So gonna pick yualls brains a bit.

What does the fuel air mixture screw on the carb do? In lamest terms. I researched it a bit and didnt understand what i was reading.

I figured out how the fast idle screw and curb idle screw worked by watching the levers and throttle plate, but i dont get the fuel air misture screw.

 

Also does ignition timing have anything to do with the vacumm system. Again in lamest terms. thank you very much

 

The fuel air mixture screw adjusts the fuel/air ratio at idle and light loads. If you turn it in (clockwise) it will lean out the mixture. If you turn it out (counter-clockwise) it will enrich the mixture.

Ignition timing and manifold vacuum are related. In its absolute simplest terms, manifold vacuum is measured to sense engine load and the distributor will vary the timing dependent on the amount of vacuum present. Factors like ported vs straight manifold vacuum signal, TVS and PVS control valves, etc further complicate and refine the amount of spark advance regulated.

 

The engine cannot run on pure gas. It needs to be in a vapor form. This is why you can stick a lit match directly into a puddle of gasoline and all it will do is go out. It's the vapor that's floating above the gasoline that is combustible. As such, the engine needs gasoline mixed with air, so that the gasoline is in vapor form. This lets it explode.

As air moves through the carburetor, the carburetor lets a metered amount of fuel in and then mixes it with the air. There is a very specific ratio of air to fuel where an engine gets the best explosion in the cylinder. The ratio is around 14.7 times as much air as there is gasoline. The term for this is stoichiometric, or "stoich".

Your goal with a carburetor is to meter the amount of fuel it releases into the air stream to try to hit this "perfect" ratio of fuel to air. If you let in too much gas, it goes rich. If you let in too little, it goes lean. The mixture screw adjusts how much fuel is let in when it's idling, or at low loads (like cruising down the road not accelerating).

Granted, there are times when richer or leaner is better. A ratio of around 12 parts of air to 1 part gasoline (12:1) is richer, and is where you will roughly get the best power when accelerating. A leaner mixture (around 16:1) will get you better gas mileage when cruising on the open road.

Vacuum:

Engine vacuum is caused by the cylinders moving up and down. The 300 has 50 cubic inch cylinders. So, whenever the pistons move down, they're creating 50 cubic inches of space within the cylinder. However, if the throttle isn't open all of the way, the cylinder might not be able to pull in 50 cubic inches of air. It may only be able to pull in, say, 5 - 10 cubic inches of air due to the throttle being mostly closed (imagine sucking in air through a coffee straw). Since there's not enough air to fill the volume of the cylinder, it creates a vacuum.

In simple terms, the higher the vacuum, the more closed the throttle is. If the throttle is 100% open, it's going to let in the maximum amount of air, and you're going to probably get that full 50 cubic inches of air in. Therefore, your vacuum will drop down to almost 0.


Engineers quickly realized that they could gauge how hard the engine was working by how much engine vacuum was present.
Lots of vacuum = very little throttle = very little work load on the engine.
Little to no vacuum = lots of throttle = lots of work load on the engine.

From this, they could make the ignition system change how much timing it's giving the engine depending on the vacuum/work load.

 

New topic: Grounds

When i think of electricity i think of a circle. Does from point A to Point B and back to A. Whats the deal with grounds? At the moment, im trying to fix my taillights. I know for certain that i have bad grounds (different thread), but as im trying to figure out solutions, i realize i dont know how or why grounds exsist or how they work. Thanks for the education

 

In your analogy the ground is the path from B back to A. So if grounds are not there, or restrictively weak due to corrosion or loose connections then not enough current can flow to properly activate your device, be it lights, motors, sensors or whatever because the path back to the electricity source is incomplete. I have purposely avoided using terms like voltage, resistance, and current - you can get a much more comprehensive tutorial of those online..

 

New question.
Can it hurt to have to many grounds?
i put a ground wire from my starter to the fram and lights and dash started acting funny. Take it off and everything is good again. I was goin thru and realized i only have 2 ground wires on my truck, battery to the engine (alternator bracket specifically) and manifold to firewall. I want to add more, but now im spooked. As always educate me. Thank you all

 

Hrmm.... I wouldn't think so. The only way I can see that you'd make yourself an issue is if you had a short somewhere else, and adding the extra ground completed a circuit.

Otherwise, adding extra grounds simply gives the electrical flow better return to the battery to complete the loop.

I have a ground from my battery directly to the fender (2 inches away), one from the negative battery terminal to the frame, from the frame to the starter, and then another from my engine block to the firewall.

 

Funny? story:

I built a car and forgot to add an engine ground. The engine was mounted in rubber. The first time I tried to start it the only path from the starter motor back to the battery was the shifter cable, which promptly melted.

$50 mistake.

 

Hmmm... a "short shifter"!

 

Heh, that cracked me up more than it should have.