Fouled left spark plug on starting.

My 72 CB 450 fouls the left plug on start up.  I use the choke sparingly and have cleaned and bench synced the carbs.  The air cleaners were rebuilt using the original boxes with new filter media installed.  I have played around with the mixture set screw turning it in to just a quarter turn but it does not seem to make any difference.  After two or three starts the plug is fuel fouled and has to be replaced.  Any suggestions on what the heck is happening?