I have a Husqvarna YTH2148 ride mower that puts out black exhaust when Throttle is in fast mode. Changed plugs, cleaned carb, Replaced ignition coil to both sides

chaeck your fuel pressure and your ignition coil.

Black smoke means it is running rich. Too much fuel or not enough air. I doubt that it is an air issue. Check the float and carb.

Check coils.

turn on device and wait for response inspect every component for malfunction

You might want to try this one but pricey thing, replace the carburetor! I have a 2008 Sears DTY 3000 25 HP two cylinder Briggs and Stratton Engine, I struggled for a year and half if it quitting when it gets hot. I replaced everything, Gas Cap, ignition module, fuel lines , filters , spark plugs, gas tank, exhaust / exhaust manifold and it all did not help. Due to the acidic (Alcohol added to the fuel) gas two the rubber inner control gaskets disintegrated causing "vapor lock" when the engine got warm/hot. Replace the Carb and then put additive into the fuel.

replace the switch on the brake/clutch it is a dual switch so it can crank and still be bad

dirty carb clean and acid dip

clean the jets inside the carb

Don't know if this helps but the owners manual for my Husq YTH2148
indicates that the transmission fluid is not to be changed, ever. If there is a problem, the whole unit is to be replaced.

These mowers, along with many craftsman rider mowers, are actually made by Roper. Perhaps if you googled around a bit you could get more info relating to service of the transaxle.

Not so many reasons she won't start. 4-Strokes are Simple!

Here they are:

Motor:
Fuel Tank-> Fuel Line-> Fuel Filter-> Carb-> Intake (Valve)-> Piston-> Exhaust (Valve)-> Muffler

Carb (Old Style):
Fuel Side:
Fuel Bowl-> Float-> Needle Seat-> Fuel Ports (Venturi)
Air Side:
Air Filter-> Carb Air Intake-> Butterflies (2)
(Clean 'em ALL!)

Hardware (Motor):
Rotor (Magnets)-> Magneto (Ignition Coil)-> Spark Plug Wire-> Spark Plug-> Cylinder Head (Spark Plug Ground)

Ignition:
Kill Switches (Seat, Clutch, etc.)-> Spark Plug/Ignition Coil-> Ground
Key Ignition-> Battery->Starter/Generator

If one of those fail.......= Non-Starting 4-Stroke!

Fixes-
Fuel:
You are either getting too much, or too little. Too much = Carb.
Too little = Carb. Therefore, tear down the Carb (COMPLETELY-Careful!) and clean it up.

Electrical:
Grounding out the System (Kill Switches)
Battery-> Ignition-> Starter.
Magneto->Plug Wire->Plug (Check 'em all!)