ok, first i place any particles, adjust them any way i want, then preview. Lets say it stops emmiting at frame 350.
now, even if set the start and the end in the particle buttons and in the timeline, after frame 250, it stops emmiting. help please 
That is the default bake length of any new emitter. You need to extend the bake length to your needs.
Go to Scene(F10) and in the Render tab, change the end to what you like. Also, if that doesn’t work, in the particles button, in the first tab, there is a bake settings option, change the end to what you like.
thx, that worked!