My 2001 Classic 700 backfires for the first 1/2 hour of use on a cold day. Went riding yesterday at -10F and had lots of problems with backfireing at 1/4 to 1/2 throttle. It ran OK at full throttle. When it becomes well warmed up the backfireing only occurs a bit when slowing up from 1/2 throttle down to 1/4 throttle. Thanks for any help.