Understanding Carbs

Now I posted this question on another forum but thought I would also hunt for some opinions here as well.

Been having a discussion with a mate on the difference float levels make to rich/lean conditions.

Now I've never paid much attention to float levels as I've always seen the floats as a simple fuel delivery device to keep things running.

Ie Set too low and run out of gas at WOT

And set too high, flooding, stains, pool on the floor etc.

Now to the hard part of my comprehension.

How does 1-2 mm make a difference in a carb bowl that is essentially sealed from the carb throats with the exception of the main jet and starter tube.

Now both of these fuel intakes are taken from the bottom 5 mm of the bowl so I don't understand how a few mm on the level can make a difference.

Lifting the fuel level still means the feed has to go through the main/pilot jet orifice so why does it make a difference.

Cheers
HG

 

If the level is a little low the jet meters fuel but can pull so much through that it sucks air. So raising the fuel level up means that no air will be sucked in now. The jet must stay immersed in fuel so that it only meters fuel. This would be the lower portion of the cone or funnel shaped part of the jet. If the level rises above the tip of that cone then fuel pools above it and the jet is no longer metering fuel. You must also remember that as the jet pulls fuel the supply starts to drop until the float opens and supply is refreshed. So the jet must be immersed enough to stay immersed until the supply is refreshed.

 

I don't think that PLUS or MINUS 1 mm or 2 does make a huge difference. The illustration in the Factory Manual says so in print.

But, the speed-is-fun designers know where that optimum level needs to be, to satisfy the engines fuel requirement when you're wide open with your right wrist cranked and your elbow down by your shin ... hanging-on tight, just shy of where your ludicrous ground speed causes you to temporarily lose control of your bladder.

They want you to keep all them fuel-sucking jets submerged in the go-fast motion-lotion so you don't go gettin' pissed-off while you're actually causing yourself to be pissed-on.

It's very highly technical stuff, as you can see!

 

Ok,

Imagine the fuel bowl with the pickup tubes in it. The fuel will rise to the same level in the tubes as it is in the bowl.

The carbs need to "lift" the fuel from that level to the carb throat where it is atomized with the incoming air. The lifting of the fuel is technically called "work". At a given air speed the carbs will do a fixed amount of work/second (power).

If the fuel is lower in the bowl it will be lower in the tubes. The carbs need to do more work to lift the same amount of fuel. Problem is they can't do more work so they lift less fuel resulting in a leaner mixture.

Likewise if the fuel level is higher in the bowl it is higher in the tubes. The carbs need to do less work to lift the same amount of fuel. They will still do the same amount of "work" so they lift more fuel making the mixture richer.

The jets are restrictions introduced into the fuel passages to increase the resistance to the fuel flowing and therefore increase the work needed to lift the fuel. Larger jet, less restriction, less work, more fuel. Smaller jet the opposite effect.

You could in theory do away with carb jets by having really deep fuel bowls with a low level. You could set fuel level to adjust mixture. Nobody wants a 30 foot tall carburetor though.

Gee, aren't you glad you asked?

 

I'm glad someone asked. I've always wondered how this part of the carbs really work.

Makes sense to me.

 

GREAT explanation MiCarl. Now, can you answer this: is the point made by BlueMaxim technically correct? Does uncovering the actual jets affect the amount of work done by a carb? It would seem to me that under all sorts of different conditions, i.e. hard accel or braking, hard cornering, etc. that some of the main jets would be uncovered at a great many times. It just seems that keeping the fuel levels exactly at a point where the cones of the jets....and only the cones of the main jets....are covered with fuel is an almost impossible situation?

 

Gentlemen:

The fuel rises above the Jets. The fuel rises to about where the lock washer is on the fuel bowl. The Jets are immersed in Fuel. The Fuel level rises above the Jet and "The Metered" amount is above the Jet available to be drawn-in to the Intake.

Rapidly moving Air, within especially designed passages, causes a differential of Air Pressures to occur and BOTH Air and the captive Fuel above the Jets react to the variations of pressure.

A strong rush of Air through the Start Air Jet rapidly increases in speed as it is drawn through a passage which is much narrower than the opening of the Air Jet.

The rush of air across the top of a small internal passage connecting the Starting Pilot Air to the fuel ... captive above the Pilot Jet ... causes pressure in the passage to be reduced ... the gas rises and is Siphoned out of its passage, is atomized and carried with the air to provide a Mixture capable of maintaining the engines need ... at Idle.

This Mixture needs to be rich enough to satisfy the engines requirement for some additional fuel ... as the Butterflys are opened and the engine is run at a higher rpm.

The Mixture needs to be sufficiently rich enough to allow the engine to accelerate without a moments hesitation while the differential in Air Pressure causes the Fuel ... captive above the Main Jet ... to be drawn into the Air Stream of the Air passing through the Main Air Jet and accelerating through the Main Air Passage surrounding the Emulsion Tube ... where it passes through the many ports in the Emulsion Tube and rushes ... along with the fuel ... captive at the bottom of the chamber ... up and into the Heavy vacuum created by the pistons on the Intake Stroke ... drawing in air from the airbox when the throttle is opened.

The Heavy rush of air across the bottoms of the Diaphragm Piston Assembly causes a decrease in pressure within the piston as the Intake air rushes past small holes in the bottom of the piston.

The Piston rises to this lowered pressure causing a vacuum to occur.
The Needle Valve rises with the Piston ... exposing some room for an additional quantity of fuel to enter the space ... AND ... reducing the resistance at the Emulsion Tubes Meter Ports to allow more air to be drawn-in ... and rush up the tube ... bringing whatever quantity of fuel being allow to rise in the chamber.

With an understanding of the role that the strictly controlled and precisely metered Air Flows have on determining the ideal Air / Fuel Mixture ... You can begin to understand the frustrations, of those who modify how the engine breathes and exhausts ... have restoring the correct "Flows" when they "Re-Jet" for Pods, Headers or Higher Performance.

How'd I do?
I wanted everybody to be able to grasp this ... not just the Wrenchers, Tuners and Pilots.

 

Excellent post Rick. I think we're probably going too technical for the average wrencher (how many licensed mechanics know this stuff?).

To answer the rest of your question Chacal - accelerating, braking, turning etc will change the level of fuel in the carburetors. Because almost all the resistance to fuel flow comes from the metering jets this will have no noticeable effect on performance. (BTW: without the metering jets the carbs would pump an insane amount of fuel and flood terribly)

As Rick stated, the metering jets are always submersed in fuel. You've got big problems if your jets are dry.

For those of you that are trying to understand what we are talking about:

Next time you are in a restaurant get a glass of water and a straw. Place the straw in the water and blow gently across the top. The water will rise in the straw. As you blow harder the water will rise higher in the straw - you will hear the whistle pitch increase. Blow hard enough and water will atomize off the top of the straw. CONGRATULATINS! You are now a carburetor! Hopefully the person sitting across the table enjoys the science experiment as much as you do.

 

SOme good posts. Micarl has nearly quoted the exact response I got on the other forum I posted this question on. The difference being the length of the straw.

Other response

A simple enough explanation in layman terms I could understand :lol: has helped me with a nagging problem on my bike.

 

Yes and no. I was trying to keep the discussion to WOT and the main jet to make the operation easier to explain and comprehend. That was a confussing way to teach it and I appologize.
The technical point that is incorrect is that fuel "does" pool above the jet. Actually about halfway up the emulsion tube. The cone can be covered but I thought it easier to understand if you restrict the discussion to just the jet. It has to be covered since the jet is at the bottom of the float bowl. This is significantly reduced at WOT. To what degree I don't know. I believe that at WOT and high engine speed the main jet produces a steady stream bypassing the holes of the emulsion tube. But I'm not sure if that is correct. Hats off to Micarl and Rick for jumping in with excellent descriptions.
If the fuel level rises too high it will cover the main air jet passage which is at the emulsion tube and suck more fuel than necessary. This will flood the engine or just create too rich a mixture. So the level must be below that but not too low or the air stream will not be able to pickup enough fuel. e.g. the straw and work needed analogy. Micarl nailed the rest as far as below WOT operation and the roles of the emulsion tube and the jet needle via vacuum and the carb piston.
One point to make is that the effects of off level operation and aceleration are minimized by the center location of the main jet. The fuel rotates around it's center point where the level changes the least. I have attached a schematic of the carbs to show the points of discussion.