I'm looking for some help on my brothers 1990 swb silverado. He cracked the heads on the factory engine and rather than just replacing the heads he bought another engine. I believe he said it was a 350 from a late 70's or early 80's van and has a hi-nickle block (I'm not sure really. Just word of mouth). He decided he liked the tbi and decided to keep the factory setup instead of goin carburated. We installed the engine with a hi-rise intake that came with it and mounted the tbi to it using an adaptor plate from summit I believe to attach itb to the carburated intake manifold. It runs great and the engine sounds beautiful! However, he's only getting 11mpg compared to the 18+ mpg he got before. I'm thinking the computer is trying to compensate for the increased airflow and dumping fuel in. It also floods the plugs. Can anyone confirm my theory or correct me? And, if I'm correct, can someone guide me as to how we can correct this problem? I'm sure we'd have to change the fuel to air ratio on the computer but we don't have the equipment or knowledge. Any help is accepted with gratitude. Thank you