500 CID Carb Issue

[dirtywaterdiver]

So I have a 1968 De ville with a '74 500 in it.  It originally had an Edelbrock Carb and the stock intake is modified with an open plenum and by way of the exhaust crossovers being welded shut at the point where it meets the heads.

It was not an issue with the Edelbrock but now that I have returned it to a Q-Jet the carb seems run cold and I have to keep managing the throttle so it will not stall.

After a while that huge cast iron intake finally gets hot and the car runs fine.  In fact, like my CI pans, it stays hot for a long time and the car will start up and run fine even after sitting at a car show for hours.  It is only a PITA when it is cold.

This Carb has an electric choke.   I took the Edelbrock off because of clearance problems, it is not going back on.

So any suggestions?  Time for new intake?

[Dave Shepherd]

I would try enriching the choke somewhat and see if it improves cold performance,  the cold intake may not be vaporizing the fuel enough, so it is possible with a stock QJ  this may only be corrected with the heat risers opened up.

[dirtywaterdiver]

Did, not much help at all.   

[chrisntam]

When you say it runs bad when it's cold, does that mean when it's cold outside it runs bad until the car is warmed up or it runs bad even when it's warm outside.

My intake is the same as yours, plenum opened up and exhaust crossover filled in.

In the summer in Dallas, my car runs fine when the car is cold.

In the winter, when it's 40 degrees outside, my car stumbles when cold, but once warmed up runs fine.

I think it's purely a choke issue.

I have a '70 500 w/ a '76 800 cfm quadrajet w/ electric choke.  I'll fiddle with it this winter...

[dirtywaterdiver]

When the engine is cold, not run that day.  Ambient temperature does not effect it.

Even when the engine gets up to normal water temp via the Stuart Warner gauge after idling for a while in it runs and stumbles.  But after about 15 to 20 minutes of road driving it goes away.

Is your cross over filled in up at the carb where the two holes are?  This one is welded closed at each end at the intake gasket end.  No hot exhaust passing through under at all.

[Scot Minesinger]

Well does the plugged (welded shut) exhaust gas crossover at least make real hot day starts after engine has run a half hour or so and shut off for a half hour easier (does not crank as much)?  If not might as well have the exhaust cross over and make the cold running better, as otherwise what is the point of the plugged exhaust cross overs?  If so I guess that is the trade off. 

Choke adjustment seems to be the way to go.

[savemy67]

Hello dirtywaterdiver,

I tend to like original components, so my vote would be that you replace the modified manifold with an unmodified manifold.  My basis for this recommendation is that a lot of engineering went into designing the original parts, and Cadillac wanted to have a reasonable expectation that their dealers would not be swamped with problematic cars.

Many car owners tend to think that engines get hot (which they do), but I have seen stock intake manifolds on running cars get cold enough to chill beer.  The idle and off-idle circuits on a Quadrajet are sensitive to small changes including changes in temperature.  If the exhaust cross-over passage was needed only for warm-up purposes, Cadillac would have avoided the expense of casting the manifold and heads with a cross-over passage, and come up with a cheaper solution.  The exhaust cross-over passage is a permanent design feature of the engine because it is needed all the time to prevent condensation/separation of the air/fuel mixture in the venturis, and throttle body section of the carburetor.

Despite the fact that the cast iron manifold gets warm eventually, the airflow through the venturis, throttle body, and through the manifold is of such a volume and velocity that under certain conditions enough heat can be carried away to render the surfaces relatively cold - cold enough to upset the air/fuel mix when the fuel condenses out of the airstream.

There may be other causes of your carb issue, and there may be other solutions.  However, if you replace the manifold and adjust/rebuild the Quadrajet correctly, you will be well on your way to enjoying one of the best engines ever manufactured.

Respectfully submitted,
Christopher Winter

[Dave Shepherd]

Well stated.

[chrisntam]

I tend to like original components, so my vote would be that you replace the modified manifold with an unmodified manifold.  My basis for this recommendation is that a lot of engineering went into designing the original parts, and Cadillac wanted to have a reasonable expectation that their dealers would not be swamped with problematic cars.

......snip.

Respectfully submitted,
Christopher Winter

For the 472 & 500 motor, my understanding is that the intake manifold design was forced due to the low hood clearance, car design won over engineering.

Doesn't some or all of the fuel flow at some point, go up?    It would have been nice had Cadillac hung that ol' A6 to the side so the engine designers could have worked their magic on that intake.  Never seen an intake as flat as that one.

The engine is an air pump, get air in, get air out as fast as possible, that is done with free (as much as possible) flowing manifolds.

["Cadillac Kid" Greg Surfas 15364]

Airflow through the intake system irregardless of its unorthodox (to those unfamiliar with Cadillac engineering) appearance was 100% purposeful to ensure proper low
Speed velocity and liquid fuel entrainment. That stated simply means thAt if you were to install a high rise single plane intake on a stock motor you would substantially decrease low speed power (torque).
Greg Surfas