Sorry, but I am using an APM flight controller now. I used to use the kk 2.1 and it flew OK with stock settings but much better after I tuned it.
My approach for tuning the P gain was to turn it up until I got oscillations and backed off from there. For the I gain, I followed the guide I linked to. BTW, this tuning has to be done in acro mode, and then you might want to adjust the self-level gains afterwards. The quad won't be stable when you let the sticks go unless you are using GPS. You need to control it all times.
I would also suggest you upgrade to the latest firmware for the kk 2.1. I couldn't get my quad flying with the 1.6 firmware but it flew like a charm when I updated. You can find the latest firmwares
here (my preference) or
here