KNOWN ISSUE on B2400 ( and others ), mine does ( did ) the same thing. Just have to find the right guy that knows about it.
Key switch circuit has developed resistance that it isn't sending full power to stater solenoid ( what you hear clicking ) solenoid doesn't pull in hard enough to energize starter and shoot out drive gear.
The fix..... there is a$20- $150 "kit" sold by Kubota and JD and a few other places, it's a relay and socket don't buy it.
you can make your own for under $10 unless you need to have things fancy and proper
10 minutes of your time and a 4 pin 20amp relay ( with mounting tab )assorted wire terminals and some 12 gauge wire.
no need to cut ANY tractor wiring can return to factory form if so desired. you're only moving one wire on the tractor. everything else is made by you.
pin 85 to ground ( bolt on engine close by )
pin 86 the small wire that goes to spade on starter solenoid put on this pin
pin 87 to spade on solenoid
pin 30 to battery terminal lug on starter.
mount relay where convenient
start your tractor EVERY time ( unless relay burns out
)
I'd take a picture of mine, but I used a relay I had laying around, and it burned out, came out of a 12000+hr fork lift, go figure, so until I get a good one I'm back to click click click
pic from similar repair on this site
http://www.orangetractortalks.com/forums/attachment.php?attachmentid=2082&d=1285680886