As others have said popping and spitting back through carb usually indicates too lean condition on that carb. Since the problem is just off idle the problem
is often idle mixture setting or idle jet dirty or small hole in throat above idle jet and under throttle butterfly plugged (spray carb cleaner up idle jet and confirm
it comes out in throat EYE PROTECTION!) Also spray up through main jet while you have the bowel off.
Before fiddling everything and introducing too many variables try to isolate the problem with logical steps
1) Confirm problem is on one or both carbs by warm engine up to temp and then pulling the throttle cable at each carb one at a time.
Is the action the same or does only one carb falter? If only one carb confine your investigation to that one.
2) Set idle a little faster than you had it - does problem persist?
Are you forced in Canada to burn the alcohol contaminated gas? If so then 5 years is entirely too long to expect carb to remain clean and idle jet clear.
3) Before you get any farther into carb investigation confirm the valve clearance is to spec.
Thanks 44006 - I will try as you suggest - Last year I switched to 91 octane as there's no ethanol in it - I've heard it water logs floats.