This is an OLD problem with my 78. When I bought the car, it was a nightmare of issues with its engine and injection. it's a bone stock 78 so stock efi and everything.
I got the car running pretty good through a hodge podge of tuning done without any prior knowledge of Z cars other than very basic info from reading the EFI bible and looking at the FSM for months before buying a Z car (looking back now I cringe at what I did)
The car at first was trickling lean from a crimped line, fixed that, and then it was pig rich. Looking at the AFM, I noticed it wasn't the stock one and it had been opened up before, so I figured it must have been tweaked. I began cranking down on that spring so much that I damn near BROKE the afm, but the car was now running GREAT (with the tps unplugged, plugging it in caused it to buck at WOT because of h ow the afm was adjusted) the afm was so tight that it was mostly closed at idle, and it didn't max out till the 6000+rpm range. This was fine and all. the car pulled awesome all the way to 6000rpm like it was nothing. One thing I noted at the time was that cylinder 3 did nothing at idle when you unplugged it, but the car ran fine on all 6 while driving. Never passed emissions cause I didn't realize I had to adjust timing.
Had to shop it cause my apartment wouldn't allow it to stay unregistered. Got a CORRECT Afm for the year, and the shop told me the leakdown test showed cylinder 3 had a really nasty exhaust valve leak. Had the head rebuilt, bought new injectors and had them install it along with the afm, car ran like crap smoking everywhere. pressure test showed it was pushing 100PSI had the seized FPR replaced. Car ran good now, but would pop out the intake at 5500rpm, found out it was because my exhaust was CRUSHED down to only a few mm. Removing the exhaust alleviated the popping issue so I replaced the exhaust. BUT, the car has ZERO power past about 4500-4800 rpm now.
And this is where it's been for the past 2 years since I've gotten it registered and have put 30,000 miles on it hahah. (and for whatever reason it burns a nasty bit of oil on start up, once warm it's fine)
So what the hell? It's not a fueling issue, I tried that with an adjustable pot that I had replaced the water temp sensor for testing. The afm is adjusted perfectly, and there are no restrictions in the intake nor exhaust. I thought for a while the shop had installed the camshaft sprocket wrong, but after finally gaining enough mechanical courage to check yesterday (and running around finding something to use as a chain stop. Home depot sells 12inx1in marking stakes, 12 for 2 bucks. hack one up a bit and wrap a wire on it and it's PERFECT)
The shop did the job right. Timing is spot on. Set to the number 1 position. Though oddly my chain has been marked in 2 separate places by someone scratching the links (on purpose from what I see) and has the shiny link line up on number 1 perfectly fine. So that rules out my ONLY hope at getting back that top end power my car is missing. I'm really at a loss now. After more or less thinking my cam had been advanced to the number 2 or 3 notches, only finding out it's perfectly fine I'm rather annoyed.
I have an MSD Blaster 2 coil installed, but could my cars ignition system be showing signs of age and isn't putting out enough spark to get good power? Plugs always look good too. I wish I took pictures of them when I pulled them yesterday. I've got something like 15000 on them and they look really good despite me constantly fiddling with my settings.
:/ I'm almost starting to think my car had a weaker cam installed when it was rebuilt, but i'm pretty sure they kept the cam and rockers and all that fun stuff, and just straightened the warped head, milled it to flat (not enough to change compression) and replaced the valves, etc. pretty basic stuff. Shop said the car was hitting 155-160 on all 6 during a compression test. too.
TL:DR, My car had is head rebuilt to spec, fuel system fine, exhaust fine, intake fine, but has no power past 4500-4800 when before, with a ghetto tune and 100psi of fuel pressure pulled like a monster to and past 6000rpm.