Rich TRX850

After 17,000 miles of running well, my TRX850 has started to run rich on both cylinders. It is rich enough to foul the plugs and cause misfiring at low revs. I believe the ignition is ok as it appears fine at higer revs I have changed the plugs and air filter, checked that the chokes are not sticking, renewed the choke mechanisms, cleaned the carbs several times, but they look spotless anyway, checked the diaphragms and throttle position sensor and checked for a blocked carb breather. I suspect that the bike has been dynojetted but as it was running fine before this problem this should not be part of the problem. I have run out of ideas, any suggestions would be more than welcome.

Thanks,

Mark.