Tim305

I am swapping out my 2bbl on my 1969 cutlass 350 for a 4bbl. The 4bbl I have according to the numbers is a 1973 quadrajet. What are the differences between a 73 and a 69 quadrajet, and will it work? I know it has an electric choke and I suspect it has more vacuum lines.

billmerbach

This probably isn't even true so don't quote me 69 at least to me had all the good things carwize so imo 69 is netter. Now on the other hand as far as more vacuum lines thryccsn always be plugged up not a problem there

MDchanic

There are specification tables showing jet sizes, etc. for various carburetors, and I don't have one in front of me now, so I can't get specific, but, in general:

GM cars used EGR starting in 1973. EGR injects exhaust gasses (up to about 15% by volume, depending on conditions) into the intake.
EGR is designed as an element in reducing exhaust emissions.
In 1973, the regulated exhaust gas components were CO, NOx, and hydrocarbons (as well as lead and evaporation), if I am not mistaken.
The leaner you run an engine, the less CO and hydrocarbons you will produce, but running an engine lean will increase combustion temperatures, which will tend to produce oxides of nitrogen (NOx).
In order to be able to run lean, thus reducing CO and hydrocarbons, but not create extra NOx, two things were done:
One was to reduce compression ratios, as increased compression increases combustion chamber temperatures.
The other was to introduce EGR.
By adding an inert gas (exhaust gas, which has already been burned) into the mixture, the mixture was cooled slightly, and NOx were reduced.
The EGR doesn't affect the mixture itself (being inert), but displaces the oxygen and fuel, in essence reducing the compression ratio or the displacement of the engine (depending on how you look at it).

Because EGR is used to allow leaner mixtures, the carburetors used with EGR are jetted leaner, and, in fact, may be jetted lean enough that they could cause some detonation under hot conditions if used without EGR.

Also, since Emission testing at the time was done at idle and cruising only, most of these carburetors have power valves that kick in at relatively high vacuum levels (slight throttle openings), enriching the mixture more than you necessarily need when you step on the gas.

While all of these things can be modified, it would seem easier to just use an earlier carburetor that was designed for use without EGR, as they are neither hard to come by nor expensive.

- Eric

Seff

So if I jet the 77 carb on my 72 350 too lean, and/or set the adjustable part throttle too lean, I'll end up making some ghastly emissions?

MDchanic

No worse than your lawn mower.

- Eric

Tim305

Thanks that answer my question. I'll just get the right carb.

billmerbach

I was way wrong dang lol

Seff

I don't drive my lawnmower 8000 miles a year. :P

MDchanic

No. I mean using your lawn mower for a few hours every 2 weeks makes more than driving your car 8,000 miles a year.

- Eric

Seff

Oh dear, I seem to have infected you with a case of the smileys. D:

But, that sets my mind at ease. And gives me ammo for when the environmentally conscious people give me crap for driving an old, smelly car.

Pardon the thread derailment.

MDchanic

Screw them.

Tell them if they don't like it, they can move to Europe.
Um... Er... Or something like that.

- Eric

Seff

It's generally accepted that I'm crazy for driving a collector's piece every day. Their loss.