10* Before TDC @ 800 RPM
 

Curious, I have spec'd out all I can so far. Valves checked and adjusted, oil pressure is good. AFM "seems" to be functioning properly, 5 gas analysis says it's mixed properly, vacuum lines replaced for the most part, no fluid leaks anywhere, compression is good and even, nothing seems out of the ordinary. Engine mounts and driveline mounts need replacement, clutch is tired but functional. Timing has been verified at idle according to the manual three or four times. When accelerating under WOT right around 4500-5500RPM the motor stumbles. Half or 3/4rs throttle, no problem to redline. 1977 280z all stock. All original. What gives? I feel like its timing (checked multiple times at idle) , fuel delivery (either way but lean is my guess, not much backfire on sudden dead pedal and not popping out of the AFM) or freakin aliens, because it doesn't happen unless the pedal is on the floor. Which is when I would prefer it not to :)

Any ideas? Any timing advice from the pros regarding an out of spec adjustment that solves this? Any AFM knowledge that will blow my mind? I've seen most of the DIY AFM stuff.

Thanks in advance. Hehe...

Would check your TPS and make sure it's actually making contact with the WOT side of the sensor at WOT...

Would also check the brain on your AFM and look for scoring on the carbon sweep thing. This may meen that at some point in the range of that potentiometer it's loosing contact and sending the wrong AFM signal to the EFI, EG instead of the AFM reporting it's 3/4 of the way open it will report nothing, which the EFI might interpret strangely.

I had a similar issue at 2800 RPM and the AFM circuit board was the culprit. I loosened the screws On the circuit board, moved the board between 0.5 and 1mm higher to get the sensor out of the 40 year old groove. I tightened the screws again and it was good.

That you say it's fine if you go through that RPM range at 3/4 throttle makes me suspect the TPS as well, or a combination of the two.

@Pez
 

Thanks for that. The TPS was next on my list. I admittedly know nothing of the Z's TPS but I assume it's relatively standard. I've often wondered if it would cause problems but everything seemed pretty right n tight after chasing down all the vacuum issues. I will give that a shot tonight :)