Hmmm... changing Emission > Bursts > Time doesnt seem to have any effect

It's based on a specific time of the particle effect. If you have the duration set to 5 seconds you could set the time for the burst at 2 seconds and when the particle effect is at 2 it'll create the burst. This allows you to time specific bursts to happen at a specific time within the particle effect.

Ok, so I cannot seen the difference because of the loop on.

Try it without the "Rate over time" emission. Just use the burst. It'll happen at the time you specify. So using the example above, set the time to 2 and it'll burst every time at the 2 second mark.