Themann101

I recently got a used gs400 engine put in mu car becausr of blown head gaskets on the original. I just got the car back today and it was running great. So I go to leave to run a couple of errands and it starts up fine but when I press down on the car peddle the car doesnt rev up or accelerate until I have the pedal to the floor and it barely moves at best. Thinking it might be a fuel problem but not sure. Waiting to hear from the mechanic who put the engine in. What else could it possibly be??