Exciting day - rebuild of 78 coupe

[johng]

It's a quadrajet Ross, pretty sure it is the original carb but someone has rebuilt it in the past with the wrong length rods (from an earlier generation carb) and wrong size jets. I've put it back to stock but obviously something is still not right. I'm going to go hunting for a vacuum leak this morning.

 

[johng]

So I've plugged each of the vacuum ports in turn and it seems I have a problem with the pcv valve. With that disconnected I get a more stable tickover and an extra 2 in hg vacuum. By the way Andy my plugs were white, they are too hot to remove at the moment.

 

[Oneball]

An exhaust leak where it draws in air will give you a false lean afr reading.

 

[Oneball]

So I've plugged each of the vacuum ports in turn and it seems I have a problem with the pcv valve. With that disconnected I get a more stable tickover and an extra 2 in hg vacuum. By the way Andy my plugs were white, they are too hot to remove at the moment.

That’s not necessarily a sign that the pcv is bad. It maybe you’ve not correctly tuned the carb to take account of the small vacuum leak the pcv is. You need to test the pcv to ensure it closes as required.

 

[johng]

Thanks Tim, the pcv doesn't shut completely, you can suck through it all the time but it does flow more when you compress the spring

 

[Roscobbc]

That’s not necessarily a sign that the pcv is bad. It maybe you’ve not correctly tuned the carb to take account of the small vacuum leak the pcv is. You need to test the pcv to ensure it closes as required.

Is the PCV connected directly to the inlet manifold (either in a rocker cap or in-line).......or to a port on the carb?

 

[johng]

I still can't seem to get the idle mixture too rich, after about 4 turns out it makes no difference.

 

[johng]

It is connected to the carb

 

[johng]

I wonder if the egr is stuck open

 

[Oneball]

Does it die if you screw the mixture in?

 

[johng]

Yes

 

[Oneball]

Vacuum leak I’d guess. If you take the filler cap off with the engine running and the revs rise significantly it could be the pcv not limiting flow correctly. Otherwise maybe intake manifold leak.

 

[Roscobbc]

It is connected to the carb

Are there other 'ports' on the carb the PCV line can be connected to John?

 

[johng]

I've sort of done that, if I set the tickover with the pcv removed then reconnect the pcv the tickover drops and becomes less stable

 

[Roscobbc]

Shame you can't 'borrow' a 'known' correctly working Quadrajet to check things out?

 

[Oneball]

I've sort of done that, if I set the tickover with the pcv removed then reconnect the pcv the tickover drops and becomes less stable

That’s kinda what I’d expect of a working pcv. If it wasn’t working I’d expect reconnecting the pcv to cause the rpm to rise significantly or completely die as it’d be getting a lot more air than it should.

 

[Oneball]

Can you get it to run rich with the PCV disconnected?

 

[johng]

No I can't get it to run too rich at all, I think reconnecting the pcv makes it run even leaner, hence the revs drop. Should the pcv shut off completely with no vacuum or pressure applied?

 

[Oneball]

No I can't get it to run too rich at all, I think reconnecting the pcv makes it run even leaner, hence the revs drop. Should the pcv shut off completely with no vacuum or pressure applied?

No

Pcv does two things:
It limits total flow from the crankcase into the carb to a set amount. (It should in theoretical flow the same amount connected to the crankcase and open to air)

It prevents blowback from the carb entering the crankcase (only flows one way)

 

[johng]

Mine doesn't prevent blow back you can blow or suck through it