Could We Have An EXPERT Discussion Of Basic Carburetors ???

[bobgritz]

Could we have a discussion of the basic operation of a carburetor (see below) from expert carb mechanics.

The gas/oil mixture enters the bowl by way of the float valve and this is fairly rudimentary but from there it becomes a bit more mysterious.

It would seem that the depth (or height) of the fuel in the carb's bowl is critical to the operation of the engine especially at idle when there is less suction due to lower rpms. We all know that the 'basic' adjustment of the float is done by holding the carb upside down and adjusting its level with the bottom of the carb's body. For something so seemingly impotant, this seems pretty sloppy. If this adjustment is important, shouldn't the measurement be kept within mm rather being done by hit-or-miss?

If I understand the operation of this type of carburetor, the gas/oil is atomized in the open space (above the fuel & within the bowl) and it is sucked upwards as a mist into the high speed nozzle. If this is correct, and this 'vacuumed' fuel feeds both the high and low speeds of the engine, why is it called the 'high speed nozzle'?

As far as I can tell, this (high speed nozzle) is the only exit area of the carb and it feeds both the high speed and low speed circuits! As it enters the nozzle I assume it is distributed in a direction relative to the RPMs of the engine. At slow speed, it is detoured by way of the Slow Speed Needle/Orifice and at higher speed it travels more directly into the throat of the carburetor.

 

[HighTrim]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

I am by no means an expert, but will try to help with a couple of the questions.

Most simply call what you are referring to as a "high speed nozzle" the fuel nozzle. Carb operation varies slightly as well depending on what model you are referring to.

I like to set my floats so that the unhinged end is just slightly higher than the hinged end, this is to ensure proper seating of the float needle. There is a tool for certain models to set an exact float setting, but for the OMCS we work on here having it parallel and square is good enough.

 

[Lyle29464]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

If you "eye ball" the float setting and get it somewhat correct you gas level will be well within operating range. I have had several floats go bad in my 28 ( bought it in 1979) never had to reset after installing new ones. Most problems are too much angle on the engine and not enough wedge under carb.

The low speed circuit is by-passed when the butterfly is opened up. So it is self tending. No improvement needed there.

 

[daselbee]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

If I understand the operation of this type of carburetor, the gas/oil is atomized in the open space (above the fuel & within the bowl) and it is sucked upwards as a mist into the high speed nozzle. If this is correct, and this 'vacuumed' fuel feeds both the high and low speeds of the engine, why is it called the 'high speed nozzle'?

Ok, not quite right here. At high speed, the air is rushing through the venturi at a high rate of speed. This develops an area of low pressure in the venturi at the top of the tube that protrudes into the venturi from the bottom, which also extends down to the bottom of the fuel bowl, and which all fuel entering that is metered with the HS jet.

So, the fuel is atomized at the top of this tube, sucked up from the fuel bowl thru the HS jet. The fuel vapors above the liquid fuel in the bowl play very little part, if any.

As far as I can tell, this (high speed nozzle) is the only exit area of the carb and it feeds both the high speed and low speed circuits! As it enters the nozzle I assume it is distributed in a direction relative to the RPMs of the engine. At slow speed, it is detoured by way of the Slow Speed Needle/Orifice and at higher speed it travels more directly into the throat of the carburetor.

No, the HS nozzle is not the only place fuel leaves the bowl. There is an idle circuit in the carb (for simplicity) that has much smaller diameter pick up tube/point. It is therefore able to feed fuel into the engine with relatively low air speed thru the venturi (compared to high speed). Also, for example, on my V6 loopers, the low speed has it's own pick up tube extending into the bowl, and the fuel is eventually delivered thru passages in the side of the carb, into the throttle body, and enters the air stream thru three small holes that are drilled into the side of the throttle body, just behind the butterflys.

Most designs are similar, differences are easy to recognize and understand.

 

[bobgritz]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Lyle, you said the "low speed circuit is by-passed when the butterfly is opened up" ... is that a physical/mechanical closure or is by-passed by the higher vacuum that is created (the increased suction simply overpowers the low speed orifices)?

 

[Haffiman]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

The correct level is critical, but not that critical. On engine with trim, the 'column' of fuel will vary with trim in the tube 12. However what is critical is that the carb does not overflow and too low level increases the risk of air-bubbles in the fuel. Mainly the carb itself when operating on high-speed circuit is nothing but an ejection pump. The airflow passing the high speed nozzle pulls the fuel up and atomize it. This is even enforced by the venturie itself as the speed increases as the flow passes the narrowest point and increases the suction.
At idle the butterfly valve is closed, the vacume behind the butterfly where the idle outlet is, pulls the fuel out. How much fuel depends on this carb, of the adjustment of the needle. On other carbs you have an air correction or fuel correction jet giving a pre-calculated amount. Carbs of the latter types often needs an intermediate system to cover the gap between closed butterfly and just open as it will loose the needed suction in the idle circuit as soon as the carb starts to open. An alternative is an acceleration pump, which is used on bigger carbs and normally 4-stroke engines.

 

[daselbee]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Lyle, you said the "low speed circuit is by-passed when the butterfly is opened up" ... is that a physical/mechanical closure or is by-passed by the higher vacuum that is created (the increased suction simply overpowers the low speed orifices)?

It is not bypassed per se, but the fuel supplied becomes very insignificant once the mains start drawing.
Look in the front of a carb when operating. You will see a large amount of fuel being sucked up and atomized in the center of the venturi.

 

[bobgritz]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Daselbee ... great response!

Take this other carb (see below): The High Speed Nozzle in first posting above (#12) is different in this second carb. Instead of calling it a High Speed Nozzle, it (#23) is referred to as an 'Orifice Plug' and it does not extend down into the liquid fuel in the float chamber. Could you, or someone, elaborate on how the fuel is fed to the upper reaches of the carb body.

 

[daselbee]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Well....

You will find this hard to believe, but they don't show the pickup tubes in the drawings. I suspect that this carb does indeed have such tube, just doesn't show it.

Go to shop.evinrude.com, pick any motor, drill down into the carb section, and you will not see the tube(s) I am referring to.

The carbs I am most familiar with are the 1990 thru 2001 V6 loopers (200, 225, 250).

I have worked on V4, V6 crossflows, all have the tubes, just do not show them on the drawings.
They should.

 

[bobgritz]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

I found it hard to believe too! But it is exactly as it shows ... I have one.

This is the carb in my 1983 Johnson 15hp and it was because of it that I was confused about how the fuel gets from the airspace above the bowl's fuel, then up into the main body. The carb in my opening post is from a 6hp Johnson (that I also have) but I should have used this 2nd pic instead.

Thus my question in the first posting [If I understand the operation of this type of carburetor, the gas/oil is atomized in the open space (above the fuel & within the bowl) and it is sucked upwards as a mist into the high speed nozzle. If this is correct, and this 'vacuumed' fuel feeds both the high and low speeds of the engine, why is it called the 'high speed nozzle'?]

I can understand how the fuel can be lifted by a tubelike device if it dips into the fuel but this one is confusing. That was why I felt that the distance from the top of the fuel, to the exit/entrance orifice, was critical. It would seem that if the fuel was being lifted by the vacuum created by the venturi, the distance would need to be specific. But if you think about it, with a vehicle wiggling and waggling over water or over potholes, the operation of the carb/float/etc. needs to be very forgiving.

Anyone?

 

[daselbee]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Something you are missing....somewhere. For instance, if the HS jet (orifice) dipped into the top surface of the fuel when assembled, the rate of fuel flow would soon make the fuel level too low for the HS jet to pick up any more.

Is there a tube that is in the bowl itself, that when assembled feeds gas up to the jet?

I would just have to see the carb to try to figure it out. But there has to be a source of liquid fuel, readily replinished (by the needle/seat action) present at that HS jet, I would think.

Even lawn mower carbs, my Honda generator, etc. have such a tube. Honda generator HS jet clogs regurlarly, and it's location is up inside the tube screwed in to the upper body of the carb, deep into the tube. Different design, but figure-out-able.

 

[daselbee]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Bob, does this carb work? Or are you trying to figure out why it doesn't work.

If not working, could someone have removed the phantom tube?

 

[AlTn]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

on the second carb what is not shown is the casting that #23 screws into. This casting also contains a nozzle that extends up into the carb throat. Common to the 2 cylinder carbs I've fooled with is a passage way to the chamber that the idle mixture needle extends into. It's either been been cast into the carb body and is not threaded but has a hole in the carb to base gasket for fuel flow, or another smaller tube or opening extends down into the carb throat right above the fuel nozzle and receives an atomized mix which is pulled into this chamber and exits through the 3 tiny holes behind the throttle plate.
With the core plug removed and the 3 exit holes blocked make a tight fit for a compressed air fitting and blow some air into the chamber the core plug covers. Observe where the air exits. This is the path fuel takes to get there.

 

[SparkieBoat]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

the carb is a great invention..simple and effective and efficient and most importantly cheap and easy to maintain.. thank you uncle sam for forcing EFI down our throats and making us pay for the much higher cost of fuel injection engines..only a idiot could screw up a perfectly fine and cost effective invention like a carb..same idiots who are screwing up our light bulbs too..if it works and is cost effective and reasonably efficient, leave it alone. morons...so stupid they just keep costing me more and more money...oil works fine too and there is plenty of it...o durn, I am venting now...but I feel better. still broke though.

 

[daselbee]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Hey sparkie...how about the good old 29 dollar chevy v8 fuel pump? Now in the name of progress, the fuel pumps are in the tank, and run about 400 bucks.

 

[SparkieBoat]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

yes...I agree...why mess with something so simple that works so great.

 

[Lone Duck]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

Yup!!

 

[Lone Duck]

Re: Could We Have An EXPERT Discussion Of Basic Carburetors ???

on the second carb what is not shown is the casting that #23 screws into. This casting also contains a nozzle that extends up into the carb throat. Common to the 2 cylinder carbs i've fooled with is a passage way to the chamber that the idle mixture needle extends into. It's either been been cast into the carb body and is not threaded but has a hole in the carb to base gasket for fuel flow, or another smaller tube or opening extends down into the carb throat right above the fuel nozzle and receives an atomized mix which is pulled into this chamber and exits through the 3 tiny holes behind the throttle plate.
With the core plug removed and the 3 exit holes blocked make a tight fit for a compressed air fitting and blow some air into the chamber the core plug covers. Observe where the air exits. This is the path fuel takes to get there.

yup!!