I have a 1996 S10 truck that had a 4.3 V6 and swapped it out to a Crate 350 260HP. Everything works with the following problem: when it is first started and until it gets warmed up and driven several miles to really get warmed up, it backfires, for example when you let off the gas and then give it gas again it backfires or tries to stall, TERRIBLE gas mileage around 10 to 11 miles to a gallon, black carbon out of both tail pipes on the driveway when you first start it up. However, it only does the backfiring when cold or until it gets real warmed up. Changed plug wires, plugs and no odd computer code when a laptop is hooked up to it. After it is warmed up, it runs great, no hesitation, no backfiring, but still get terrible gas mileage.
So, what is WRONG? By the way, it is fuel-injected, LS1 edit program was used to program everything, and at this point I feel it is something hopefully simple that has been overlooked or whatever.
Thanks for any info.

Rich

 

have you messed with the timing?