Skimmer weir not rising

Ok, so Im a new pool owner of a used pool. The pool seems to always have real real small trash that my regular and fine net cant get. I think it because the weir in not rising to break the water as the pump is running. Since its always down when the pump turns off all the small trash gets back out to the pool.

If I shut the pump off and lift on the weir, it seems to work and rise and fall to break the water. The minute I turn the pump on it gets slammed to the bottom. It does not seem to have a foam on the back. Is that required or does it have internal springs?

It sounds like you have the wrong size weir, or it's installed wrong, or the skimmer's sockets for the weir's pins are damaged. The weir should float on the water and uses no springs. It should be loose like it's on a hinge.

Should it float while the pump is running? Does it use foam to float?

The weir should be pulled in toward the skimmer's drain by the flow of the water, but it is always on top of the water. Depending on how deep the water level in the skimmer is, the angle of the weir can vary. If the water is high, the weir will be a bit off perpendicular. If the water is lower, the weir flattens out. When the pump is off the weir will go up to perpendicular but bounce in and out as the water bounces in and out. It may even fall forward. The plastic side should be toward the pool and the foam side toward the skimmer.

You see that the thing....I cant see the foam on the weir. Maybe its internal

Really? Usually one side is plastic and the other is styrofoam, that floats it. If they are both plastic it is probably the smoother side that faces the pool, with an angle. Go to some on line pool sellers and search for "weir" and look at them.

I have weirs that are all plastic and sealed. Presumably the air inside provides the floatation. Could be this type and filled with water. They are generally available for replacement through a pool supply house.

Good suggestion! Weirs aren't too expensive and replacing it with one the proper size cannot hurt. I've replaced mine at least once.

Skimmer weirs, at least the Sta-Rite/Pentair (U-3) have, in the past 5+ or so years, have become increasingly thin. I assume, like you said, that they are a pocket of air as opposed to the "tried and true" Styrofoam. If the weir floats when the pump is off, then it is probably ok. It should hover between the floor of the throat and the waters surface. You can try adjusting the water level in the pool to increase/decrease that amount of water in the throat that the weir has to deal with. If you decide to get an aftermarket weir, it will have Styrofoam, and perhaps perform better for you.