Sure sounds to me like there is some play somewhere in your time, Either at the Chain or in the distributor. Could be the weights not returning fast enough or a sticking diaphragm in the vacuum advance.

 

I did not consider that, but that would make sense... I can guarantee that it's not at the chain, as this is a newly rebuilt original engine with appx. 13,000 miles on the rebuild... But I have not checked the distributor, could I possibly spray a lubricant on the springs and vacuum advance to see if that would free it up, or what would you recommend?...

 

I wouldn't spray anything on the weights. They should remain clean and dry. I'd pull the rotor and simply check the weights by pulling out on them, and see if they return normally. If they don't then new springs and cleaning should fix them. But I'm not certain the weights are an issue. The weights should be fully extended at anything over 2000 rpm, and decelerating will still have them out above that rpm range, and shouldn't cause any problem unless they were out lower than that rpm. If your issue was a lower rpm problem, then I'd suspect weights.
Not sure that it "always working before" means nothing failed now. Floats can stick, even after years of working. The fact that it gets worse when you try to throttle it tells me it's starving for fuel and when you open the throttle without enough fuel you're adding more air, and it makes things even worse. I'd remove any inline filters ahead of the carb, or in the inlet, and drive it to see if it helps. If it does, then replace them with new. Might also pull the fuel line from the carb and hook a hose to it, then put the hose in a can and have someone crank the engine to see if it's getting a good fuel supply to the carb. Might be a fuel pump starting to get weak.

 

You sure are locked in that there is no way anything recently repaired can be the issue. That is a mistake. Anything can break, I spent 6 years doing warranty work for GM and a large percentage of cars I worked on had less then 1000 miles on them. The after market world has less, yes less strict standards then the factory does.

Pins in chains are not heat treated right, bolts are not to spec, holes are not tapped deep enough. It happens all the time. When working on cars you need to approach the problem with no perceived notions. "I know it can't be that" without a test to back it up will mess you up. Anything made by man can be made wrong or break.

 

No, not at all, I do understand how even new parts can fail, being a car collector and restorer for over 25 years, TRUST ME, I've seen it all...lol I still have 7 collector muscle cars that I work on regularly and there's ALWAYS something failing...lol I'm just 100% certain that it's not the timing chain, as I've dealt with stretched chains, chains that have jumped, and this issue doesn't fit the bill, as the car runs perfectly all the way up until deceleration, that's when it acts starved for fuel... As per 1971BB427's recommendation, I'll go ahead and pull the fuel filter and recheck all fuel lines and such, I may go ahead and replace the new fuel pump that I recently replaced as it takes waaaay to long to get the fuel from the tank to the carb when starting after a week or two, so I've always considered that I'd gotten a bad or failing pump... Just trying to get all the info that I can cause even though I've replaced and trouble shot damn near everything there is, there's always someone with a better idea or recent issue that matches mine... I REALLY do appreciate all the great feedback, hopefully I'll have in pinpointed and fixed in the next couple days so I can share the results...lol

 

Just a FYI:

The test for a timing system is simple, Hook up a timing light and see how the timing reacts to reving the motor. A bad chain will show up as erratic timing. Bad advance will show a slow advanced return.

If you think the motor is running lean then just run the car and get it to miss and kill the motor. Pull a plug it should show a white tint to it.

 

PROBLEM SOLVED... Alright, figured that issue out, it was the fuel line at the pump, under acceleration it was getting suck pinched shut, so by the time I was under deceleration, it was just running out of fuel from the pinch off, once it returned to idle or low rpm, the line would then open back up and flow normally... I replaced the hose, so good to go, anyone have any input as to the tach not reading correctly?... Also, the Z has a Hurst Dual Gate shifter in it... It uses the stock cable and mounting locations, but appears to be a gear off, can't get it all the way back into first... I'm assuming it's probably just years of wear and could probably use a cable replacement, yes?... Or is there a way to adjust these stock cables and brackets?...