To alter the position of the gas injection core in the injection tube, first open up the lid of the Bangalore.
The injection tube is the blue cylinder at the rear of the chassis. You should see the gas elbow poking out of the top attached to the gas pipe:
If you look down into the injection tube, you should see the steel gas injection core.
Loosen the hexagonal bolt on the front of the injection tube so that the injection core is free to move up and down.
Try moving it upwards a few milimetres and re-tightening the bolt. Try a test fire and see how the Bangalore sounds.
If it doesn't fire at all, or it still doesn't sound right, try moving the gas injection core a little more. Alternatively, try moving it downwards.
You'll need to experiment a little to find the best position that produces the best bang.