B200E starting problem
 

Occasionally in the past the engine has make a "pop" sound on turning over and immediately died. On opening the bonnet there is a strange smell and one or other of the pipes from the auxiliary air valve has popped off. Once replaced everything works as normal.
Yesterday having run several miles earlier in the day the same thing happened but this time after replacing a different end of the pipe the engine continues to start, revs up a bit and immediately dies. It continues to do this every time thereafter and did so after a nights rest
I've checked and rechecked both the hoses from the air valve and everything seems tight and in place. Today I changed the auxiliary air valve for another one with no difference.
Has anyone any ideas ??