Fuel pump problem

[A7novice]

I have a manual a c type fuel pump.
It displays an intermittent fault when it fails to pump fuel into carb.
Basically, when I operate the priming lever, no fuel gets pumped into carb, but (using a temporary one pint tank) I can see and hear air bubbles being pumped INTO tank.
This seems to suggest a problem with one of valves.
If I then remove outlet pipe from carb and operate priming lever, after a few seconds pump begins to pump out of Outlet (carb) pipe correctly and no bubbles into fuel tank. 
Start engine, runs fine.
Anyone any idea what the problem might be.....I don't want to trust the pump on the car on a run just  yet.

["Slack Alice" Simon]

If the operating arm is on the high part of the cam, you can operated the priming lever, but there is little or no movement of the diaphragm - so no output.

If this is the case, if you turn the engine half a turn on the handle, you will find the pump operating.

It is important, if you take the pump off, to make sure the operating arm goes between the crankcase and the camshaft.

If it is on the wrong side, the engine won't turn over on the handle, and you will break something if you try and force it.

I know this, I must have removed and replace this one particular pump 20 times, and still got it wrong the 21st time.

Realised I had it wrong before any damage, fortunately.

[Andy Bennett]

Our cherished suppliers supply kits to refurbish fuel pumps including diaphragm if you have an intermittent problem. it isn't a bad idea anyway if you don't know how old the diaphragm is or what it is made from as they do deteriorate.