It sounds as if you need to go through the overhaul procedure for the idle system. Start by checking whether it is the same as the WYA covered by this Walbro overhaul manual:
http://wem.walbro.com/distributors/servicemanuals/WYAseries.pdf

Look at Page 5, which tells you how to unseal and adjust the coarse idle mixture adjustment. Then clean out the idle system, and follow that adjustment procedure.