Float Height vs. Altitude

jrp

jrp

Joined
Nov 25, 2008
Messages
535
Age
60
Location
Colorado
Bike
ST1100AW
STOC #
8991
Hello fellow Coloradoans,

I recently serviced the carbs on my '98 ST1100a for the first time since I've owned her, and found the float heights set to 8-9mm, rather than at 7mm which is the specification given by Honda. So I went ahead and set them all to 7mm (which means the mixture will be more richer). I have not yet put the carb assembly back in the bike.

Now I'm wondering if perhaps these carbs were set to be more lean than spec because it is a Colorado bike. What do you have your float heights set at?

Thanks,
Jeff
 
Interesting question but I think mixture would be adjusted by jets not float level.
That being said , I guess if you didn't have jets to swap the floats MIGHT be a stopgap measure.
Check which Jets you have. Little lettering on the Jets (Both Idle jets and Main jets) I have a small assortment if you want to make a change.
 
I have the #40 pilot/#125 main/.020" washer shim on the needle.

Slydynbye said:
Interesting question but I think mixture would be adjusted by jets not float level.
That being said , I guess if you didn't have jets to swap the floats MIGHT be a stopgap measure.
Check which Jets you have. Little lettering on the Jets (Both Idle jets and Main jets) I have a small assortment if you want to make a change.
Click to expand...
 
By the way, the height level of the fuel in the bowl does have an effect on the amount of fuel that flows at certain throttle openings.

Slydynbye said:
Interesting question but I think mixture would be adjusted by jets not float level.
That being said , I guess if you didn't have jets to swap the floats MIGHT be a stopgap measure.
Check which Jets you have. Little lettering on the Jets (Both Idle jets and Main jets) I have a small assortment if you want to make a change.
Click to expand...
 
Slydynbye said:
Interesting question but I think mixture would be adjusted by jets not float level.
That being said , I guess if you didn't have jets to swap the floats MIGHT be a stopgap measure.
Check which Jets you have. Little lettering on the Jets (Both Idle jets and Main jets) I have a small assortment if you want to make a change.
Click to expand...

:plus1:
 
A lot of good stuff for sure, and I have used that page, however it does not answer my question about float level versus altitude. I live in Denver, Adam lives in NY. Here is what Adam says:

"As with anything else, the float level is very important. All of mine measured different from the factory. Factory spec is 7mm. 3 of mine were at 8mm and one was at 9mm. Very lean emission settings. I set them all to 7mm "

That worked well for him where he lives. This is why I posted in the Colorado forum. I want to hear from people who live a mile or more above sea level.


Al st1100 said:
http://www.st-riders.net/index.php?topic=10249.0 all the carb stuff you need to know:thumb:
Click to expand...
 
The float level controls how much/deep the fuel is in the float bowls. As the level of fuel increase the floats rise and the float needle shuts the flow of fuel off to the bowls. That is all that the floats do. Control how much fuel is in the bowls.
It has nothing to do with fuel/air mixture.
My experience with Colorado riding is off road on "dirt" bikes. I lived in Iowa and 800' ASL. We road at a starting elevation of 8500" ASL. We would change the main jets to a smaller size to adjust for the thinner air and make a needle jet height adjustment. Not once did we change the float level.
 
Kevin_56 said:
The float level controls how much/deep the fuel is in the float bowls. As the level of fuel increase the floats rise and the float needle shuts the flow of fuel off to the bowls. That is all that the floats do. Control how much fuel is in the bowls.
It has nothing to do with fuel/air mixture.
Click to expand...
Won't a higher fuel level mean it takes less intake air velocity to draw fuel into the air stream, effectively making the mixture richer, and vice versa?
 
  • Like
Reactions: jrp
Yes, you are right, the float controls the level of the fuel in the float bowl.

The fuel level in the bowl and what effect it has in CV carbs is commonly misunderstood.

The fuel level height in the float bowl affects richness or leanness at cruise/low rpm, along with the main jet and needle clip position (or level of shimming).

Kevin_56 said:
The float level controls how much/deep the fuel is in the float bowls. As the level of fuel increase the floats rise and the float needle shuts the flow of fuel off to the bowls. That is all that the floats do. Control how much fuel is in the bowls.
It has nothing to do with fuel/air mixture.
My experience with Colorado riding is off road on "dirt" bikes. I lived in Iowa and 800' ASL. We road at a starting elevation of 8500" ASL. We would change the main jets to a smaller size to adjust for the thinner air and make a needle jet height adjustment. Not once did we change the float level.
Click to expand...
 
Kevin_56 said:
The float level controls how much/deep the fuel is in the float bowls. As the level of fuel increase the floats rise and the float needle shuts the flow of fuel off to the bowls.
Click to expand...

true

That is all that the floats do. Control how much fuel is in the bowls. It has nothing to do with fuel/air mixture.
Click to expand...

false, the distance between the jet opening and the fuel level affects air/fuel mixture, this is why it is specified to +/-1mm in a service manual.

edit: I see two others replied while I was slowly typing my reply
 
The reduced air pressure at altitude also lowers the bowl ambient pressure causing less pressure to move fuel to the engine. If the fuel level in the bowl is higher there would be less head pressure to move fuel to the jet exit, so I would agree that less fuel in the bowl would cause a slight loss of fuel delivery.
 
dwalby said:
false, the distance between the jet opening and the fuel level affects air/fuel mixture, this is why it is specified to +/-1mm in a service manual.

edit: I see two others replied while I was slowly typing my reply
Click to expand...
I will agree if the level gets lower than the intake of the MAIN jet then the fuel/air mixture will be effected. The height adjustment is to insure that this will not happen. The Main/jet size along with the needle setting controls the correct mixture. The float level setting makes sure there is enough fuel at the main jet opening. IF that level gets so low that fuel does not flow through main jet your bike will run very erratic. Like it is running out of fuel, which it really is.
Back to the original question. Can float height make your fuel richer, no, the main jet controls how much fuel flows through the carb to get the correct fuel/air mixture.

Larry Fine said:
In other words, jetting recommendations depend on correct float level.
Click to expand...
Jetting is determined by altitude.

Consult a mechanic. I have a friend that is a motorcycle mechanic that has guided me and many others that adjusted our carbs for high altitude riding. Those adjustments never had us change the float height. We did change the main jets and needle setting.

Maybe Uncle Phil can chime in, or George.
 
Larry, it's actually the other way around.

When jetting a CV carb, you first settle on the main jet size by testing at full throttle performance.

Once you determine the main jet size, then you determine the optimum needle height at mid-range throttle.

After you've done the above, only then do you determine the optimum float height for low end power.


Larry Fine said:
In other words, jetting recommendations depend on correct float level.
Click to expand...
 
I believe you. I usually follow the directions and don't try to modify unless I know what I'm doing.

When it comes to wiring, I know what I'm doing. :thumb:
 
I scanned replies and didn’t see any mention of the Service Manual “High Altitude Adjustment” procedure, page 5-16 in mine. If that’s of any interest here...

John
 
Kevin_56 said:
Consult a mechanic. I have a friend that is a motorcycle mechanic that has guided me and many others that adjusted our carbs for high altitude riding. Those adjustments never had us change the float height. We did change the main jets and needle setting.
Click to expand...
I think you may have misunderstood some of the replies in this thread. Nobody is suggesting you adjust fuel mixture for altitude by changing the float level (although I think the OP did ask that question in post #1, which may have led to the confusion). What has been said multiple times, (because its true) is before doing anything else on a bike of unknown history make sure the float levels are correct, that's the baseline for which the jet sizes were originally determined.

Larry Fine's comment "jetting recommendations depend on correct float level" is 100% correct. After that variable is confirmed, then you change jet sizes to make it richer/leaner. You never change the float level to make it richer/leaner once its set to the level specified by the mfr.
 
changing the float level does not change the amount of fuel that goes thru a jet. The needle (on the diaphragm) regulates the fuel that come out of the jet under different throttle/load conditions.
.
 
You must log in or register to reply here.
Back
Top Bottom