>>68889
I'm using pollymc, but this should all apply to anything: Java 8 openjdk (64 bit), 6GB max memory, minecraft version 1.12.2, furge 14.23.5.2859. Nothing changed from default besides JVM memory.
Your issue of not even being able to start a world is weird, and makes me suspect something else is wrong besides something with the above (in which case game probably wouldn't start). I would guess something is up with a mod. Basic troubleshooting: can you start a minecraft world with all mods disabled? With one mod (eg. mekanism) enabled? What error message (if any) do you get in minecraft? can you check the logs?
Sorry I can't be more help. Maybe somethings changed with the modpack, in which case server owner will be more help.
Good luck!