I have a zero-turn mower with a Kawasaki FR631V that stopped running while mowing and won't restart.
 
TROUBLESHOOTING - Fuel, air, and spark right?  OH - and timing and compression too.
 
Fuel- all seemed well but we went ahead and replaced the fuel line, filter, and carb.
Air - brand new air filter, intake is clear
Spark - brand new battery, plugs and coils
 
Here is where it get's interesting ....
 
Timing - did find the woodruff key sheared, thought AHA!  The flywheel was 30-40 degrees off. Replaced the key and it still won't start.
Compression - tests at 80lbs, both cylinders equally.
 
I decided to check the valves, so I drained the oil, pulled the valve covers and heads.  No issues that I can see (broken parts) but there is a LOT of carbon built up.  Top end was cleaned when a bent push rod was replaced last fall.
 
QUESTION 1:  What should I look for as to the cause of the carbon before I put the motor back together?
QUESTION 2: Why won't it start?
 
I am at a loss on this one, any help you can give is appreciated!
 
-Jim

I gather it's carbureted?
 
Pull spark plugs, attach to leads and ground to head. Have someone crank the engine and visually 
look for spark.  Just cause it's new, doesn't mean it's good.
 
Should you have spark, try spraying some carb cleaner/starting fluid down the carb throat and see if it starts up/kicks for a couple of seconds..
 
You mentioned the valves, did you check clearances on them?  
 
Has the machine been using up oil?
 
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I gather it's carbureted? - YES
 
Pull spark plugs, attach to leads and ground to head. Have someone crank the engine and visually 
look for spark.  Just cause it's new, doesn't mean it's good. - ALREADY DID, GOOD SPARK
 
Should you have spark, try spraying some carb cleaner/starting fluid down the carb throat and see if it starts up/kicks for a couple of seconds.. - DID THAT TOO, WON'T FIRE ON STARTING FLUID EITHER
 
You mentioned the valves, did you check clearances on them?  - NOT YET
 
Has the machine been using up oil? - USING?  DUNNO.  I THINK IT IS LEAKING THOUGH.  THERE IS A RESIDUE AROUND THE BASE ON THE MOWER

Check valve clearances first.
 
A leak down test would be next.
 
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