ORIGINAL: jedijody

An accelerator pump, is intended for applications where the engine needs to idle slow, consistent, and reliable for extended periods then punch the throttle to full or nearly full throttle, the extra shot of gas carries the engine over until the airflow through the carb accelerates to the point it can pull enough fuel with it to sustain the high RPM. The problem with our engine application is they are very lightly loaded in the off idle and midrange RPM band and at least for most aerobatic and 3D flight the engines are run between off idle and about 3/4 throttle most of the time. If they had accelerator pumps they would be constantly pumping fuel into the engine and would flood it out. Also, accelerator pump carbs almost have to be designed for the specific size engine and use application, there is a very fine balance in correctly metering the fuel at off idle and midrange for small gas engines with accelerator pumps, that's why they are so rare. So, if the carb and accelerator pump isn't calibrated specifically for our engine size, type, and use, and we only need the engine to idle for relatively short periods and most of the running is at a throttle valve positions that would constantly be pumping in extra fuel they won't work all that well.

There's a little more to it than that but those are biggest reasons as I view it. I've used accelerator pump carbs on small single cylinder 2-stroke engines in the past and in every case the metering of the pump volume had to made adjustable to suit the application,....you think people have a hard time adjusting a carburetor now with just two little needles, throw in an adjustable accelerator pump and for many the engine would become unusable.