So.. I was trying to find a manual for the 270 because I only have a manual for the 250/300, but I found this picture of the inside of the carb and it looks like that hole by the red arrow is normal on these things.. Weird but true.
I also found a reference to some "pair solenoid", which it turns out the 250/300 have, and yours might too. It's not an electrical solenoid but a vacuum operated solenoid, and if it had a diaphragm with a hole in it , it could let too much air in..
MXU300.pdf