I have a very original '77Z with 45,000 miles. When I first got it about 5 years ago, I took everything off the engine, and sort of "cleaned it with a toothbrush" and replaced most worn out wires, leaving the original connectors. I did rebuild the fuel rail, and replaced the injectors, plugs, wires, and fuel and vacuum hoses, new fuel pump and drained the gas tank. Put headers on it and it has run like clockwork, seemingly timed perfectly. I take it down the road about once a month.

Last week, when I started it, I immediately noticed it to be misfiring. It started right up, and I took it down the road, misfiring 90% of the time, unless I was WOT 4000rpm or so, and it would seem to run well for a few seconds only. 3 weeks prior, it had run perfectly.

I put new gas in, and replaced all of the plugs and plug wires, checked to make shure all wires had spark, but same result. The old plugs looked like the car may have been running a little lean, with the exception of plug 4, which did not.

In this old car, any ideas about the most common culprit? All sensors are original.

I have the fuel injection "bible," but I wanted to float it here first.

Especially the injector connectors. There is probably a green scale inside the connectors. Then clean the tsp, thermotime and temp sensor

wires. Then pull the main connector off the efi computer and clean the terminals. Try to get a good electrical spray cleaner and corrosion inhibitor

since you let the car sit for long periods and especially if the storage area has high humidity.

That's what i'de do to start with since your car ran good before.

p.s. are you sure thats not 145k miles?

I agree with hr369, clean the connections then coat them with dielectric grease. Cheap and easy to do.

Check the spark plugs, I had a mysterious miss with my car and it was as simple as bad plugs. I bough a brand new set and one of them was bad so it is something that happens. Pull the wires off to see which one is not firing and then try a fresh set of plugs. Start simple cause it's usually things that are simple and overlooked.

You don't mention adjusting valves in the last 5 years. I adjust mine once or twice a year no matter how much I drive it.