The problem is that i was using kartdlphax because I already have a hacked 3ds but want to hack another 3ds . I don't have Mario kart 7 in a cartridge so I downloaded it with a CIA into my already hacked 3ds. Then I tried kartdlphax but when I went to create a room to continue the hack it says "a communication error has occurred". Do you know any other way of hacking a old 3ds?Perhaps you should consider trying kartminer7 instead?
Otherwise it's kind of hard to say what you should do differently if you don't say exactly what you've done.

The more precise you can be, the more likely someone will be able to help you. Don't just say you "tried kartdlphax". What, exactly, did you do?Then I tried kartdlphax but when I went to create a room to continue the hack it says "a communication error has occurred".
Pretty much everything is on https://3ds.guide already.Do you know any other way of hacking a old 3ds?
same region and yes i followed them correctly. Il try again tomorrowAre your 3DS's both the same region? And are you following the guide accurately?