Hey guys,

I have an 86 Z-28 Iroc that had the LG4 in it from the factory, it's since been swapped out to a 350, however, the shop that did the swap used the factory carb, ecu, etc when they did the swap. I get a code thrown that it's running rich, it falls on it's face at WOT after a few seconds, but otherwise runs excellent, starts everytime, warm or cold, and drives fine under about 3/4ths throttle. I'm guessing it's a carb tuning issue, however, I've never worked on a computer controlled carb, any suggestions?

I have worked on the non-computer controlled version of the Quadrajet, I have one in my truck I recently rebuilt, so I'm pretty familiar with that version. I just don't want to start messing with it and make it run worse.

 

You don't really have to worry about the computer-controlled part, at WOT the computer doesn't do anything and it kind of regresses back to being a non-computer carb.

Code 45 is the only code you're getting?

 

I'm getting a code for the VSS too, but those are the only two, 45 and 24.

 

I've noticed lately that if it sits for around 15mins-1.5 hours and I start it back up and drive, it runs really crappy for a few miles, then clears right up. I still haven't adjusted the carb, so I'm going to try that tonight, and take it for a nice long drive, and see how it runs. I'll report back if it doesn't clear up the issue and see if anyone has any ideas.

 

I could be wrong since I've never worked on CC's but with the work that I've done on the non-CC's and EFI that really sounds like a carb that isn't tuned right for the motor it is sitting on. The tuning in the ECU still thinks it is the LG4 rather than the 350 that it sits on now. I personally would try to do what adjustments you can and then if that fails to work, move one way or the other, full cave-man carb or full-EFI.

 

That's really not a problem for carbed setups.

 

Sounds like it's just starved for fuel at WOT, maybe need to adjust spring tension on the air valve, and/or swap out the secondary metering rods for something a little richer.

 

I hadn't had time to look at this really for a while, and just got a chance on my lunch break today to dig in. Found several very large vacuum leaks, appears the engine swap wasn't done as well as I was lead on to believe. I'll be changing plugs, wires, cap, and rotor this weekend, and hope to have her the rest of the way sorted after I look a bit more. I adjusted teh carb, but it was about dead on, although, with those vacuum leaks, probably need to retune it now.

 

I found that the carb isn't opening the secondary butterflies. The vacuum lockout is keeping them shut even once the car is warmed up. I have basically the same carb on my truck, so I'm going to look at it's setup, and see if I can figure out what they installed backwards on the Camaro. I rebuilt the trucks carb not that long ago, and it was pretty easy. THe only difference is teh truck isn't a computer controlled carb. I also have a plugged up cat, and had some other issues, most of that stuff is fixed, cat gets replaced tomorrow, going to swap out my o2 and vss next week sometime, once I find a friend that will let me do it in their driveway, shops full, and I don't want to lay in the dirt and do my vss, did that to do plugs the other day...

 

Wait, wait, wait! We need to explain some first principles here.

The "vacuum break", commonly known as the choke pull-off, is doing exactly what it is supposed to do. It needs to hold the secondary air valve closed until vacuum drops. You won't see that happen looking at it under the hood. On a chassis dyno, perhaps, but it ain't gonna happen in the driveway. What you should check is with the engine off (no vacuum), open the air valve by hand (don't need to open the throttle to do this) and make sure the secondary rod hanger is pulling the rods up as you open the AV. There have been reports of the plastic cam that does this breaking and not pulling the rods up.

However, I agree with the fuel delivery suggestions, usually either the fuel pump and/or fuel filter. And, you need to know whether or not you've got an in-tank electric pump - some of these cars had them added by the dealer to resolve customer vapor lock complaints, and they seem to be failure prone. If there is an in-tank electric and it isn't operating, it'll cause exactly what you're describing.

FWIW, the first issue I had with my stock LG4 after it was swapped into my '82 was the fuel pump - acted exactly as you described what yours is doing. New mechanical fuel pump (mine doesn't have the in-tank electric) and the problem went away and never came back in the 10 years since.

 

I have a mechanical, looks like it's easy enough to get to. I have a pressure testing tool, what's the necessary fuel pressure on this car? 8psi? Or there abouts?

 

Around 6.5, or less. Too much pressure is a bad thing.

 

If anyone else is following this, it's actually 9-13psi according to my shop manual for this car. It's at the exhaust shop getting a cat on her today, so I haven't had a chance to test, hopefully tonight after work.

 

I'm following, but I'm not sure you are.

The shop manual is either talking about the TBI fuel pressure, or it's wrong.

What you want is 4-7 psi at the carb. That's what the typical factory-type mechanical fuel pump puts out.

You said in your first post you have a non-CC q-jet. That is NOT mechanical secondary. The secondary throttle blades are open mechanically by the throttle arm, true, but air flow is controlled by the air valves at the top of the secondaries. Therefore, it is not considered a "mechanical secondary" carb. There is a mechanical lock-out for the secondary throttle blades which engages when the choke is on, but you said your secondaries were being held closed by vacuum, which is the air valves like I said already.

 

In my first post I said I have a non cc version on my truck I've rebuilt, cars is a cc'd one. I checked my book again, it must be wrong, it shows 9-13 psi for h code engines. I'll check it tonight, see what I'm actually getting. The only different psi it gives is for tpi engines close to 50 psi.

 

Fuel pump seemed fine if the numbers from five7kid are right. It was at 7.5. With the new cat it doesn't hesitate as badly, but still does right near WOT, I took it up to around 3500 and floored it too, as a test, still bogged down. Also, when I said I have a mechanical up there, I meant fuel pump. Sorry if that was confusing. I'm almost thinking my issue is secondary meetering rod size. But, I'm still not 100%.

Is there a way to absolutely test the secondaries function without putting it on a dyno and going WOT in gear? I guess I haven't ever had issues with the secondaries on any of my carbs, so I don't really know how that portion operates.