The main problem I have had with getting these to prime is the bowl lip being too high and bottoming out on the carby body before sealing the o-ring (hence the pressure leaks out before it can force fuel up the venturi. Test this by tipping it on its side and seeing if petrol leaks out of the bowl seal. Found this is common on chinese quantums.
I ground the lip down on a flat surface with sand paper and it worked
The other is needing 2x gaskets on the air filter housing and evenly torquing down the bolts progressively. The crap plastic housing warps and the air pressure leaks
