Repairing a leaky fuel tank.
The fuel tank on the 70's to late model Motobecanes is part of the frame, so if your tank start leaking you have to repair it. You first have to locate the leak, you may have to remove the engine to access the under belly of the fuel tank. Fill the tank with water and inspect for leakage. You can see the little hole on the very bottom of the tank on the picture below.
Cleaning the tank
First by flushing it with water to get rid of all the crud and loose rust flakes, use the garden hose for this procedure. Rock the moped and tip it to have the dirt come out while flushing and let it dry.
You may then want to epoxy the hole on the outside of the tank and let it cure.
Use whatever product you like for patching the hole, I used "Steel Weld" you can get this product from your car parts store.
Now , plug the petcock outlet with tape or a piece of rubber. Fill the tank with vinegar and let it do its work for a day or so.
Then empty the vinegar out of the tank and rinse thoroughly, fill the tank with a mixture of baking soda and water to neutralize the acid effect of the vinegar, repeat a couple of times then rinse again with the garden hose. Tip the moped over to let all the water out of the tank, then pour some rubbing alcohol into it. The alcohol will bind the water molecule and get rid of all the water. Open the petcock outlet and release the alcohol in a proper container and dispose of properly.
Use a hair dryer to primarily dry the tank, but letting it dry in a warm place for a couple of days is a good idea.
I used this product to seal my tank, Let the moped tank dry for a day or so, don't store it in your house the fumes are potent. I used about a third of the can, after smearing the seal in the tank I poured the excess back in the can and it can be reused.
Now reinstall your petcock and fill the tank.