Hi Vietvet,

When you press the primer bulb, if it does not fill smoothly or continues to generate bubbles, it may mean the pump is not functioning correctly or that there’s a blockage.

Fuel Flow Issues: The presence of air bubbles in the return line is concerning and could indicate a few things:

Ensure that all fuel lines are securely connected and not cracked or damaged

It always pays to replace the fuel hoses , if the old hoses have a very small hole or crack that you can't see the carby won't work correctly.

below are a couple service manuals ,even if your carby is slightly different the operating principle is the same.

https://www.walbro.com/wp-content/uploads/2016/01/WYJ.pdf

https://www.miniplane-usa.com/pdf/WalbroServiceManual.pdf


Cheers
Max.