How To Set Your Motorcycle Carburetor Float Level (more commentary).

[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 is 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.

[yz250guy]

Well done! How much do I owe you for that? That's the way all "how to's" should be done! Nice job.

[Faded]

Originally posted by yz250guy
How much do I owe you for that?

T,

This one is free.

[Woody_393]

Thank you very much Faded, That was/is the very best explanation I've ever read.

[CDN_YZee]

very nice

[Faded]

Originally posted by Woody_CRF
Thank you very much Faded, That was/is the very best explanation I've ever read.

Any time, hopefully this will clear up some of the confusion and get everybody on the right track.

[cr2504life]

Thanks alot, the manual doesnt' even explain it that clearly. Excellent job.

[Mecrazyperson]

As always 110% Andrew, Keep up the killer job bro.

[Frakke]

Great write up mate!!!! Excellent job!!

Euh, I still can need a mechanic at the races........??

[Faded]

Originally posted by Frakke
Euh, I still can need a mechanic at the races........??

LOL, do you cover traveling expenses?

[Frakke]

Originally posted by Faded
LOL, do you cover traveling expenses?

Maybe we can share the prize money, lol!!!!

[yz250guy]

Originally posted by Frakke
Maybe we can share the prize money, lol!!!!

So you'll cover his breakfast expenses then?

[Frakke]

Originally posted by yz250guy
So you'll cover his breakfast expenses then?

Yep, coffee not included!!!!

[Dirtbikindad393]

Andrew awesome job Bro I think I am going to check my floats now.

[TexasDirtNap393]

Great writeup and I now know how to set the float level but what exactly does the float do.

The float sits in the gas held by the bowl. Does it control the needle valve and does the needle valve modulate the gas coming in to the bowl from the tank ? Is this correct and is this the function of the float. So setting the float level determines when gas is brought in to the bowl. As it gets pushed/sucked out of the bowl the float opens the needle valve allowing gas to enter the bowl and keep it at a constant level.

I would guess that given all the bouncing around that a bike goes thru that the level of gas in the bowl can never bounce around to the extent that there is no gas available at the main jet to be used for combustion

Am I confused ?

[Faded]

Originally posted by TexasDirtNap393
Great writeup and I now know how to set the float level but what exactly does the float do.

Sorry for the confussion, maybe I should have clarified. 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.

[Frakke]

Faded, if you already haven't, I think you better open a bike shop!! I'm sure you will have a lot of customers because you're mechanical skills seem SO GOOD to me, really!!! great job you've done here!!!

[schwen_108]

This is probably one of the best "how to" articles I have ever read!
thanks Faded!

[Dr. Destructo]

I have a question. I just tried to adjust my floats, which are shaped just like the ones in your pics, to 16mm. The previous adjustment was so far from 16mm when aiming for the very tip top of the float, that I began wondering if the very top of the float was the spot you were supposed to measure at. Just for the heck of it, I adjusted the float , setting the very top of it to 16mm, and it was quiet a huge change /adjustment to get it there! Something didnt seem right, -even with the carb completely upside down the fuel needle didnt seem like it was seated at all after this major float adjustment. I verified this by hooking a vacuum gauge to the fuel inlet and verified indeed the needle was not seating.

So I thought to myself, there's no way you measure at the very top of this float, but maybe closer to the linear line about 2/3 up, where its a normal "shape". I ended up adjusting my 16mm to line up with where the metal arms mold into the plastic float, and doing it this way gave me a sealed needle as judged by my vacuum gauge.

Im not questioning your writeup, Im questioning why did setting my float to 16mm the way you say cause some weird things to happen and actually hang my fuel pin up from seating?

Note= the bike spits whitish exhaust smoke at all times when reving from idle to full throttle in neutral (87 cr125)

Now i will go back to the garage and try this adjustment again.

[Faded]

Two things come to mind...and I'm not saying it's either of these but definitely something to check. First verify that the floats are installed correctly and not upside down (it could happen). Second, especially on a bike that old, how sure are you that someone didn't do you a favor and use a set of floats from a different carb?

It is also a commonly accepted practice to set the float level by having those 'mold lines' parallel to the gasket surface. Usually this is used in a pinch or if you can't obtain the factory spec. With your current setting how far off from parallel are you?