Dripping Zenith 26va

[John Mason]

On my Ruby the spring on the primer has not worked for some time. I can still prime the engine with the priming lever pushing it both up and down ( the spring fails to take it down again) I have to remember to leave the lever in the down position otherwise if left in the up position the pump fails to work.

John Mason

[e-richard]

Still struggling with a leak. If the suggested cause is indeed the new pump being too strong, then what does the team think of this solution using a small bore copper bypass as suggested by my local grease monkey?

It should take the excess pressure off the carb, but what else?
Too complicated ?



  bypass.jpg (Size: 24.26 KB / Downloads: 223)

[David Stepney]

It probably would work. My VW Polo has a mechanical fuel pump that delivers pressurised fuel to the carb all the time. There is a return pipe from the top of the carb inlet back to the tank, so the fuel goes round and round, whilst the carb just takes what it needs. It works fine.

[John Mason]

Not trying to show Grandmar how to suck eggs but my understanding of how the petrol pump works. Is that when the pressure builds up because the needle valve in the carb is closed the pump goes into neutral until the petrol level in the float chamber drops and the valve is open again. I would look at the pump again and check that it is working correctly with regards to the neutral aspect of the pump.

John Mason

[e-richard]

John, please don't be afraid. This Grandpa has no idea how to suck eggs. 

I've looked at the pump and cannot see what I can do to stop it forcing too much fuel into the carb. The carb has been completely rejuvenated and for the moment has extra washer under the inlet valve. Happy to remove that if it will help. Clearly something is going wrong but I cannot see what to fix ?? Its been mentioned that the "pump spring" is too strong, but how does one make a spring less strong?  I don't have an old pump to hand.

I do have Woodrow's exploded picture of the pump (page B-5), so feel free to reference any of these bits that need fixing/tinkering/adjusting or what.

The bypass seemed like a logical getaround, rather than an elegant solution.

[Ruairidh Dunford]

Hi Richard,

If you contact a second hand supplier like Tony Betts, I am confident he will be able to salvage one from a scrap body.

[morrisminor]

It definitely works Richard,  I've done 200 miles since I've put a old spring in,  no leaks and it does prime well, wasn't sure but it does

[David Stepney]

An AC type pump works as follows:
The lever running on the cam pulls the diaphragm down, sucking the fuel from the tank. Th spring underneath the diaphragm pushes it back up, thus providing the pressure to force the fuel to the carb. If the diaphragm remains in the down position (because the carb needle valve is closed) then the pump 'idles' until the needle valve opens again, allowing the spring to push the diaphragm up.

If the spring is too strong, it will overcome the needle valve, allowing more fuel into the carb, effectively flooding it at bit. Whilst the engine is running, all that happens is that the mixture will be a bit rich, but as soon as the engine stops, then the carb will be overfull and the fuel level in the float chamber will be above the top of the beak, which will drip until the level drops to the same height as the beak.

The current replacement pumps are based on the AC 'U' type (post-war) design and were designed to run with post-war downdraft carbs. Any pump 'over-pressure' would not be noticed, as the excess fuel would run into the inlet manifold. the only way one can tell if this is happening is that the car will require full throttle to start hot. On a side-draft carb, this doesn't happen and the excess fuel tends to drip out of the air intake and (unless one has the engine compartment side plates in place) onto the exhaust.

I hvae replaced the spring in my replacement 'U' type pump with the spring from my old 'T' type pump As Ruiaridh has suggested, which appears to have cured the problem, although my carb only dripped intermittently.

However, whilst looking for something entirely different, I came across a box of bits within which was a complete set of valves, cage and valve springs for a 'T' type pump, so I am going to order an new diaphragm and rebuild my 'T' type pump and see how that behaves.

[e-richard]

Very helpful everyone.

In my naivete, I kind of expected a replacement pump from an Austin dealer to be "suitable" for an A7. I'm not complaining about the supplier, but I'm a lot wiser now and love this forum even more. When I eventually get Jane running again, I hope to be able to reciprocate. I bought her in 1968, but this is my first foray since then back into auto-mechanics. She has been on stocks since early seventies, and my brain has been involved elsewhere (IT type stuff).

Off to hunt a 'T' Type spring....

[John Mason]

Thanks David I knew it idled as you put it (I said it went into neutral) but I didn't know how it did it. Your explaination made it clear as to why it didn't work correctly.

John Mason