I have a Carrier furnace 201BB and the pilot comes on but theburners do not ignite all the time. What can I do. Is there a part that needs changed. HELP

Does this model have the hot surfice ignitor, if it does here is my advice. Find the flame sensor, it is a wire probe that sticks into the burner, looks like a wire sticking out of a cerramic rod, it has one wire coming out of it and it is attached to the furnace with a screw. Take the screw out so that you can get to the ignitor, use a fine sandpaper and clean the metal rod, re-insert it into place and screw it in place, and now try it.

I hope this helps if so let me know or give me a rating on this answer.