70 Z28 Under Revving Engine

[RMTZ28]

As long as you know what's what, so nobody rips you off. Have you set your valve lash with it running? Its not that hard and you may find your problem, or at least eliminate one.

 

[COPO]

The carb is a Holley 4555, original to the car.

If you have the stock fuel filters then they need to go. PS, I have you beat, I’m 66 and still wrench on my cars. If you were local I’d love to wrench on it and figure out the problem 1 step at a time. A fuel pressure gauge would sure let you know if that fuel pump is failing and a timing light on each wire would let you know if all cyl are firing. Have you ever opened up the throttle by hand including the secondaries and looked down the intake, to see if someone left a rag down in the hole or blocked with duct tape? Yes sir those are true stories.

 

[biker]

To all that have responded, took the car out today for a short drive, and things are getting worse, now down to around 3500 to 4000 rpm when it hits a wall. What’s weird is the the engine sounds and runs great at low rpm’s, then feels like it’s going to come apart. Broken hearted, will probably wind up taking it to a reputable engine builder to figure out what’s going on. Don’t want to cause any unnecessary damage. As a teenager I would work on my cars including my new 1970 Z28 I purchased when I was 17. I purchased this car at 65 years old, wanted to have another one before I get to old. So unfortunately I am not comfortable trying to work on the car, it was not cheap.

Hey Tony, cool old picture.
Don't despair. We all go through episodes like this with these cars, and the first thing we all jump to is " well that's it, the crank must be broken in half". If this is a recently rebuilt engine and was built and broken in as u describe, plus the fact that it runs so well at low rpm, I would guess like everyone else that it is most likely an ignition or fuel issue. Spark is a funny thing. A weak spark can fire a low rpm low pressure mixture and start/ run just fine. At higher rpm, a strong spark is more important to light off a mixture under higher combustion chamber pressure and with more advance needed.
DO NOT discount the crappy ignition wire advice here! Spend 100 bucks, take an hour, and carefully replace each wire, including the coil wire. Make sure each boot is lined, each terminal clicks on and replace your cap and rotor while u are at it. My car ran great to 5000 a few years ago and would intermittently poop the bed. Wires were all good. Popped the cap off, and half the rotor was sitting on top of the advance mechanism. And it ran! Weak fuel pump or fuel restriction will do the same thing.
If you aren't comfortable doing that, take it back to the builder if that is an option. Find a good hot rod shop or a member on this board with the skills to walk through proper diagnosis. Start simple, ignition and fuel. Then valve lash, compression and so on. I'm betting you wont need to go that far though.

Edit*: when I say 100 bucks, that is Canadian.... good plug wires in the States are probably way less.

 

[Richard Schultz]

Double check your cam specs,alot of times engine shops put in the cam they want,not what you want.

 

[biker]

Double check your cam specs,alot of times engine shops put in the cam they want,not what you want.

Agreed, but if the condition is getting worse, there is something else deteriorating quickly. And since it still runs perfect at low rpm, I'm thinking (like others) ignition or fuel.

 

[maguyvr]

I agree with whoever said the cam may be going flat. It will run good at low RPM but not at higher RPM's. Obviously you should check for a fuel supply issue first though.

 

[Larry Holmes]

A friend of mine had an issue like this and it turned out that his new hi tech distributor and ignition system had a rev limiter that was factory pre-set at 5000 rpm. He had just installed it without checking this. Once we figure this out he just raised the setting and the engine ran fine after that. If its all new this might be something to check.

 

[mallard]

Rotisserie restored? Does it have the factory smog pump and A.I.R. tubes? If so, you're lucky it turns 5500. The factory cast iron exhaust manifolds are far from idea for a 6500 rpm engine anyway, and with that smog system pumping air into the manifolds, where is the exhaust supposed to go? The A.I.R. systems effectively choke a high rpm engine.

Hence why back in the day we removed those systems and tossed them in the garbage.

 

[450bench]

Sounds like valve springs to me.

 

[Tony P]

Hey Tony, cool old picture.
Don't despair. We all go through episodes like this with these cars, and the first thing we all jump to is " well that's it, the crank must be broken in half". If this is a recently rebuilt engine and was built and broken in as u describe, plus the fact that it runs so well at low rpm, I would guess like everyone else that it is most likely an ignition or fuel issue. Spark is a funny thing. A weak spark can fire a low rpm low pressure mixture and start/ run just fine. At higher rpm, a strong spark is more important to light off a mixture under higher combustion chamber pressure and with more advance needed.
DO NOT discount the crappy ignition wire advice here! Spend 100 bucks, take an hour, and carefully replace each wire, including the coil wire. Make sure each boot is lined, each terminal clicks on and replace your cap and rotor while u are at it. My car ran great to 5000 a few years ago and would intermittently poop the bed. Wires were all good. Popped the cap off, and half the rotor was sitting on top of the advance mechanism. And it ran! Weak fuel pump or fuel restriction will do the same thing.
If you aren't comfortable doing that, take it back to the builder if that is an option. Find a good hot rod shop or a member on this board with the skills to walk through proper diagnosis. Start simple, ignition and fuel. Then valve lash, compression and so on. I'm betting you wont need to go that far though.

Edit*: when I say 100 bucks, that is Canadian.... good plug wires in the States are probably way less.

When the distributor was installed so we're new MSD spark plug wires. Took it out for a drive yesterday and it's worse, is at around 3500 rpm it feels like it's going to fly apart. I believe something is going on internally, cam shaft, etc. Going to have it towed to the best engine machine shop in Tucson and have them figure out what's napping and fix it. Anybody reading this in Tucson, AZ??