Carb Back fire

I've just re installed the carbs after installing a rebuild kit, did a valve adjustment, service and tune. Went for a short drive yesterday, all OK.

Just heading off for Day of here in Melb when the fasty just didn't want to go. Fortunately I wasn't far from home so I limped her back to the garage. The left carb has an intermittant flutter in the air flow and then an occasional backfire. When looking down the throat small flashes of blue flame are visible accompanied by a crack sound.

While waiting for her to cool down so I can re check the valves, I thought I would see if anybody knows what causes carb backfires. Anything I should check for?

Looks like I won't make Day of VW this year, F@#$!

Ain't nothin' like the wind in your hair.