20-06-2019, 07:49 PM
(This post was last modified: 20-06-2019, 07:50 PM by 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.
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.