Hi OP
I was also adopted in the 70s. I met my birth mother when I was 18. we had contact for years but I stopped it about five years ago as it had never really felt "right." I'm glad I met her, but also glad we aren't in contact any more.
Very gently - whether your mum would be upset or not, if you want to track down your birth parents you should do it.
In my case, I really needed to see where my physical characteristics came from, as I look very, very different from my family! (in fact I'm a different ethnicity which suddenly became glaringly obvious once we knew the facts!). It helped me feel more secure in myself. I wasn't looking for my mother - I already had one of those and she was wonderful - but I was definitely looking for a part of me.
I'm totally at peace with the fact that we are no longer in contact. When I met her (my birth mother) I was excited and fascinated and felt instantly that I knew her really well. What I didn't acknowledge for many years was the fact that I didn't actually feel comfortable with her, or like her! I'm not sure if we are too different or too similar, but I know that if she'd been a work colleague or someone I'd met in the pub, I would never have wanted to spend time with her.
Annoyingly she pops up on facebook every so often as "someone I might know.' God knows how, we don't have any friends in common and I've never visited the city she lives in. She is quite high-profile and very well-known in her public sector job, so she pops up on TV occasionally. Which is why I never watch the news now. I'm just happier without her in my life, but as a teenager I needed to know her and I'm glad I had the opportunity.