TBI help wanted

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

Welcome to CHR. I'm not following what you've done so far. Your brother bought another engine, but did he get the computer and the engine, chassis and interior wiring with it or just the engine? Did he do a R&R on the engine, or just swap the heads, intake & TBI from the "new" engine onto the old? Not following why an adapter plate was needed - why not just use the heads, manifold & TBI as a unit as opposed to mixing parts old & new?

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Originally Posted by thefordbrother

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

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Gm stock TBI unit will not take any cam what so ever. put the stock Gm cam back in it for a TBI and it will run better.i been there many times with customers the map scensor will not let much of any thing work.that and a dual plane intake. cam must idle under 700 rpm and clean with good vacuum . i do not care what you seen i know to many that have tried and failed