Quadrajet transplant

[trevbeadle]

I'm about to attempt a carb transplant on my 350 Mexican Goodwrench engined Corvette. The current carb is, I'm told, a mid eighties ECM contolled truck carb with no adjustments. The car as far as I am aware does not have ECM! Can someone answer the following questions?


What is the vacuum diaphragm on the right of the pic on the existing car? What did it do/should it do/ does it need to do? it looks like a vacuum pulloff that should have a linkage to the vacant post on the throttle lever.

the rear vacuum pulloff on the choke is not present on the replacement carb. Do most Qjets have 2 or is this an ECM abberation? Is one sufficient and normal?.

I think the vacuum port on the right will be redundant as there is and electrical connection in that position on the old carb. Do I just cap it off with a rubber piece/bolt or should it connect to something?

Likewise, this top port seems redundant?



Grateful for any advice or additions. Thanks.

 

[trevbeadle]

The old carb has an electrical commector (top right). What does this do?...not on replacement carb

 

[Oneball]

It’s the plug that connects to the computer on the computer controlled quadrajets. Came in on GM cars at the beginning of the 1980s

 

[trevbeadle]

It’s the plug that connects to the computer on the computer controlled quadrajets. Came in on GM cars at the beginning of the 1980s

How would I know if the car had ECM? Car was a 79/80 but rebuilt from salvage mid 80s, new chassis and perhaps new loom? There are certainly connected plugs on carb!

 

[johng]

The vacuum actuator in your first photo is for cruise control.

This diagram shows the vacuum ports I have on my 78, not sure if an 80 is the same.

 

[trevbeadle]

It’s the plug that connects to the computer on the computer controlled quadrajets. Came in on GM cars at the beginning of the 1980s

Where would the computer be located if one WERE fitted?

 

[Oneball]

Where would the computer be located if one WERE fitted?

I think it’s in the bulkhead but without the exhaust, temp sensor etc and appropriate dissy and carb it’s useless.

The carb you’ve got will work fine. It’s designed to meter the fuel without input from an ECU.

 

[trevbeadle]

I think it’s in the bulkhead but without the exhaust, temp sensor etc and appropriate dissy and carb it’s useless.

The carb you’ve got will work fine. It’s designed to meter the fuel without input from an ECU.

The new one......hope so!!

 

[Oneball]

The new one......hope so!!

Yep new one

 

[trevbeadle]

Now that I (we all) seem to have more time at home due to Covid-19, I think I have fathomed the vacuum connections/ports on both the existing and new carb. If someone can confirm what I think it would make me a tad more comfortable in tackling this swap job myself.
Firstly the old carb: The diagram that Johng provided fits pretty well EXCEPT that the EGR hose to the TVS and EGR valve has had a tee fitted which goes to the distributor! Shortly after that T is another that takes vacuum around and through the bulkhead into behind the dash (heater??)
Secondly, in the larger hose that that goes from a large port to the PCV valve has been teed which takes a small hose to the canister in the engine bay inside the left wing.
There is also a feed to the canister from a large port.
Lastly there is only one vacuum port on the back (low down) on the existing carb that goes to a secondary choke brake. No connections to the air cleaner although it does have 2 diaphragms and temp sensor.


Now, referring to the new carb photo I think connections should be, on the front from pax to drivers side , left to right, EGR; EFE; PCV (large low); Canister (large high): distributor. Total of 5 ports.



Ports on the rear just 2, top air cleaner snorkel intake flaps and bottom secondary chock brake (which I haven't got on new carb, so I will cap off).

 

[johng]

Trev, your plan for the front of your new carb is how I've set mine up (but as I'm probably a few months away from firing up the engine, I can't guarantee it's right!). At the rear of the carb, I've got the choke connected to the top port and the air cleaner was at the bottom, although I've capped that off now.

Not sure which carbon cannister you've got, as there are a few different ones, but if you have the one with 2 valves on top then this diagram might be useful as well.

 

[trevbeadle]

Trev, your plan for the front of your new carb is how I've set mine up (but as I'm probably a few months away from firing up the engine, I can't guarantee it's right!). At the rear of the carb, I've got the choke connected to the top port and the air cleaner was at the bottom, although I've capped that off now.

Not sure which carbon cannister you've got, as there are a few different ones, but if you have the one with 2 valves on top then this diagram might be useful as well.

View attachment 4649

How many choke pulloffs do you have? There is only one on this new carb and pre-plumbed in! Ported vacuum comes from the bottom of the carb, yeah, and is not present at idle? As opposed to manifold vacuum which is?

 

[johng]

You can see my setup in this photo, the choke connection on the back of the carb was just about to be pushed on when I took this. With the solid pipe there's no way it could attach to the bottom port