Backfire under loads

[D13]

TCC on 700R4 does nothing in 1st gear. If you have the problem in 1st then it's not the TCC.
TCC is fed by 2nd gear oil. If there is a problem where it locks on in 2nd the motor will lug horribly at the1-2 shift.
Yes, the TCC is wired. Power thru the brake switch so touching brake pedal will dump it.

My guess is that the cam is off a tooth on the chain, or my favorite - someone put a high RPM cam in a stock motor and converter won't flash enough to let it run right. Neighbor had that on his Ferd, put in a junkyard motor that turn out to have been built to run 4500-7000. Explained why they couldn't get it to idle at 800. Set the idle at 1500 and added a ton o timng and it made scary power.