Austin Big Seven starting problem

[Thowdfella]

Sorry. The latest entry is underneath this old reply
I have mentioned this problem before but never managed to sort out the problem. The carb is a Zenith vm-4 which appears to be in good condition with no worn spindles. It has now got to the state where it will not start at all. I have done a valve grind; there is no sideways movement on the pistons; the compressions are all around 70 when turning the engine over on the starter motor; the timing has been checked again and again and ignition laser indicators on the plugs are all  flashing. When the head was off we checked the camshaft timing which appeared to be correct. but still wont start. Last night when I was doing a last ditch attempt to start it - I forgot to turn the ignition switch on. When I did turn it on and tried it - it eventually kicked into life!
To me this suggests petrol starvation so its back to the carb. on previous occasions when it wouldn't start, I have removed the float chamber - filled it to a higher level than what was in there and then it started. Can anyone help / I am using fresh fuel and priming with the fuel pump lever and its a new pump which gushes fuel out .

This problem will to go away. You may remember that I eventually managed to start the car using fresh fuel and a separate tank. And it has started ok since then. Yesterday going for a short run it stopped and wouldn't start again. Petrol was leaking out between the float chamber of the vertical downdraft Zenith carb. This has happened many times when I previously couldn't get it started so today I changed the carb for a spare I bought on eBay. When I prime the float chamber with the pump it fills the chamber and then stops with the float full. THEN - petrol starts leaking out again in the same place previously described. How can this be? There is no adjustment for float level. It's a completely different carb but I can only think that the fuel level most be too high for it to leak out. I have a new fuel pump fitted - is it possible that the fuel pump is too strong and overrides the cut off valve. I think I need some expert advice please!!!