So, as clutch starts to engage and engine takes a load it stalls? ... It sounds like a fuel problem; water in your filter, water or blockage at the pickup in tank, badly pinched line, etc. If you can get it running add a gallon of fuel heavily laced with fuel conditioner and just let it sit there and run for an hour while you check over the topside tank connections for air leaks and trace the lines for a pinch or loose fitting at the filter or pump. [Youll have to take off the left engine cover.] The conditioner alone should improve it if its water causing it.
larry