I should probably tell you not to mess with it, but I had the exact same problem with the 173cc Powermore engine on my Huskee mower. It was set from the factory to run at low idle.

I'm not sure how different your 140cc engine is, but here is what I did:
After I remove the air filter and cover I removed 2 nuts and 1 bolt that hold the air filter box in place. I left the crankcase breather hose attached and flopped the air filter box out of the way.
There is a funky "D" head black plastic idle adjustment screw on top of the carb next to the throttle plate shaft. Turn it in to raise engine rpm's and back it out to lower them.
I cut a notch in the shroud on the air filter box that covers the carb so I could easily make adjustments after I put it all back together and had the engine running. I used a flat head screwdriver that fit snugly in the "D" hole.