engine problem

[ks147j111928]

Hey guys, I normally post in the 67-72 area but this is for my project truck. It's a 1980 K10 I got for $600 with a blown engine. I rebuilt a donor engine from a former parts truck that ran well. The engine is a mystery, it was injected (no fuel pump rod hole in block) but I got it with a 2-bbl on top. The casting # is 14082508 (if I remember right) It has been totally resealed and is running. The problem is this: it backfires through the carb on the driver's side intermittently but not erratically. It's a steady pop, like something hitting inside the carb. I have double checked everything I can think of: plug gap, wiring, firing order, timing, TDC, jets, needle/seats but it still pops. It's noticeable at idle but gets real bad under load/higher idle. I am at my wit's end here, this is the only thing holding me up from getting it on the road. More info: I redid the heads (factory iron) with new valve seats and seals, 3 angle job and decked it .010. Also the pop didn't show up until after I had the engine running for a day or two, not longer than @ 15 minutes at a time. Either that or I didn't notice it. I also had to switch to points-type dizzy to clear the old school Edelbrock intake (C4B) I put on. Points gap and dwell angle are within factory specs (Haynes manual specs anyway...) and I get good spark at the coil and all wires too. I am out of ideas here!

Thanks,
Dan