We used to operate both 152s and 150s at a flying school I used to work for. By the time winter came around, we always had to do a briefing on the differences in the priming systems as the students were very much used to the 152s, where the priming fuel is deposited a lot closer to the cylinder than on the 150s. To make matters worse, someone thought it a good idea to have all the dual flights use the 152s as they had a bit more power, while the 150s were scheduled for students flying solo. Anyway, that was a whole different discussion.
The cold starting procedure on the 150 (if my memory is correct) has you turning the prop over by hand while priming, so that the suction will draw the fuel up, from the point down in the carb where it is injected, towards the intake manifold. On the O-200, the carb is of course down below the engine and you've got an airbox at the bottom, the carb directly above it and from that point the intake manifold goes straight up and turns outboard to end up underneath the cylinders. Gravity will of course always take over and fuel will inevitably drain down again towards the air box. Because of this sometimes the procedure was modified to turning the engine over on the starter while priming, but this was not in accordance with the POH. It did get the job done though. We also had the inevitable carb fire at times, one time when I was on board. The first clue was a burning smell and smoke coming up through the vents. My student was very quick and starting shutting things down before I could tell him to keep cranking so the sensible next thing to do was: get out. Fortunately, there was no big fire, but you could tell something had gone wrong, see below.
