No power, exhaust backfire

Pulling out of an intersection my truck backfires through the exhaust and acts like it wants to stall for a minute and then takes off, but it has very very poor power. If I hold the brake and the gas all the way all it does is backfire a lot and wants to shut off. The exhaust smells bad too, not like a gas smell. It is just very strong smelling and usually is more white than it should be in my opinion. I know the head gasket is good so I am at a loss here.