I am trying to find out what happens to the wieght of a body 2-40
hours after death. Does the weight of a body increase or decrease, by
how much? Is it a fixed wieght or does it have a percent up / down?

Assuming it is in a sealed container (so no evaporation of fluids)
then it stays the same.

It is quite common when someone expires that their bowels "let loose".
 So if they are just lying there then any water could evaporate and
thus they would be lighter.  If you are looking for evidence that the
sould leaves the body and now the body weighs less there is no such
scientificly validated evidence. (cause it doesn't happen)

This is a hard one to answer as there could be many factors affecting
the putrefaction process. For instance the process will differ if the
body is in a moist environment compared to a dry environment. In
general the composition process starts rapidly after death and hence
effectively means that there is a gradual loss of wieght and body
fluids. The chronological processes are;
1.Greenish discoloration on righ flank of abdomen. (due to changes in
the blood due to action of bacteria.
2. Staining spreads over whole of abdomen and begins to appear on
other parts of body.
3. face begins to swell and discolour.
4. Swelling and discoloration of the scrotum or vulva
5. Abdomen distended with gases (generall the gases ariseing from
putrefaction are sulphenated hydrogen, phosphoretted hydrogen and
amonia)
7. Skin develops blisters of various sizes which subsequently burst
accompanie by shedding of large areas of epidemis.
8. Fluids escape from nostrils and mouth, eyeballs liquefy.
9. Whole body discoloured abdomen greatly distended
10 Insect infestation.
11. Finger and toe nails loosen and shed. Hair loosens and falls away.
12. Tissue begins to soften and liquefy, face becomes unrecognisable.
13. Abdominal and thoracic cavities burst
14. Tissues progressively dissolve.

Factors affecting speed of process include
Presence of microorganisms
presence of air
Temperature
Moisture
Type of atmosphere e.g. air, earth, water
Age
Size
Nature of death

In terms of your timescale i.e. upto 40 hours the following happens.

0-12 hours Cooling of body
10-12 body feels cold to touch
20-24 body is cold internally
3-5 hours Lividity begins to develop
5-7 hours Rigor mortis begins in face, jaw and neck
7-9 hours amrs and trunk then legs
12-18 hours full rigor mortis
24-36 hours rigor mortis leaves the body in the same order.

Putrefaction then commences as detailed above but depends on ambient conditions etc.