I had replaced the intake/exhaust manifold gasket earlier that day, and replaced the fuel filter, pcv valve, oil breather cap, and vacuum lines. I drove it to go hunting to break the gasket in so I could re-torque all the bolts. We'll on the way home it started to stutter while acceleration, I backed off then pushed it again and it ran ok. Got home and it wouldn't idle. I popped the hood and the air filter and housing was sitting sideways on the carb. Pulled it up and the small brass nut that locks the threaded rod in the carb was missing.
I pulled the intake/exhaust manifold back off, no nut in them or on the valves. Pulled all spark plugs and checked pistons by having the wife bump the starter while I looked in with a flashlight. No nut visible in any!!
Not sure what to do next. Can't see where the nut could have gotten to. Really don't want to pull head off. Just don't have time, what little spare time I have is squeezed to the woods.

Need some outside advice!!

 

I once had something very small, about a quarter inch in size fall down the intake and get sucked into the piston. It slammed and pounded so loud I thought my engine was going to come apart. I would think if the nut came off, you would have known it right away if it went in the intake.
My guess is it's laying on the road somewhere.

 

Another possibility is that it did go thru one of the cyls. The "stuttering" could have been caused by it getting caught between a valve and the head. Then go thru, into and back out of the combustion chamber, then out the exhaust.
You said you examined the piston tops as best you could, and didn't see any sign of it. Perhaps it went thru without doing any harm. You stated it was brass, which is rather soft, so no damage is quite feasible.

 

I only saw one cylinder that had a slight bit of the carbon build up knocked off the top. But no noticeable "damage" was present. I plan on doing a compression test once I get the manifolds back on. I'm hoping the stuttering and dying is just from a dirty carb that needs adjusting. I plan on doing a more detailed cleaning of everything. But am wanting to wait until after hunting season is over.