Over-carbing an engine

[Stallion]

What happens when you put setup of carb(s) on your engine where there is too much volume and air flow? What exactly happens when you "over-carb" and engine?

Thanks!

Stallion

 

[MaineShark]

Re: Get your mechanic's license here....

For a gasoline engine, the perfect air to fuel ratio is 15:1 (15 parts air to 1 part fuel by weight).

14.7:1

That's the "perfect ratio" (ever bit of air matches up to a bit of fuel, so there is no unused air or unburned fuel). However, peak power tends to be closer to 12.5:1.

Stallion, basically, the issue is definitely one of getting the correct air:fuel ratio. Carbs only really meter the fuel correctly when they are flowing near their peak flowrate. That's why we have "primaries" and "secondaries." The primary bores are smaller, so they are flowing closer to their peak, in low-throttle situations. Then, if everything is set up right, the secondaries will open just as the primaries are reaching their peak, but before they become a restriction, so that the engine now has new, larger bores to flow the extra air and fuel.

Having a carb that is too large, means that the area where it is near its peak airflow (and, therefore, metering fuel most accurately) is smaller. So, when you're at idle, the mixture can vary greatly, resulting in poor idle. And, since most "in-town" driving is done at part-throttle, it is also subject to that improper mixture.

The air-valve secondary used by the Q-Jet eliminates much of that problem, since it meters based on the force of the air entering, not merely its velocity through a venturi. It's much harder to over-carb with a Q-Jet than with other carbs, but if you were to stick a big ol' 800-CFM monster Q-Jet on a tiny 4-cylinder, you could manage it. If you stuck that same carb on your Shark's engine, you'd probably not notice the difference, in msot driving.

Joe

 

[Stallion]

But say you "over-carb" an engine, and you're pumping too much air. Instead of getting a lower CFM carb, couldn't you just re-adjust the choke?

 

[MaineShark]

But say you "over-carb" an engine, and you're pumping too much air.

There's no pumping involved. The engine sucks air through the carb. In order for the carb to meter fuel, it uses a venturi - as the air flows through the venturi, it speeds up and creates a low-pressure area. The carb compares that pressure to the pressure outside the venturi, to determine how much air is being drawn in, and then administers the correct amount of fuel. Over-carbing simply means that the carb is too big to create enough of a pressure differential. It's not that the carb is letting in too much air - it's just that the carb is too big to get a good reading.

Think of it like a foot-long stick. You can use it to measure things that are longer than one foot, and even estimate things that are smaller. But the smaller the size of the thing you are trying to measure, the less accurate you're going to be. Then do the same with a six-inch stick, and a one-inch stick. You'll be much more accurate with the smaller stick.

Instead of getting a lower CFM carb, couldn't you just re-adjust the choke?

The choke is only active when the engine is cold. The issue is the size of the venturis, though, not the airflow.

Joe

 

[JohnZ]

The differential between the low-pressure area in the carb's venturi and the atmospheric pressure acting on the fuel in the float bowl determines the strength of the "metering signal" which the fuel flow through the booster venturi responds to; the strength of the metering signal determines how rapidly the carb's metering system responds to changes in airflow and load. The bigger the venturi (at constant airflow), the weaker the metering signal is, and vice versa. That's why an engine that's "over-carbed" (has more venturi area than it needs) has poor throttle response and doesn't meter fuel accurately under normal street driving conditions.