Ryan is right, I wanted to add a clarification though. A lot of people dont know what reserve is.
First. The Fuel Valve is Called a "Petcock"
Your maxi has "ON", "OFF", and "RESERVE"
When in the "on" position, gas will flow through untill there is around two inches left in the tank. At this point, youre bike will run out of gas. Then you switch the petcock to Reserve, and it will allow the last 2 inches of gas in the tank to go through. The idea is that the reserve allows you to get to a gas station.
It is always a Very good idea to turn the Gas off when not using youre ped. So you really should get a new one.
Its possible that if yours hasnt been used in a long time that the rubbers have shrung, and will eventually expand when they come into contact with gas again. this usually takes a couple of hours. So if it still isnt working, than get a new one. I beleive you gan get a new one to fit for less than $15. Check Ebay.
The Postitons on youre maxi Carb are as follows:
When the little handle is pointing towards the Gas line, then it is in the "ON" position. When it is Perpendicular to the Gas line at a 90 degree angle, it is in the "OFF" position. When is it Pointing oposite, or away from the Gas line, it is in the "RESERVE" position.
When I say pointing, I imagine the little lever like the hand of a clock. Some petcocks actually have an arrow on them ging the opposite way. But for this refernce, And i beleive for all Puchs, the lever is like a clock hand.