When I ride the bike at say, 30mph in 3rd gear the bike pulls fine, but at the same speed in 4th gear, it will ride ok for 50 metres or so and then the bike will start juddering, backfiring and sometimes completely stall. I then have to wait a minute or two before I'm able to start the ignition.

The mechanic said it can't be engine flooding as there would be visible signs of petrol overflowing and petrol smells, but I'm not convinced.

Any ideas if it is engine flooding and if so, what can be done about it? I've had to adjust my riding so that the bike is constantly pulling hard (not cruising) but this isn't very economical on petrol.