The carb tip is a good one - but beware that over-tightening the carb onto the head can create a similar problem - the carb base bends and creates an air-gap.
The base (or flange) bends very easily and it is a very common problem. Many people don't realise what a light touch it takes to reach a high torque, even with a small spanner.
To check if there is an air gap due to a bent carb, take off the carb, rub the base with lipstick or similar and press it on a very flat surface like a piece of glass or a mirror. the lipstick will leave a "footprint" on the glass so you can see where the gaps are. If the base is not flat on the glass, gently use a bench vice to straighten it up as far as you can, and a file it to finish off.
Before all that, keep trying the timing - it often needs to be a bit advanced to get it right, especially with unleaded fuel. Or it may be too advanced already