I just registered so I can provide a solution to someones problem on an old post - as I had the exact same problem and fixed it.
But your damn stupid rules say I have to jump through all kinds of hoops to be permitted to post a reply, got to first post 10 discussions, wait 2 months etc etc.
It is no wonder that many problems are left unsolved for years, as who could be bothered to register just to help someone out??
This is the post I want to reply to :
"Honda GX120 shuts off after 10-25 seconds of running. Cant figure it out!"
I have same engine with same problem. Solution - remove the carby fuel cup by unscrewing small bolt. Insert a fine wire down screwhole to clean out the jet hole through to the venturi throat of the carby. If you removed the air intake you should be able to see the wire poking through and hitting the side wall of the throat. Now the fuel should have no trouble flowing into the venturi and engine. This fixed the problem for me. I found this solution on a youtube video on carby cleaning. An indicator that this is the problem is if you can keep the engine running a bit by spraying carby cleaner into the air intake, this indicates fuel starvation, and a seemingly clean carby is actually partially blocked. This was what happened in my case.