4th cylinder back fire problem

Hello everyone,

I have problem as per video http://www.youtube.com/watch?v=76XVMPyr0VE

Happens only on cold (without choke), and when idle revs.
Problem goes away when warm or on higher revs.

Before done: oil/filter change, valve clearance, carb clean-up. Carbs are set 3 rev's back on idle screw (by the Haynes book) and are due for adjusting and balancing.

looks like you have a split inlet rubber....

It seems to be a bit better after re-checking valve clearance. One of inlets was too tight. Maybe it was too cold when I done it first time?!?
Waiting for colortune to adjust carbs as I think they run to rich. Will update later :)