Keep in mind the more you pre load the spring the more pressure it will take to activate the switch. Whicu will need a smaller gap between the arm and switch. It's a delicate balance. In the past I have found a couple of different items that cause problems.1, debris be hind the comb plate roller. 2, the bolt that goes through the roller being bent.
I do not remember the measuremental but there is a certain length the spring is pre loaded to. I think 21mm but it's been a a couple of years
