EDIT: Solved.
I have a simple problem but I need some advice.
My brother found some of our old games, and with the lot there was a bootleg NES multicart that came with a Subor I used to have back in the day.
It also came with a NES to Famicom adapter, so I tried it with my AV Famicom but I only got a green screen. So...
I opened the cart and inside I found a Famicom PCB and a Famicom to NES adapter. I then took that apart, popped the Famicom PCB to my console and it works nicely.
Now, I forgot which way the Famicom to NES adapter was inside the cart. Is there a way to check other than putting it all back together and trying? I couldn't get the combo working in the first place so I don't know how to check if it's the right way in.