No its not a weak spring . Once its in gear it will stay in gear unless something is worn . the detent spring only works on the selector fork , and the syncro springs only work on the syncro hub . I have had this problem before . You could try tightening the nut on the back of the main shaft ( I think its 200 ft.lbs but check first ) but you need the tool ( or make one ) . There is a bronze bush inside the box on the main shaft which breakes if the main shaft has any end float . If the bush is broken you need to fix it or put up with the problem . If you have end float you need to eliminate it and maybe the bush is OK . .......Al