Thanks for the reply AV8TOR. I was hoping to hear from you.
the HobbyKing velocity stacks are too small for my carburettor. Mine has a 22 mm bore. All the ones I could find listed were only 18 mm.
I do like the configuration of this one which has the stack integrated into the filter mounting. :

https://adaracing.com/hobby-rc/air-f...cheap/gpafk-rc

I'm even thinking I might be able to make something similar big enough for my engine. What kind of foam is suitable for a filter, do you happen to know?