Loud backfire POP when shut down?
 

Whenever I shut down the car, I get a LOUD explosion pop right from inside the muffler (a custom "non-blocking" i.e. I can see thru it from 1 end to the other).

Any suggestion on what can I do to get rid of that? (For background - my idle AFR is around 14.8 so she's not running rich there).

thanks...

possibly a wasted spark discharging a spark on a cylinder when the exhaust valve open igniting the unspent gasses in the exhaust?

umm... you could be right! But if that's the case, then it means there's nothing I can do to stop that then? :(

Are you twinplug and what coils are you running?

Yes, I'm twin plug; wasted spark. Coils are Mitsubishi ones (for 3000GT).

Are you firing the coils to these respected cylinders?
coil A 1&4 coil B 3&6 coil C 2&5
And the same for the second set of coils?

Yes...

What are your timing values from idle to 0" of vacuum? (Or 101kPa) Or let's see your timing map. Not sure but maybe too advanced from idle to 0"?

Going from memory, my idle timing is around 8 deg.
I don't think other timing could cause this as I'm nowhere near off-idle vacuum?