Okay so for starters, this is not my car, this is my cousins 84’ corvette. I know this isnt the correct forum to be looking on and that i should be on the corvette forums, which we have tried. Nothing has helped off that forum and now im turning to my Third gen friends for help. Below is his whole smeal on the issue and things he’s replaced in the process of trying to fix the issue.

So let me first begin with what caused this issue to arise. As a lot of you know the 84s are slow by today's standards. I get that. So before I did the little bit of work to it I did, I never really beat on the car so to speak. So in reality I don't know if my problem was occurring before I did this. Now the work I did was very mild. I swapped in a very mild cam and a renegade manifold as well as new lifters, rockers, timing chain, etc. My cam choice was made after careful research to make sure it would play well with the stock computer. It would. I ended up with a very very mild upgrade. It is comparable to the 89 l98 cam specs with a little more lift.

Next I made all of my adjustments then broke in the cam. After break in I set the base timing to 8 degrees and took it for a test drive. Had a check engine light. Okay. Code was for coolant sensor. I still had the old knock sensor style plug and it was questionable at best. Replaced it with a new style sensor and plug then it was good and went into closed loop with no more light. Awesome. Now I start driving and I can actually smell the car leaning out. Okay. Adjust fuel pressure. No good. New fuel pump fixed pressure issue but still a tad lean. So I swapped in larger injectors and dialed back the pressure. lean out was then fixed. Now the oxygen sensor isn't mapping great. Checked connections and they were good so I replaced the O2 sensor. Nice smooth voltage swings now.

Now all of this was occurring while I had a break up starting around 2500 rpms and clearing out around 3100-3300. The best way to describe it is that it feels like it is hitting a rev limiter. The car just stops pulling and it actually almost sounds like is being cut to some cylinders. Alright. Check the plug wires. They got cooked during cam break in (manifolds turned cherry red. Like cherry red.) and were arcing. New wireshelped a lot then the car really started bogging a day later. Culprit was the coil. New coil and it ran like a top again. New ignition module for safe measures since it was original. Still missing or breaking up starting at 2500. Checked vacuum 19.5 at idle. Soaked the intakein starter fluid with no change in rpms. No vacuumleaks... For the heck of it I swapped the plugs. I had heard of issues with the delco #2 plugs so i went with the regular coppers. (rs44t or something like that) Ran so much better but still does it. Balanced the throttlebodies and the idle became perfect but it still does it....

Now at this point the car behaves good. Linear throttleresponse and get this, it doesn't break up every time. I can take off from a light and max out first and it is good then shift second and then it hits 2500 and starts doing it. then short shift third and bury it again and it is fine. Other times I can row straight through third without missing a beat. I am literally losing it now.

To recap in a list what was done for reference:
-cap and rotor (old age mainly)
-coil
-plugs and -O2 sensor
-balanced throttle bodies
-adjusted and checked throttle position sensor
-fuel pump and pressure adjusted
-Coolant temp sensor replaced
-Checked for vacuum leaks
-Firing order (just in case I messed up the 4 times I checked.)
-I may be forgetting more, it has been a month since I really tinkered with it or even drove it because this is getting annoying.

Now I know the throttle bodies are a little worn where the shaft goes through but I got them to hold balancepretty well. But if I am just mashing the skinny pedal a couple thousandths of difference between the two blades really shouldn't make a difference like this. I am at the point where I don't know what else to check. Maybe the pick up for the distributor maybe the ecm I really don't know what else to do. I am out of things I am willing to parts change as nothing else tests questionable in my opinion. The car runs perfect aside from the break up and a lot of times you can back off and then power through or just power through. It is really strange and no one can seem to figure out what the deal is. Any help is much appreciated and I will be getting my win aldl back soon so I can run some tests on that as well.

Hopefully one of you guys on the third gen forum can be a big help and help resolve this issue with us.

Original corvette forum thread: https://www.corvetteforum.com/forums...2500-rpms.html

5500 as long as you hammer it. If you take it easy that's when it happens.

Distributor has a new mag pickup, condenser, gear endplay shimed. Everything on the distributor is new except for the housing and shaft. No measurable play side to side.

You can hear the “burbleyness” in this video.

BLM stays around 120 when you're really on it.

O2 readout stays a relatively smooth sinewave with the tip in flatlines you'd expect when you first stomp it.

TPS was replaced due to a grinding noise the old one had, no change. Good voltage sweep.

No tcc. It's a four speed.

ECM doesn't update as fast as modern ones so it only updates a few times a second. but even when it acts up I don't see anything that is glaringly obvious. The resolution may not be enough to catch it but o2 readings still switch like you'd expect while accelerating, map readout stays steady, blm remains constant, tps holds constant voltage if the pedal isn't moving.

I don't really think I'm good enough to power brake a manual...

Egr seems to work. Car runs worse with it's vacuum line plugged vs when it is connected. If you tap the underside of the valve on the diaphragm while it's idling it'll drop the idle.

I ran timing anywhere between 6 and 14 degrees. IIRC more narrowed the band it seems to occur in a little but overall made no difference. Car feels like it has more power at 12 vs 6 though so that is where I've left it.

Knock sensor disconnected made no difference as well.

I guess it's not getting too much EGR.

I've been there since August man lol.

If you read through the corvette forum post you can see pretty much every little thing I've tried. I'm thinking it could be the ECM at this point. I don't know if wide open goes to solely a written table for acceleration and ignores some other parameters at that tps reading. If it does it, usually hammering the throttle can clear it or sometimes it just ram effects like it's on a rev limiter. It's messed up and it ran sweet before hand too.