So I have a KA swapped Datsun 510 and I spun a rod bearing in it... it had been running fantastically before this happened. Pulled it for a full rebuild, block machining and everything. Long story short, it runs like s***. Rough, extremely low idle barely above stalling. Revving it up it sounds good but it seems to be running very rich from the exhaust smell. Sounds like the timing is off, but when I force safe mode by unplugging the MAF it idles and runs really smoothly. Tried a different maf, but both are perfectly functional as tested in on another KA. Tried another ECU, no change. Wiring harness has been scoured with no worn wires or anything and the only wires I've cut are fuel pump stuff and the ECU power. Mechanical timing was all installed correctly. Doesn't have any obvious vacuum leaks or anything. One thing that's confusing is unplugging the IACV doesn't change anything, and opening/closing the idle adjust screw is also useless. ECU throws no codes. Pretty lost here...

http://www.youtube.com/watch?v=UaD5jfcUkC4
Video where you can hear how rough and low the idle is.

You may want to run back through the timing again. Connect a light to the 1st cylinder as the engine idles and see if its correct. I don't know the number off the top of my head, but if you search, it should be found pretty easy. You could also pop off the valve cover, spin the engine by hand to TDC and see where the cams, gear markings, and rotor in the distributor are pointing. At TDC, Intake Cam is at 12 o’clock, Exhaust Cam is at 1 o’clock and the distributor should be on the #1 cylinder. If this is off, consult an FSM for details on how to reset the cam gear and timing. You could also try loosening the hold down bolt on the distributor and twisting it ever so lightly to see if the idling gets any better.

I will need to get a timing light, but the mechanical timing has been checked and rechecked and it's spot on... Distributor rotor lines up almost horizontal, right on the #1 spark plug post. Cam gears are at the same angles as stock TDC, and were all lined up to the chain markings originally. I'm pretty sure something sensor wise is retarding the timing almost a ridiculous amount, considering how well it runs in safe mode. Advancing the distributor all the way helps a tiny bit, retarding it all the way it won't idle at all.

Copy and paste a link to this thread into this sticky in the 240sx section:
the-i-m-new-here-and-i-have-a-simple-qu ... -1680.html

Papasmurf will be better able to help you trouble shoot than I can and he checks it almost daily.

First off, holy crap man that car looks AWESOME. Great job on the engine install. I wish my KAs looked that good haha.

If you say the cam lobes are correct for TDC (intake around the 9:30 position, exhaust around the 2:30 position), and you've tried rotating the dizzy to adjust timing with no real good result, I believe you. It SOUNDS like it runs/picks up well when you nail the throttle. The IACV part has me worried.

If possible, try inducing a vacuum leak after the MAF to see if it helps (between the MAF and throttle). You can do this by attaching the MAF/filter to the pipe loosely/ a tiny bit open so some air comes through the MAF, and some sneaks by it. If that doesn't work well enough, try inducing a vacuum leak past the throttle body (like T off from your fuel pressure regulator vacuum line and just leave a vacuum line open). That will sort of simulate an idle air control valve. Both should fix the richness issue.

Let me know how that works.