[TR] The big bang

[John Dunham]

Hi all,
Finally enjoying my newly restored 62 TR4.  I do have one question -
usually after driving it for more than 15 minutes, when I shut it off, the
motor stops & it doesn't run on / diesel, but after maybe two or three
seconds - Bang!  Not a very loud backfire, (almost like a backfire under
water).
I just installed a Pertronix kit (what a difference!) but still a backfire.
I have SU carbs that may be running a bit rich, but I am not sure how to
quantify.  I have been playing with the idle as it has been fairly high
(1200 rpms at idle)  I do have 87mm pistons & a 270 cam with a slightly
milled head for higher compression.
Any advice or  would help.
Thanks as always,
John D