Hi,
Been a while since I operated V2500's but I do remember this problem. I once selected BUMP after starting the first engine and then when I selected it on the second (after start) it "flip-flopped" between the two! (indicated by a "B" next to the EPR gauge for each engine on the Upper ECAM). Only solution was to shut down "un-bumped" engine and restart.
I think you answered your own question - let the start sequence FULLY finish before selecting BUMP.
For those that have no idea what we're on about - BUMP is a facility on V2500 engines that puts an extra ring of injectors into the combustion chamber. Used to increase thrust at limiting fields...... but it cooks the hot section of the engine so is very detrimental to engine life.
If you watch the EPR gauges after TOGA is set you can see the EPR take another step up about 10 seconds after it has initially stabilised - presumably this is the "after burners"
kicking in.
A4