I have fitted the OS hyper 50 helicarb to my OS 91 fx. The carb is one big ***** ***** and actually have a 10mm venturi. Stock 91 fx is 9.5mm. On my teststand it seemed to spit alot of fuel in midrange even though I have leaned the low needle all the way (it is only possible to turn the low needle 90 degrees either way) I just finished a new plane fitted with this particulare engine so if the weather allows I will give it a shot this weekend to see how it performs in flight. I would suspect that a 50 engine with this HUGE carb on an airplane would experience even richer midrange than my 91 fx