Small Engines (Lawn Mowers, etc.): Carburator problem, speed jet, photographic memory

Question
Hi Ben,
I have an Encore Pro-Line mower with a 52" deck with 550 hours on it. It's a commercial grade mower I purchased new in 1996 to cut my 2 acre lot. It has a water cooled Kawasaki FD620D-AQS03 engine,
E/NO FD620D062115.
It has started to idle hunt up and down in rpm and loosing power under load. Today to cut the grass I found that if I clamped the coke at halfway it would cut fine as long as I didn't load it up too much , at which point it would start to slow down in RPM. I have sprayed carb cleaner into it and made sure the external fuel pump feeding it is providing gas to the downdraft carburator. I thought I might try you before I started taking the carb apart. It also backfires when I turn it off, possibly needing a new solenoid?
Can you steer me in the right direction? I would greatly appreciate any advice you could give me, I've had this mower since new in '96 and try to keep it like new.
Thanks for any help you can give me.
Bill

Answer
Bill
The carburetor high speed jet may have dirt in it as the mower demands more power (more Fuel) to keep up with the demand of the engine it is not getting enough fuel. Take the carb apart and clean and replace with new parts which you can get in a carb kit.Make sure you have a diagram or a camera  so you remember what goes where,unless you have a photographic memory. You may have problems putting it back together.