What are we seeing, flames from the tail pipe, or is that the catalytic converter ?
If flames, it must be be running incredibly rich. Is the Check Engine Light ON ? Have you pulled any CEL trouble codes ? The obvious suspects would be worn, defective, or incorrect tune-up/ignition/emissions parts, or faulty fuel injectors.
How short is your exhaust ? because if it is that short that is normal all motors will do that . also at full throttle it will also run with a blue flame about 10 inches long .