I found a few threads that are similar to my problem but not the same. My girlfriend has (she bought the car New) an 83 Z28 with a carb 305 and a 5sp manual and the motor is Bone stock. when you hit 4000 RPM's you can hear the motor bog like it's starving for air or gas. it runs GREAT up to that point. the motor has 159,000 mi on it i rebuilt the carb, new plugs, wires, cap, rotor, ignition module, and all vac lines. I read on 1 post that it might be the valve springs starting to float. But after that post the guy said he was going to do a valve job and he never posted again. that was 9 years ago. Another guy had a post where he said it might be the magnetic pick up in the dist. But again no more post after that so i don't know if these suggestions fixed the problems. Any help would be Greatly Appreciated.

 

Sounds like it's running out of fuel.

Changed the fuel filter lately?

Start with the simple stuff first.

 

I changed the filter when i rebuilt the carb. there was rust in the tank so i replaced it along with the sock filter in the tank and added a filter back by the tank also. Replaced the fuel pump last year. I really don't think it has anything to do with fuel. I had the same problem before i changed the gas tank and everything is new now and same problem.

 

Could it be the catalytic converter?

 

I have heard of rubber bellows collapsing and air filters getting wet and old and collapsing when enough suction hits them.

 

"and added a filter back by the tank also"

Take it out.

To test for fuel delivery issues you hook up a fuel pressure gague THAT YOU CAN ACTUALLY SEE WHILE YOU ARE DRIVING. It's the only way to know for sure what's happening at WOT under load at high RPMs.

 

Take it out.

To test for fuel delivery issues you hook up a fuel pressure gague THAT YOU CAN ACTUALLY SEE WHILE YOU ARE DRIVING. It's the only way to know for sure what's happening at WOT under load at high RPMs.[/QUOTE]

I put the filter in the back because the tank had rust in it. I'll take it out and get a fuel gage and see how that goes.

 

This is old but it sounds like you do infact need stiffer springs. I had my 383 cutting out like that and that's what it ended up being valve float.

 

Sounds like a plugged cat

 

I wish people would come back on the forum and give updates on what the problem turned out to be. This was an odd case and would have been nice to find out the fix.

 

I agree. We will probably never know since the OP's last post in this thread was the last time he was logged into the forum.
I placed my bets on air restriction (not necessarily the filter though). Lots of other good ideas were offered though.

 

That's the first thing I also thought of.

Bad ignition/distributor or fuel supply could also be the problem. Each needs to be tested to pinpoint the actual source of the problem. Being a keyboard mechanic, we can only guess.