I just got a 94 discovery, I have a problem it wants to backfire and wants to die, and it has a smell of fuell after it turns off.

I have been told it was the throtel positon swich?

 

If it is backfiring then it is because your timing is off.
If it will run fine with your foot on the gas pedal but will not idle on its own then it is the idle control valve, also known as the stepper motor.
Have the timing set, if it continues to backfire then your timing chain may have jumped a tooth.

The throttle position sensor will not make it backfire. Start by having the timing adjusted.

 

How many miles on the truck. Start with the basics, check and replace your wires and plugs, use 8 mm Magnacor wires, install a new, factory rotor and cap, flush the engine, do an induction cleaning. If it still happens do a compression and or leak down test to see if you have a fried exhaust valve.