Friday, April 8, 2011

carburetor float height adjustment?

To adjust float height:

Carburetor Measurements

  • The Mikuni BS32SS carburetor had a 1.3-inch bore size, with a standard idle rpm of 1,150, plus or minus 100 rpm. The fuel level measured at 0.2 inch, plus or minus 0.02 inch, and the float height measured 0.88 inch, plus or minus 0.04 inches. The main air jet measured 1.7 millimeters. The by-pass measured 0.8 millimeter and the pilot outlet measured 0.7 millimeter. The valve seat measured 2 millimeters. The recommended cable play for both the throttle cable and choke cable measured 0.5 to 1 millimeter, or 0.02 to 0.04 inches.

---------------

You read the carb kit instructions and they specify a 15mm float height( for example). Float height? What's a float height? 15mm? How much is that in inches? (it's .590")

Does that mean I must do it???
YES!

Gee whiz! What do I measure with??

You can use a set of vernier calipers, if you have them - they are great for measuring all sorts of things, but you

can make it even easier with our...

Float Level Setting Gauge

Measures from 0mm to more than you will ever need! Easier to use than vernier calipers on carbs. The vertical legs adjust in width and the thin post slides up and down so you can slide the post to the required

clearly marked float height measurement.

----------------------------

Remove float bowl screws. Factory Pro gives you replacement allen screws in most carb kits to replace usual Phillips screws.

---------

Slide the lower movable leg so that both legs will rest on what would be called the gasket surface of the carb body (if there was a gasket, anyway).

------------------

Tilt the carbs so that the floats just flop over towards and into the carb body.

Tilt just enough to touch and close the float the float valve, but...

Do NOT compress the float valve spring when measuring .

Test adjust the "L" horizontal arm in the usual 1mm increments and test fit the gauge to see what the float is set at now. (or not).

(If we say 15mm and you measure 4mm less than that - you might be compressing the little float valve spring.)

At this time, you can set the Float Height Gauge to what you want to reset the floats to.
If you are tuning, you'd usually use 1mm increments.

----------------

CV Carbs: Too RICH at full throttle / low rpm (that's the most common in CV carbs)
Slide carbs (FCR's) - Float height controls 25% throttle at ALL rpms.

If the float height is too low (small float height measurement), bend the tab slightly to increase the heightmeasurement. It's a 5:1 movement ratio.

CV: If the float sticks out further, down, into the float bowl, the carb will deliver less fuel (leaner), especially at low rpms and at cruise.
Slide carbs: Float height controls 25% throttle at all rpm's, even at redline.

You'd generally change the float height in 1mm increments when tuning.

It will affect the topend slightly. Maybe 1.5mm leaner float height would require 1 size larger main jet to keep equal main jet fuel delivery.

--------------

CV Carbs: Too LEAN at full throttle / low rpm (not too common in CV carbs)
Slide carbs (FCR's) - Float height controls 25% throttle at ALL rpms.

If the float height is too LARGE, bend the tab slightly OUT to decrease the height measurement.

If the float sticks out further, UP, into the carb body, the carb will deliver MORE fuel (richer), especially at CV low rpms and at cruise.

It will affect the topend slightly. Maybe 1.5mm richer float height would require 1 size smaller main jet to keep equal main jet fuel delivery.

You'd generally change the float height in 1mm increments when tuning.

When you are done, a multi-cylinder must be all within .5mm (1/2mm) or .020" range of each other.

------------------

This carb type has been around since the early 80's! It's had some updates over the years.

-------------

A view from the engine side, butterfly open.

------------

Why adjust the float height?

Changing the float height changes the level of the fuel in the float bowl. The fuel height adjusts the full throttle, 2k-3k rpm and part throttle cruise, as in cruising around town, trying to be quiet... To give a scale of change, if the bike runs well when cold, but gets a bit sloppy when fully warmed up, lower the fuel level 1mm (i.e. go from 15mm to 16mm float height - remember the float measurement is "backwards").

When do I adjust the float height?

When installing a Carb Recalibration Kit tm.
When rebuilding carbs.
To adjust low rpm / part throttle operation that isn't pilot jet, fuel screw or needle height related.
When a bike starts running rich at low rpm after a mishap or
After an overwhelming gravity equalization process occurs.

--------------------

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.)

Right click on the image and select option "open image in new web" if the image is not seen full or its very small to see.-----------

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, its recommended replacing the fuel inlet needle/seat assembly every two years. I’ve found that the average cost is around $15 for both parts.

Right click on the image and select option "open image in new web" if the image is not seen full or its very small to see.-----------


Right click on the image and select option "open image in new web" if the image is not seen full or its very small to see.-----------


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.-------------

Full history of Mikuni BS32SS

carb:-------------

  • The Mikuni BS32SS was a 32-millimeter, constant-velocity carburetor. A CV carburetor creates a more fluid air flow travel than some other carburetor types, which results in maintaining proper mixture levels and better engine performance. Because of its efficient design, the BS32SS carburetor was used in the 1980s in a variety of vehicles and motorcycle brands, such as Suzuki and Yamaha.

Carburetor Measurements

  • The Mikuni BS32SS carburetor had a 1.3-inch bore size, with a standard idle rpm of 1,150, plus or minus 100 rpm. The fuel level measured at 0.2 inch, plus or minus 0.02 inch, and the float height measured 0.88 inch, plus or minus 0.04 inches. The main air jet measured 1.7 millimeters. The by-pass measured 0.8 millimeter and the pilot outlet measured 0.7 millimeter. The valve seat measured 2 millimeters. The recommended cable play for both the throttle cable and choke cable measured 0.5 to 1 millimeter, or 0.02 to 0.04 inches.

Designations and Carburetor Settings

  • In the Suzuki GS 700, the BS32SS used ID No. 06A00 in most E-03 models, while in California it was ID No. 06A10. In the Mikuni naming conventions, the main jet was #122.5 and the pilot jet was #37.5. The jet needle designation used in the Mikuni BS32SS was 5C-28 and the needle jet specification was Y-5. Mikuni used a starter jet identified as #45 and a throttle valve specified as #135. The pilot screw and pilot air jet came pre-set.

Emissions Control

  • The Mikuni carburetor was designed to meet the 1980 United States Federal Emission Regulations. Any motorcycle having over 50 cubic centimeters of engine displacement was required to adhere to this emissions level. This called for a hydrocarbon level no greater than 8 grams per mile and a carbon monoxide level no greater than 19.3 grams per mile. The main jet, needle jet and pilot jet were all components specifically calibrated to allow the Mikuni BS32SS to meet the required emission level requirements.

Carburetor Troubleshooting

  • According to Suzuki, if the air cleaner fails in the Mikuni BS32SS carburetor, it could be due to the presence of dirt or excess oil lubrication. A faulty fuel level in the carburetor could be caused by a sticking float, a leaking needle or an incorrect carburetor setting. A leaking or sticking linkage could cause the choke to malfunction. Clogged or restricted carburetor jets, a too lean or too rich float level, leaking choke plunger or sticking linkage, or air leaks in the intake manifolds or engine gaskets could all cause mixture malfunctions in the carburetion system.