how do i pull out the gas tank? how do i clean it?

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1970 Valiant 4 door - 225ci slant six 1bb

Well, not knowing the year or model, here is a GENERAL guideline. It helps to have the tank near empty - its a lot easier to handle. Disconnect the fuel line to the sender. The easiest way is to cut the line. Fuel line is cheap and you want to replace it anyway. There MAY be a grounding strap on the line. If there is, it just unclips. Unplug the sending unit wire from the sender, or the plug, depending on year/model. With a floorjack and a piece of plywood on the cradle, jack the wood up to the bottom of the tank. Loosen/remove the 'J' bolts and the straps. Slowly lower the jack. The tank will cock to one side. Twist, pull the the tank off the fill tube grommet.
Once the tank is out remove the locking ring for the sending unit with a BRASS drift (No Sparks) The sending unit then comes out. You may have to twist it around do to the pick-up tube.
Cleaning the tank can be done with lacquer tninner and a small length of chain, a handfull of marbles or rocks or soap and water. It would depend on the type/amount of deposits.
You may want to flush the lines. Again, this depends on the type/amount of deposits. When you are done, change your fuel fiter and pick up a carb kit. Some debris always makes it to the carb.
Good Luck