Small Engines (Lawn Mowers, etc.): honda 3813 riding mower, throttle plate, air passage

Question
engine runs fine until the blades are engaged.then engine won't increase rpm because it runs rich [black smoke] disengage the blades and it will clear out and gain rpm.it drives just fine even in 5th gear. carb was cleaned 5 months ago &worked fine, problem came on gradually inlast 2 months. checked carb bowl & needle for dirt -none.

Answer
Clean the carb again.  It sounds like an air passage is clogged.  When you engage the blade, the governor opens the throttle plate more utilizing different carb circuits.  It sounds like the main air delivery or atmospheric vent is clogged.
Do you have an air compressor to blow compressed air through the carburetor orfices?
Let me know.
Eric