torque converter won't lock up
 

I bought a used 86 Z28 that used to be tbi but now is carbed. The previous owner said the trans was replaced 10000 miles ago with a new trans from GM. Said it also had some work done to it, possible shift kit? Trans shifts firm through the gears but doesn't lock up. I tried the jumper wire bit at the diagnostic terminals but that didn't work either. What's next to try? I'd like to get my mileage up above 14mpg.

Make sure it has a good connection at the trans.

Make sure it's getting voltage down at the transmission. The stock setup routed 12v through the brake light switch through the engine harness from the driver's side fenderwell down to the transmission. If it was carb'd with a non-computer controlled carb, that part of the harness may very well have been removed, in which case you need to route it yourself and provide the grounding function (which energizes the lockup solenoid and locks the TC) by some other means.