Float Level setting and how it affects jetting

[skipro3]

I have been doing some reading here and have found that some KDX riders have problems with their bike peeing (float bowl overflow) when leaned over. Suggestions to stop this is to set the float level lower so that the bowl doesn't fill to near capacity when upright and overfill when leaned over. Further searchs and reading suggest that jetting in all circuits are affected a lot by how the float level is set with a low float level making the bike leaner and a high float level making the bike richer. By a lot, the searchs say as much as 2 jet sizes. Is this true and if so, how does float level affect jetting on any circuit let alone on every circuit?

 

[cfr1970]

Theoretically, the float level CANNOT change your jetting requirements in the carb. The only way it could make your bike lean is if your gasket sealing the bowl to the carb is bad. If that were the case, changing the jetting is just masking a problem. Just set your float level a little lower (just enough to stop the overflow) and ride on.

 

[skipro3]

Now that's what I thought too. So this stuff I've been reading about float level affecting jetting is not true I guess. Anyone else hear of this theory and have any thoughts on the subject?

 

[canyncarvr]

Sorry guys.

'..the float level CANNOT change your jetting requirements in the carb.'

All well and good. Wrong, but nice.

It most certainly does change the EFFECT of the air/fuel mixing process and resultantly, the air/fuel mixture.

How does the fuel get from the bowl to the airstream in the first place?


It is pushed by atmospheric pressure into a space of lesser pressure (caused by the venturi effect of the carb).

The farther the fuel has to be pushed, the more pressure is required, or....with the same pressure being applied, the less fuel will come out the other end.

Put a 1 1/2HP well pump on a 2' long piece of 1 1/2" pipe. Turn it on. Get a LOT of water.

Put that same pump 300' down, connect it with 1/2" pipe. Turn it on. You won't get squat. Well, comparatively.

Point of fact: Lowering the float level (and thus the fuel level) will indeed have the effect of leaning the A/F mixture. Less fuel gets 'pushed' out cuz it has farther to be pushed.

In my case, a change from 16mm to 17mm float drop made the difference pee-wise. Two jet sizes? Nope. Not even close.

 

[skipro3]

Thanks CC!
How much did your change from 16 to 17mm drop affect your bike and how was this determined? Did you notice a performance change, a plug read change, etc? Is adjusting the float cause to verify correct jetting over again? How about A/S adjustment with a change in float setting? Would that need to be readjusted? Thanks for taking the time to respond.

I am asking because I recently reset my float using the tip method where I measured the angle my bike could be leaned over before peeing, adjusting the float and leaning again to measure the lean angle before again overflow is noted. My carb would leak just on the kickstand, now it can lean about 10 degees more without spilling.

 

[canyncarvr]

1. Form Follows Function

Function: Need bread.

Form: Take some wheat flour, salt, sugar, shortening and yeast. Make bread.

A good thing.

2. Function Follows Form

Form: Someone used too much flour, no salt sugar or yeast.

Function: I have a brick

But, I'm hungry!!

Not a good thing.

The purpose of setting the float level in the carb is to allow proper operation of the carb..its fuel delivery, mixing of such with air to provide a properly atomized ratio of fuel/air to the engine.

The purpose of the carb is not to not pee.

If you have some other problem (bent float arms, corroded/gummy needle seat, degraded needle tip your carb may well pee with a float level of 6" (hyperbole for effect ;) )

While that may be at odds with the whole idea of dropping the float level to improve bike continence, it depends on which comes first...form or function.

Sheesh..there is a point here someplace....

It is changing your float level based on whether it pees or not isn't a good idea.

That was much simpler, 'eh? (but not as much fun...and I get paid by the word!)

If a float level of 17mm (spec is after all 16mm+/- 1mm) still pees, another problem needs to be addressed.

Honestly, I didn't go through a jetting sequence (plug chop, subsequent plug inspection and such) to check the result of my float level change. It's within spec and the carb otherwise is operating properly.

Jetting changes taking thousands of feet el. change and huge temp variants into account keep me busy enough as it is.

Cheers!

 

[skipro3]

You better get paid by the word. Thanks for answering my question. Yes, I did all the regular checks to see why it leaks so much and it must just be the tolarances in my specific carb, besides the fact that many KDX owners experiance this problem. I just wanted to be sure the change wasn't enough to worry about jetting. The way I ride, that bowl has got to look like the inside of a blender at times anyway and I can't understand how they should work at all with the beating they take while on the track or trail. Send me your PayPal account and I will deposit your remitance. (How much again per word?)

 

[Jim Crenca]

Let me take the short reply to this thread.
I've set float level at stock height & slightly lower; no signifcant jetting change (ok maybe once it seemed off but it could have just as easily been because of barometer, elevation, etc.); if any change it would have been clip on needle
Put a new float needle in; mine was inconsistent & that's not good
My $.02; put in a new needle, set the float at 17ish; jet carefully; go riding :aj:

 

[skipro3]

Thanks for your input Jim. I agree with you and CC. Why spend too much time anilyzing this when I can just replace a few simple cheap parts and go riding. I do however enjoy the discussions and appreciate the insight provided by members here on how things work. I now have a better appreciation for how the carb is working for me and a much fuller comprehension on how pressure changes can affect carb performance.

 

[Rich Graham]

Great thread. My '97 200 carb needs fresh parts to. Point: I bought this bike recently with very low mileage. This also means this bike probably sat quite a bit with aged fuel. I'm so close with the jetting but I will adress the jet needle first. Then, a DFII is next !!!!!! Thankx for the heads up!