BSA 250 stopping after 10 seconds after starting?

i just finished rebuilding my bsa c25 but after starting and running for around 10 seconds cuts out and maybye fires once or twice while dieing. i reconed that the float chamber wasnt filling back up so i tried holding the tickler but it didnt help, maybye added a few seconds before cutting out. the ignition is fine and never misses a beat. even at high revs it wont miss a beat and then again cuts out and fires couple of times on way down. its a amal concentric mk 1. ive rebuilt many times but cant find any problems. has no trouble starting but if you try to start after it died it will just fire once. ive rebuilt many bikes including this one before with success, so if your thinking im a idiot who hasnt rebuilt it propely im not.