First time poster, having learned a lot from the forums over the years. I’m posting with a problem with the function of a Shimano Deore XT front derailleur. This is the original on an ’83 Specialized Stumpjumper (triple chainrings, friction shifting). Regardless of the shifter position, the spring on the derailleur wants to pull the chain inward (towards the seat tube) and onto the smallest (granny) gear. Thumb shifter seems to work well –I can shift up to a higher gear/larger chainring but have to hold it in place in order to stay in said gear. If I let go, the spring on the derailleur pulls it back onto the little chaining. I can ride the bike, but I just have to hold the shifter in place, which makes for a very sore thumb!
Any suggestions? How does a derailleur resist the force of its spring? Could the derailleur be worn out or broken? It looks to be in good condition but I don’t understand the spring mechanism enough to know if there’s a problem. I can post more pictures if that helps.
Thanks in advance!