Small Engines (Lawn Mowers, etc.): Engine surging, twin cylinder engine, craftsman lawn tractor

Question
I have a 2002 Craftsman lawn tractor (model #917-271842) with a Briggs 21 HP twin cylinder engine.  I recently changed spark plugs, air cleaner and fuel filter.  Now when the engine is anywhere below top speed the engine surges.  Any ideas?
Thanks,
Gary LaGrange

Answer
It sounds like a blockage or adjustment in the low speed circuit in the carb.  At wide open throttle and under load the high speed circuit handles all of the fuel demands and below this, the low speed circuit becomes effective.  You will need to first try to adjust the low speed mixture screw (if equipped) starting at 1 1/2 turns out from lightly seated.  Depending on how the engine responds, you will determine whether you need to open or close the mixture screw from the initial setting.  If the low speed mixture has no effect or the carb is not equipped with one, you will need to thoroughly clean the carb using a piece of tag wire to push through all of the internal passages.  Hope this helps.