Wonderful engine!
The Walbro carb used on the GF40 is the WT-1024. It's unlikely you'd find one anywhere other than an OS dealer. Walbro will design a carb for a specific application if you can fund it, as OS and Saito have done. With some effort, one can track down a suitable carb, though Walbro doesn't make it easy. In the past their site was more searchable. Four strokes really like an accelerator pump in the carb. The trick is finding a carb model which is currently available with the specs required and there is no resource which lists venturi size and other features.
Might say that a diaphragm carb can only get fuel by the impulses working the pump side of carb. If you can pull fuel through that carb then your needle/seat is leaking
This is not how a Walbro works. If there is venturi suction, the diaphragm opens the inlet needle. Obviously, suction also has to be great enough to pull fuel through the inactive pump. I would not try to fly a model with the carb setup like this, and certainly needle settings will change if the pump is working. It's working, seems quite well, as the carb is probably on the smaller side for the displacement.
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