tip on the adjustments. I won't be discarding my old pump. I hadn't seen the Moss video so I will make sure to comply with the adjustments. It does sound like that might be the issue.
Over the years I've known many helicopter pilots who routinely tap certain instruments to assure they won't stick when they start up a ship and I always thought it would be slick to have a weighted pendulum that you could swing so as to bump the entire instrument panel to "tap" all the gages at once (not necessary with all the new digital instruments) and maybe I should build the prototype and put it on the panel that the fuel pump is mounted to so I can grab it and just lift it and let it go to give the pump a knock now and then:highly_amused:
 
I had a similar problem, intermittent fuel pump and sputtering engine. I believe ethanol gas caused my fuel pump to start disintegrating. Found the little screen under the banjo on the carb full of little black bits, some got past the screen and lodged under the diaphgram. I replaced the pump with an ECCO pump. Moss sells it as a replacement pump for an MGB/C. It looks alot like a SU pump with similar output. It is unaffected by ethanol, has the same ticking noise as an SU, but even quieter. Steve
 
roscoe said:
... I always thought it would be slick to have a weighted pendulum that you could swing so as to bump the entire instrument panel to "tap" all the gages at once (not necessary with all the new digital instruments) and maybe I should build the prototype and put it on the panel that the fuel pump is mounted to so I can grab it and just lift it and let it go to give the pump a knock now and then:highly_amused:
Click to expand...

You mean like this:https://www.youtube.com/watch?v=2qp5CPUIenM
 
One issue with points-switched pumps is the tungsten points oxidize when not in use; if you plan on using a points pump as a spare be aware it needs regular cleaning or be prepared to pull the cap and clean the points if you need it.
 
Hi Bob,

I agree that tungsten points do oxidize when unused. However, the oxidation is quickly eliminated (burned off) by the initial flow of power passing across the points. Since the pump's coil will induce an increase in voltage across the points of as much as 200 volts, an selected TVS must allows sufficient voltage to clean the points but not enough to flash and burn the points. I have had a TVS that allows as much as 21 volts but shunts to ground anything greater then 27 volts. Over the years, I have found no issues when initially starting my Healey after its winter slumber and the few pump ticks to build pressure seem to be sufficient to clean the points. No problems in years.

I do definitely agree that any back-up system be tested and/or switched on periodically to validate its readiness to perform when needed.

All the best,
Ray(64BJ8P1)
 
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Hi Ray,

Don't disagree, but the 'initial flow' across the points will only be 12V. Not sure if the points will oxidize if they're in contact, but it was Dave DuBois who told me this can happen (and he sees more pumps than everyone on this forum put together). It may be a rare occurrence, but according to Dave it happens; I've long since lost the email, but IIRC he said points-switched pumps that have been sitting on the shelf for a long time may fail to work initially.

If points and a TSV--I tried one, it didn't seem to work any better than diodes--work for you, great. But I've had too many fail on me in 140K miles of driving my BJ8--I now carry TWO spares--so I go with SS. The last set of points I bought were burned after only a few hours of operation; they appeared to be made from silver instead of tungsten.

Here's Dave's 'howto' on cleaning points: https://userwebs.donobi.net/sufuelpumps/Fuel_Pumps/Cleaning_Points.pdf

Bob
 
I put diodes across the two sets of points in my Bentley's SU pump about ten years ago and they're still unmarked. Sadly the diaphragms have hardened and blistered, presumably modern fuel damage, and I've had to replace it. It was new in 1998 and lasted a few years before the points burnt away and I had to replace them. A diode makes a terrific difference to longevity.

Use a 1N 5401 Diode.
 
A Diode maintains power flow in one direction where a TVS Transient-Voltage Suppression will also control the maximum amount power allowed to flow. Keep in mind that a diode will work well and Tungsten was selected for the points to handle the induced voltage created by the coil effect. However, original points are now hard to come by and quality can be questionable. The TVS I use (P6KE20CA manufactured by Vishay) will pass up to 21 volts across the points but will shunt to ground the portion over 27 volts. This power control definitely stays well within the limits of the available tungsten points while having sufficient power to burn off any oxidation with no points flash and burn.

Bob, I have not talked to Dave in a few years and trust he is in good health. I believe Dave will also state that the initial voltage passing over the points is sufficient to burn off the oxidation. It is when the power level is substantially reduced, as with Dave's original plan for the implementation of a points saving transistor (quickly withdrawn). This addition was meant to cut back power passing across the points to extend longevity. However, the level of voltage passing the points was insufficient to burn off the oxidation and it was not long that the oxidation buildup insulated the points and caused a pump failure. Realizing this condition, Dave tried to eliminate the plan from the internet but it can still be found.

I carry very few spares, however, I do carry a points setup in case my electronic ignition fails, a spare tire, and the top half of an SU fuel pump. If my pump fails, it is probable that the failure will be located from the diaphragm forward to the trigger and not in the lower body. By separating at the single ring of screws securing the diaphragm, coil, and trigger and replacing with a spare, you cover almost every issue without risking cross-threading fuel lines when reconnecting to the pump body. Yes, a hot backup would be easier and faster, but see little necessity for the added complexity.

As stated, I am not against an SS pump replacement but see no cost equivalent added value provided to replace a points unit that is reparable.

Ray(64BJ8P1)


 
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