Stock 78 Z28 Running Poorly. Help!

[78Z28350]

Might as well put my 2 cents in, check the vacuum line to the distributor. I believe its supposed to be on the passenger front of the carb ABOVE the throttle plate.

Correct. It is connected where it should be. I removed the vacuum line from the distributor to verify the vacuum advance was working and it did function correctly.

 

[78Z28350]

Replace/rebuild the distributor. Make sure to check lobes and replace all springs if you rebuild. Additionally, replace HEI unit and module if so equipped. I did this on my 81Z and It solved the off idle stuttering.

I put a new module in the current distributor, along with a cap and rotor button. I have a spare HEI that I could put in it just to eliminate the entire distributor. May try that tomorrow if time allows.

 

[92RS305]

I also have a ‘78 Camaro Z/28 with the exact same problem. Mine is a 350 with a 4 speed. Very low actual miles of 9,300. I’ve been going through it since getting last December.


So far I’ve found two small fuel leaks (fixed), low compression on one cylinder at 100 while the other 7 have 130-140. Pulled the manifold and the lifter and rocker are both fine so thinking maybe there is a cylinder wall scuff?

I recently removed the fuel tank and the sending unit is so rusty bad it’s being replaced. I took the fuel tank to a radiator shop to get hot tanked and cleaned out. Once it’s done I will get all of that back in place.

I also have both factory mufflers rotted, the pipes going back of the mufflers with holes, and the original catalytic converter has some holes. I have a real nice (like new) catalytic converter, one NOS muffler (need another), and some correct factory exhaust pipes from another Z/28, and will replace all of that.

I have the exact same issue when driving it. I will track your progress.

Hoping I can get mine corrected soon too.

Very nice ride. When I bout my 81 Turbo TA, couldn't find a Z28 anywhere. 1978 was the last of the good ones IMO.

 

[92RS305]

Part 1

Where did you get the module? Who made it. I lost track but sounds like you replaced it. Pull it and have it tested. Accel had problems with them most all regardless of brand, are made in China.

On your first description, my first suspect is the module. It has a low side and high side, and typically it's on the high side that fails. The high speed section controls acceleration and WOT.

Pick up coil... Go bad but hardly ever intermittent. It either works or it doesn't. If pick up is bad it will not start.

Someone mentioned possible crossed wires.... Start it and with foot on brake (assuming you have good brakes) put trans in drive and stab the gas. Does it pop (back fire) or bog down? Yes or No? If no backfire..., wires are good.

Idle screws at base of carb 3 out is too much typically, about 2.5 max is typical with a well driven car. If 3 out is what it needs to idle and stay running... You have a vacuum leak.

 

[73 RS Z28]

Never been a fan of quadrajunk, get a Holley type carb. I prefer QFT. Just my 2-cents. Glad your making progress tho!

 

[mallard]

Part 1

Where did you get the module? Who made it. I lost track but sounds like you replaced it. Pull it and have it tested. Accel had problems with them most all regardless of brand, are made in China.

On your first description, my first suspect is the module. It has a low side and high side, and typically it's on the high side that fails. The high speed section controls acceleration and WOT.

Pick up coil... Go bad but hardly ever intermittent. It either works or it doesn't. If pick up is bad it will not start.

Someone mentioned possible crossed wires.... Start it and with foot on brake (assuming you have good brakes) put trans in drive and stab the gas. Does it pop (back fire) or bog down? Yes or No? If no backfire..., wires are good.

Idle screws at base of carb 3 out is too much typically, about 2.5 max is typical with a well driven car. If 3 out is what it needs to idle and stay running... You have a vacuum leak.

IMHO, the pick up coil (or pole piece) is very commonly intermittent. A frayed or broken wire will cause an intermittent problem, commonly with acceleration due to the action of the distributor advancing.

 

[xten]

Over the winter I went through the same thing. Last fall I bought a 77 Z28 with a 1971 LS 5 BBC. it would stutter and miss at the transition from the primaries to secondary's. Thought it had to be carb, distributor or cam lobe(s). Had the carb apart and found some problems (QFT), thought that was it. Nope. Had distributor apart, nothing there either. Then I remembered the PO told me the car had only been driven 9 miles between 2005 and 2021. It was the fuel. After a can of seafoam and 3 tanks of 93 (and some help from a 5 gal can of leaded 110 race gas) it rips. Blows the tires off. When the carb would get a pump shot and you looked down the carb, the fuel was yellow. I'm surprised it even ran at all on that gas. It was a lot of time and effort, and there was really nothing wrong but the fuel. Guess you can't expect a big block to run on piss...

 

[78Z28350]

Part 1

Where did you get the module? Who made it. I lost track but sounds like you replaced it. Pull it and have it tested. Accel had problems with them most all regardless of brand, are made in China.

On your first description, my first suspect is the module. It has a low side and high side, and typically it's on the high side that fails. The high speed section controls acceleration and WOT.

Pick up coil... Go bad but hardly ever intermittent. It either works or it doesn't. If pick up is bad it will not start.

Someone mentioned possible crossed wires.... Start it and with foot on brake (assuming you have good brakes) put trans in drive and stab the gas. Does it pop (back fire) or bog down? Yes or No? If no backfire..., wires are good.

Idle screws at base of carb 3 out is too much typically, about 2.5 max is typical with a well driven car. If 3 out is what it needs to idle and stay running... You have a vacuum leak.

The module was sourced from O'Reillys. No idea who made it, but it wasn't Accel. It's a stock type replacement. I'll have to do the check for crossed wires at lunch today.

 

[mallard]

If there's crossed wires, you should experience a rather prominent missfire even at idle.

 

[73chevylt]

Just throwing this out there, I’ve had a similar issue after having my Hei module go out during a very hot summer while in heavy traffic,after figuring out the issue of why I had no spark going through I smelt a burn smell from the distributor and traced it down to the module, I bought a stock replacement from i want to say was a 78 camaro at oriellys, granted my distributor was more of a performance one though the car only had a small cam and headers, i had issues with my car stumbling under any acceleration under load, ended up burning it up, went for a replacement and was having the same issue, luckily a buddy of mine had a box of distributor parts laying that i was able to test around one being a module that after I swapped the module and the car worked perfect. Either I just had bad luck getting faulty modules but I purchased a different higher quality module after and never had the issue again.