Why is my Lime mortar crumbling HELP URGENT !

this is not lime mortar this is just cement with sand/lime in it and a very weak mix at that, I am presuming you are using hydrated lime rather than hydraulic lime, which is what you require to make lime mortar. depends exactly what you are trying to achieve, 4-1 will definitely give you a strong enough mix to build a wall, older house are solid brick no cavity, built with lime mortar this allows the walls to breathe stopping dampness internally, if they are then rendered. painted, pointed with cement as there is no cavity any water ingress gets trapped in the structure of the building showing up as dampness internally, this tends to be mistaken for rising damp. damp proof courses seldom work in this type of property as it is the wrong solution for the problem. lime mortar has no cement in it, it is hydraulic lime, sand, aggregates. it will take 92 days to completely go off and will set hard on the outside, hydrated lime will never go of without the use of cement, it is generally a cheap training tool in bricklaying courses as it can be continually re-used by just adding water good luck Alex