How to adjust float heights and why would I want to do that?

17mm float height setting

To adjust float height:       Sooo....

You read the carb kit instructions and they specify a 15mm float height. 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 (only exception is 95-97 zx6r)

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 height measurement. 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.

 

Nifty baffle plates in the 05 KLX300!
This carb type has been around since the early 80's! It's had some updates over the years. It's similar to the Ninja 900'a and ex250 styles.
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.



How much is the nifty Float Valve Measuring Tool? How much is that nifty Float Valve Measuring Tool?

About $49.95, direct from Factory Pro

415 491-5920 voice

415 492-8803 fax

 



carb,float adj,38Mik.jpg (197016 bytes)
(for larger view, use your browser "view image".

 

This is a picture of a set of 38mm Mikuni CV carbs as supplied on the:

Ducati 600/750/900
Yamaha YZF750, fzr1000
Yamaha TDM/TRX 850

showing the Float Height Tool in use.

Note: Doing 38mm Mikuni downdrafts?
 Get the nifty float cage holder to make your life easy!
Click here

 

Note:
Measure the "rectangular", taller float (not the lower, "diamond / parallelogram" shaped one) There is ~3mm difference between the 2 different floats!
Remember to measure the float height - tilt the carbs till the floats just barely flop over and close the float valve, but don't compress the float valve spring!