You may have noticed that the main jets you received were labeled as GX340 and GX390.
The GX340 "stock" main jet size is .036" and the "stock" main jet for a GX390 is .037".
We have found these are the correct size when adding an open air filter and header to the 6.5HP engines, such as the Predator 212, Honda GX200, and the 196cc Clones.
These jets have the same overall diameter and thread pitch to fit most Chinese carburetors on engines ranging from 160cc's to 460cc's. The inside diameter is the only difference!
We like to start with the .036" jet (GX340). If the engine is still lean, swap to the .037" jet (GX390).
Thanks for your purchase!