I have a Brison 3.2 with a Walbro Carb WT 551. I was flying today and after the 3rd flight the engine would not start, I checked the spark plug and it has plenty of spark. But I did notice that the diaphram on top of the carb was not moving at all. I checked the screen and fuel is flowing from the inlet. If I put my finger over the choke plate then the diaphram moves but as soon as I take my finger off no movement. Does anyone have any ideas.

Thanks

Ray