This means that the carburetor is flooding and when this happens the gas runs down the carburetor and out the 1st place it can, which is the throttle shaft.
Some of the things to look for:
Check your fuel pump pressure and compare with what is in your motors manual. For the YF probably around 4 lbs.
The needle in the needle and seat may be damaged. This can happen when putting pressure on it when adjusting the float. Check the viton tip for any signs of scaring.
Wipe the needle viton tip off with mineral spirits. Wipe until you don't get any black.
Dirt may have gotten into the needle and seat. Blow out and re-try.
Gas could be coming in the bowl around the seat. Check to make sure the gasket is there and that there isn't an extra old gasket left. Check for any cracks in the bowl top.
Check the floats for leaks. Heat up some water and dip the float into the water. Any leaks will bubble.
Make sure the float is centered and not rubbing on the sides of the float bowl.
Move the float up & down by hand to see if it catches anywhere.
Check the float level to make sure it is correct.
Run thin wire down the idle tube to be sure it is clear.
Move the float up and down to see if you can feel any resistance. The float pin could be worn causing the float to hang up.
Check the main discharge. Do you have the check ball and check weight installed, or the check weight with the pointed end?
Dirt may have entered the carburetor after it was rebuilt. Make sure you have a good in line filter.