Remember what the WB3A was originally designed for; chain saws. They were designed to run very rich wide open to cool the engine. You can use it and it won't harm your motor.
DK
For the off throttle lean condition, you can put a notch in the shutter right at the low side fuel port like an IBEA creating a signal boost off throttle. a third jet like in the IBEA is just a high speed enrichment and will be no help for off idle, if you need more down low you can pick up the idle circuit, that along with a shutter notch should be enough. Mike , there's not a lot of room in a Walbro to have a very large t.Bore to V. ratio if you want to increase the venturi any. Jon