Help new alternater not running

Hi fitted a new alternator now my car starts with a boost then dies shortly after and goes completely flat, pulled fuses ect but can't find any problems as to why it might do this can any one help please, alternater is ok as I've tried another and still the same and the battery is ok many thanks

if you have tried both alternators and its still the same i would think it would be something else.

maybe fueling or spark?

just my opinion

is the battery light coming on with ignition then going out when started? if no check blue wire from alternator to spade connection near battery +