Don't be fooled by petcock valve.
Some kits have a fuel petcock valve that has a "reserve" spot on it. I ran into one very recently and I tested it on the tank it came with.
There is no reserve and the "reserve" position works identically as the on position. I proved this point by filling fuel tank and running fuel from on position until non trickled out and then switching to reserve. Tank was then moved into a variety of positions to see if its position would allow reserve to flow out. NONE DID.