For those that use the double reserve trick, try this instead.
1) Run with only one petcock "ON"
2) When that one sputters, turn the other petcock to "ON" (1st reserve)
3) At the next sputter, turn the FIRST petcock to "Res" ( 2nd reserve)
4) Lastly, you turn the SECOND petcock to "Res" (3rd reserve)
Now, you must know, & THIS IS IMPORTANT, 3rd reserve isn't going to get you real far. About the best I ever did with my '75 R75 with a small tank, was about 10-12 miles. Much more than that, & you'll be draining pre-mix from the trials bike you're pulling on the trailer. (I'm not kidding.) DAMHIK.
Anyway, it's a great way to maximize your range, without wondering if you're about to run dry.