The back one was accessible. I got plugs from a local Kubota dealerfor about $55 for four. They are ngk brand. I finally got around to changing them. I had to take the back injector line off. I couldn't get the second from back line off. I took all the intake manifold bolts out and could get access to the other three plugs, including the front hidden one. I've gotta try it a few more times, but it still didn't start well. I didn't do any fuel bleeding. I only tried it the one time, and haven't run it long. Hopefully it'll smooth out.
I'm glad you managed to get them in.
What is it doing exactly? Does it start then stop, puff white smoke while cranking? Slow cranking?
Make sure your battery is good, if the battery is low, the starter will spin slower and the compression won't be quite high enough to start easily. It may be worth while taking the battery to your local battery shop for them to do a load test. Give it a good charge first!.
How long are you glowing?
Sorry for the million questions, but any of them could be causing you problems.
As for bleeding the injector line, just run it for a few seconds and it will have bled its self out.
For me, i generally glow for say 20 seconds, longer if its cold. If the engine has a few hours up i would start at say 30 seconds and work from there, Kubota engines really do like a nice glow. If i don't glow long enough, it will start and run rough blowing black smoke then it will catch and work well. On a machine i have with a lot of hours up, if you don't glow it enough it will start then just die, give it a good glow and it will start almost smoke-less.
My starter has a bit of noise in it so i changed it with another one i had, the new one spins much faster. Your starter may want a new set of brushes too.
Start with the battery and glow time, work from there.