I need some help to figure out this problem, I got an glow engine Evolution 46

Here's what happens:

I've replaced the factory muffler for a pits muffler, everything works fine, except that for some reason when I close the carburator all the way ( I mean fully close 100%) the engine does not quit at all, I even did it manually just pulling the rod back and the engine keeps running, it just don't stop.

I am out of ideas, anyone knows what this can be..?

Thanks
Carlos