If you have a carb fire on the Cessna 150/152 good chance you have set fire to the air filter. Any excess fuel will pool at the lowest point which is the carb heat box/valve assembly. Fuel will be forced to run towards the oil soaked filter and if blow back occurs this will catch fire. Carb heat box has a hole in bottom so most fuel should drain out onto the nose wheel.
In winter i usually start the engine before the students/PPL's get their hands on it. Mixture rich, carb heat to hot posn so the engine can breath easier, couple of primes then third stroke of the primer as i crank the engine.
Of course this assumes that the priming lines are all connected and the jets unblocked.
Club using a Cessna at moment with only one primer line plumbed in to one cylinder, fortunately the a/c carb is fitted with an accelerator pump so several pumps of the throttle AS THE ENGINE IS BEING CRANKED, gets it started.