That sounds like a great offer from Fish.
I thought i would post up my response to your pm to me here so others can add/disagree etc.
The tank vent in your kind of saw tends to be a piece of orange tubing with two screws in (grubs);
(note this is a 025 not the 028 you have).
The job of the tank vent is to allow the fuel to remain at atmospheric pressure, if blocked as fuel is sucked out a vacuum would form in the tank. Which would result in not enough fuel being able to be drawn from the tank and the saw will starve and die.
The impulse tube as shown below (the short black tube which pokes out of the casing, centre right) (note this is an ms240, just shown to illustrate)
attaches to the underside of the carb, travels through the plastic casing and joins to the crankcase.
The carb above is out of an ms240 (again to illustrate, underside of 028 carb should be similar), the impulse tube joins the brass looking adapter of the carb. You will have to take the rear handle/tank assembly off to replace the impulse tube.
The impulse tube uses the movement of the piston, to work the pump diaphragm and thus draw fuel from the tank. If there is a hole in the tube, however, i have found that sometimes the saw will run untill hot and then die as a result of the adverse effect the heat has had on the hole in the impulse tube.