How To Set Your Motorcycle Carburetor Float Level

[Faded]

Here are a few words and some pictures I put together to help eliminate the confusion in setting your float level. Obtaining the correct float level is of the utmost importance as it can affect all jetting circuits. THE FLOAT LEVEL IS THE FIRST STEP TO PROPERLY DIALING IN YOUR JETTING.

To help you understand what is going on inside your float bowl I’ll first explain the theory behind it. Contrary to popular belief engine intake vacuum alone is not strong enough to ingest the required amount of fuel needed for proper running, much less running at all. All engines require some sort of pressure to force fuel into the intake tract where it can mix with a metered volume of air prior to combustion. This pressure is obtained by the use of a venturi and the mechanics described by the Bernoulli Effect. The venturi is used to speed the airflow of the incoming intake charge which also happens to decrease the pressure present in the venturi. The pressure differential between the venturi and the float bowl has a natural tendency to try equalize, and it’s this pressure differential that is responsible for creating the ‘fuel pressure’ that is required for proper running.

The fuel system on most off-road and MX bikes makes use of a carburetor that is of the gravity feed variety. As it suggests you’re relying on the forces of gravity to pull fuel from the gas tank down to the carb and into the float bowl. Since your float bowl is vented, atmospheric pressure (14.7psi) is constantly acting on the predetermined volume, or weight, of fuel inside your float bowl. As air flows through the carb bore (venturi) a pressure drop is realized and the fuel in float bowl is ‘pressurized’ as the pressure between the float bowl and the carb bore (venturi) try to equalize. By altering the volume, or weight of fuel in the float bowl you’re basically varying the fuel pressure that is acting on your jets. More fuel in the float bowl will create more fuel pressure and result in rich(er) running conditions for a given set of jets.

Okay, enough theory, how do you do it? You’ll need to first start off by removing your carb. Be sure to clean the surrounding area to the best of your ability to avoid dirt and debris falling into your carb; or worse, your engine. After you’ve removed your carb I would suggest a thorough cleaning using carb cleaner (or equivalent) and compressed air to ensure that all jets and passageways are spotless. Avoid using wire or other tools to clean orifices of jets; it’s all too easy to alter their original designed dimensions.

After your carb is clean you can now set your float level. The picture below will allow you to become familiar with the parts that are responsible for maintaining the correct float level in your carb. There are four basic parts, the floats themselves (part of the float assembly), the float assembly tang, the fuel inlet needle valve, and the fuel inlet valve seat.


(Float assembly pivot pin not shown.)


It is always a good idea to remove the float assembly pivot pin (already shown removed) and extract the float assembly and the fuel inlet needle. The fuel inlet needle is a wearable part and over time can deteriorate. A worn fuel inlet needle can contribute to an irregular float level. Most fuel inlet needles consist of an internal spring loaded bumper (which contacts the float assembly tang) and a plastic or Viton (rubber) tip. Inspect the fuel inlet needle tip for wear and/or damage. To give you an idea, Eric Gorr recommends replacing the fuel inlet needle/seat assembly every two years. I’ve found that the average cost it around $15 for both parts.


(Fuel inlet needle shown with Viton (rubber) tip. The Viton is used to isolate the fuel inlet needle from vibration and to create a better seal against the fuel inlet valve seat.)

Now that you’ve made sure you aren’t going to have any issues from worn parts you can reinstall your needle, float assembly and float assembly pivot pin and continue on to set your float level. The float level measurement is taken from the top of the floats (when the carb is positioned upside down) to the gasket surface of the float bowl as illustrated in the next picture. You can use an open-end wrench (sized per your spec), a small metric ruler, or a float level gauge. The tolerance for your float level is usually around +/- 0.50mm.




When setting the float level be aware that the spring loaded bumper on the fuel inlet needle valve may have a tendency to compress under the weight of the float assembly which will skew your measurement. Before you obtain your measurement you’ll need to make sure that the float assembly tang just barely makes contact with the spring-loaded bumper. Sometimes it is easier to hold the carb body at a 45-degree angle to avoid compressing the spring in the fuel inlet needle.




If you find that your measurement does not match your float level spec then you can carefully bend the float assembly tang to achieve your desired measurement. Be sure to recheck your work, and if you feel confident that your float level is spot on then you can reinstall your carb and get back to riding.

[Woody_393]

:

That my friend is AWESOME work. Thanks for posting that.

[CDN_YZee]

wel now i can actualy do the work and know what im doing...awsome post

[TexasDirtNap393]

Good post but i'm still unclear about exactly what function the float performs but i'm gonna guess.

It would seem that the float is pushed down into the gas held in the bowl of the carb. This would force the gas to rise up in the bowl as the float displaces the gas. The set point is achieved when the gas is at a specified height above the intake jet. As you explained this would then cause the pressure on the fluid at the intake jet to be set to an engineered spec and the system is ready to go.

...but this would imply that there is also air trapped in the top of the bowl. What is it that keeps the bowl from filling up with gas and overflowing constantly. Is there a valve that the float controls that shuts off the gas inlet ?

wow. thought i was gettin it but now i'm really confused.

[Woody_393]

TDN, how does your toilet keep from over flowing the holding tank? It's the same concept. When the float is down it opens the valve to allow more fuel in, when it gets enough it stops.

[KTM250rm393]

Woody_CRF,

Great concept. I never thought of using the ballcock of a toliet as a good example of how the float works in the carb.

[Faded]

Originally posted by TexasDirtNap393
Good post but i'm still unclear about exactly what function the float performs but i'm gonna guess.

It would seem that the float is pushed down into the gas held in the bowl of the carb. This would force the gas to rise up in the bowl as the float displaces the gas. The set point is achieved when the gas is at a specified height above the intake jet. As you explained this would then cause the pressure on the fluid at the intake jet to be set to an engineered spec and the system is ready to go.

...but this would imply that there is also air trapped in the top of the bowl. What is it that keeps the bowl from filling up with gas and overflowing constantly. Is there a valve that the float controls that shuts off the gas inlet ?

Sorry for the confussion, maybe I should have clarified. Woody's example is great in helping explain this too! The "floats" as the name implys are meant to float in the gas that is filling the bowl. As the fuel enters the float bowl it causes the floats to rise which push against the fuel inlet needle. Once there is enough fuel in the float bowl to raise the floats to the specified "float level" the the fuel inlet needle makes a seal with the fuel inlet valve seat cutting off the fuel supply. As the carb uses fuel and reduces the ammount in the float bowl, the floats are lowered and the seal is broken, allowing more fuel into the float bowl until the desired level is reached and the needle contacts the seat and shuts off the fuel supply.

Whew, I hope you can understand that.

Also, there is a small air pocket that resides on top of the fuel, that is why the carb is vented, and this is where atomospheric pressure comes into play. The atmospheric pressure is helping the force the fuel through the jets in order to equalize the pressure difference between the float bowl and the low pressure zone in the venturi of the carb. Atmospheric pressure, plus the weight of the fuel equals the amount of fuel pressure acting on the jets.

These questions are great, if you have more keep them coming! Let's help everyone understand.

[KTM250rm393]

From Faded:

Sorry for the confussion, maybe I should have clarified. Woody's example is great in helping explain this too! The "floats" as the name implys are meant to float in the gas that is filling the bowl. As the fuel enters the float bowl it causes the floats to rise which push against the fuel inlet needle. Once there is enough fuel in the float bowl to raise the floats to the specified "float level" the the fuel inlet needle makes a seal with the fuel inlet valve seat cutting off the fuel supply. As the carb uses fuel and reduces the ammount in the float bowl, the floats are lowered and the seal is broken, allowing more fuel into the float bowl until the desired level is reached and the needle contacts the seat and shuts off the fuel supply.

Whew, I hope you can understand that.

Also, there is a small air pocket that resides on top of the fuel, that is why the carb is vented, and this is where atomospheric pressure comes into play. The atmospheric pressure is helping the force the fuel through the jets in order to equalize the pressure difference between the float bowl and the low pressure zone in the venturi of the carb. Atmospheric pressure, plus the weight of the fuel equals the amount of fuel pressure acting on the jets.

[Honda_Rider_XR]

Not to mention that there is air constantly being sucked above the carburetor jets creating the Burneulli affect (I can't spell his name lol). That is, when the crank is turning and the piston is going up and down.

[TexasDirtNap393]

Faded, your explanation is dead on. I get it. Kinda figured it out last nite and posted to the other version of this thread. Woody, your toilet analogy is about as good as it gets.

Ok. one more question. With all the bouncing around that a bike does seems to me that fuel in the bowl would mix with the air pocket you mention. Its gotta be true that the air pocket does not have enough volume to ever allow air to be present at the main jet instead of fuel when the bike is going thru a really rough section. True ?

[crdude]

is it just me or are the pics no longer working?