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New fuel pump

by

Mike Meakin

Pembletons can be contrary machines, particularly if you have something other than the 2CV motor and its mechanical fuel pump.

Most bike engines that we use (unless fuel injected) rely on a gravity fuel feed to the carb(s) and even with a full tank, on the bike, the delivery pressure is going to be low, plus the float valves, inevitably, can only be expected to function at that low supply pressure. With the BMW engine I have in "Bolide", that’s about 1 to 1.5 psi.

That, on its own, is not a problem. For normal "pootling about" up to 1.5 psi suffices. However, when really pulling hard, the engine needs a much greater fuel flow (but not above the pressure that the float valves can tolerate). The BMW will rev out in the gears (including 4th) to 7,000 rpm. The problem occurs when pulling hard above 6,000 – but only relatively recently. In fact since our last French visit in May/June/July 2017, when it was frequently above 40 degrees C.

The "problem" such as it was, was spasmodic. The car would drive normally, until, caught behind a 25kph local tractor with no overtaking, you come across an "overtaking opportunity" and pull out, followed by "le Monde et sa femme", only to find that the motor will only pull to 2,000 rpm. Any throttle above that and both pots die, ease off and they return. Pull off the road, switch the engine off but leave the pump running for some minutes – plenty of fuel and no overflows – fire up again and all the hooligan capabilities are back! The scary bit is that in mainstream motoring, if the glitch occurs giving you only 2,000 rpm you can gradually reach 35 mph by short-shifting, but faced with any sort of incline.......

At the time, I was running a Facet( suction) pump, located under the battery carrier, slightly below the fuel tank bottom level, so it self-primed. It had sat there for some 8+ years and given no cause for concern. Since I was carrying a new Facet, "Posiflow" 60104 suction pump, I installed that, as a replacement and as a precaution, I removed the Filter King regulator (set at 1.3 psi in 2008) in case it had been compromised by the high temperatures - we drove the 1,100 miles home without fuelling glitches.

I’ve since installed an in-line fuel pressure gauge: it shows the Facet Posiflow 60104 is supplying a consistent 1.1 psi fuel pressure. The sad thing is that when "going for it", the fuel flow is inadequate. The engine gathers itself together robustly, then suddenly "gags", but resumes normality when the throttle is eased – IMHO, fuel starvation.

Following recent trends, I thought I’d go for an electronic (no breaker points) SU type, diaphragm pump – huge flow capacity (sufficient for a 5 litre engine), but a maximum pressure of 4 psi which the float needles will tolerate, with the advantage that when float needles are shut, the pump shuts down.

My Facet is on a mount on the chassis, underneath the 2CV battery carrier, which also carries the twin-tone horns. Getting to the pump involves the removal of all that. Having removed the Facet pump (which has in-line supply/delivery) I found that the SU type Fuel flow has supply/ delivery at 90 degrees ( through space occupied by discs and frame members ) and would not therefore fit as a direct replacement, so everything had to be replaced, as it was.

Enquiries now show that the Facet Posiflow, in-line offers only partial flow resistance if inactive, so the plan is to install the Fuelflow, SU type pump as a pusher at the tank, through the lines containing the Facet, so that only the positive feed to the pump needs to be transferred. That leaves the Facet and the various, new in-line fliters "in place", should the Fuelflow pump fail, simply by transferring the power lead.

What I don’t understand is that we’ve had some 8+ years of the engine performing well, in all conditions. I have to say I’m half expecting to have to replace all of the "suitable for unleaded" rubber fuel lines, in case they’ve expanded (automotive furred arteries!) a consequence of the increased incidence of ethanol, especially in French fuel.


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