Checking all those things mentioned = good of course but you have mentioned what I think is a clue to your issue: [Note this has been written up in other threads on TBN about starting issues esp with Kubota]
You said you connected the battery charger (to an already well charged battery) and the thing started right up. That is a big clue I think to what has been an issue with MANY small tractors incl. Deere and Kubota. Works like this -- you get a small voltage drop across 6 or 8 safety interconnect switches (the seat, the transmission being out of gear, the PTO not engaged, etc.) required to enable starting. All those voltage drops add up sequentially to drop battery voltage low enough to where it is barely enough to operate the solenoid by the time it gets down the chain that far. You may get a click or some noise but no cranking. Key fact is that the charger output is
MORE than 12v so when you attach the charger instead of applying 12v you are applying around
13.8v. That overcomes the disease of many small voltage drops on the way to the solenoid. [I did that on my
BX2200 Kubota for a couple of years before finally having a dealer install a 'hard start' kit. Problem is GONE now. ]
Of course cleaning every contact, connection, etc. always helps but just improves it rather than fixing it. Deere and Kubota dealers sell what Kubota calls a
"hard start kit" which bypasses all those small voltage drops, uses the "all is OK signal" from your start key switch to trigger an added relay which then applies 100% of the battery voltage to the solenoid and fixes the issue. Many savvy folks have put that in as a "do it yourself" mod. Some have added an extra starter switch that does the same bypass thing, etc.
