Crappy running 460 update.

Let her warm up a bit then moved the throttle lever. She still stumbles alot. BUT There is a noticable improvement from before.
I found that it runs kinda ok with the choke pulled out. Does not stumble or die.
I take that as a good sign.
Gives me hope that after the carb rebuild and proper adjusting it may be ok.
I priced a valve job today and it about $400 so I got my fingers crossed.

Don't know if you have checked this yet, but check the opening to the sediment bowl, if you have one. I put a new one on my Super M some years ago--a generic one from TSC--and this tractor ran like crap a long time. Couldn't figure out this aggravation until one day when the tank was empty, I removed the bowl and found the answer. The port that treads into the fule tank was nearly completely sealed due to a manufacturing flaw that left excess metal behind. I drilled it out and that ended the problem. I should have checked this way back when I bought it, but when something is fresh of the shelf, I tend to assume that it is ready for service. Shame on me :)

From what your posting it sounds just like a 560 I worked on a couple years ago.
Finally pulled the manifold(s) and found that the exhaust side had rotted out and was leaking into the intake side.
Pretty common problem but not too spendy to fix.
Just make sure if you end up getting a new exhaust manifold that you bolt the two pieces together on a flat surface and then take the assembly to a machine shop and have them plane the surfaces that attach to the head.
Otherwise the old and new won't match and you'll end up with the same problem again!

Doc