DM Nedvidek

I have a 1975 yamaha dt175, the bike is in great shape; I bought it from the original owner who kept it in his garage, it only has 1300 miles on it. When I bought it, the bike started on the second kick and was immediately drivable. He did tell me that the petcock had begun to leak, and so the fuel line was constant to the carburetor. He said it would be easy to fix, just an o-ring. I took off the cover(the one that says on, off, res)and replaced the rubber gasket with the four holes in it and i had to configure my own o-ring to go around the turn knob, but I am still having a leaking problem. Any ideas as to where I should look next. I have always been a huge fan of motorcycles, and the bike I have always wanted is a mid seventies enduro, but I am a total newbie and need all the help I can get, so any help will be great. -David