fuel screw on L carb .......need a memory refresher.


12 replies to this topic
  • Huffa 2

Posted July 13, 2007 - 02:03 PM

#1

I know it's the same on all four strokes and as time goes by I always forget :thumbsup: which way does what but I think it's ..........

Clockwise/turn in = Less fuel/more air

CC/turn out = more fuel/less air

....and on a four stroke it's technically called a fuel screw and a 2 stroke a air screw because everything is opposite on it

Is all this correct ?

  • martinfan30

Posted July 13, 2007 - 02:27 PM

#2

I know it's the same on all four strokes and as time goes by I always forget :thumbsup: which way does what but I think it's ..........

Clockwise/turn in = Less fuel/more air

CC/turn out = more fuel/less air

....and on a four stroke it's technically called a fuel screw and a 2 stroke a air screw because everything is opposite on it

Is all this correct ?


ya clockwise you are turning the screw into the pilot jet allowing less fuel thru the pj. likewise opposite for counter c.wise.

  • Bibleman

Posted July 13, 2007 - 02:32 PM

#3

...and if I'm not mistaken, it doesn't adjust the amount of air on the "L" - right?

  • martinfan30

Posted July 13, 2007 - 02:37 PM

#4

...and if I'm not mistaken, it doesn't adjust the amount of air on the "L" - right?


correct. just the fuel allowed intp the pj.

  • Huffa 2

Posted July 13, 2007 - 02:54 PM

#5

correct. just the fuel allowed intp the pj.


OH, I didn't know that, thanks buddy and thanks for asking BM :thumbsup:

I'll get down your way one of these weekends :thumbsup: ( not you martin:lol: )

Although in 97 I flew out to the colusium to see Doug Henry win the 1st 4 stroke championship on his 400 Yammie.

Stayed at the pyramid in Vegas.

You by chance go too? Hmmm, not sure if it was 97 or not, now that I think more about it.

  • HawkGT

Posted July 13, 2007 - 04:35 PM

#6

Not that it really matters, but for the sake of accuracy....a fuel screw doesn't actually adjust the amount of fuel entering the pilot jet. Fuel screws adjust the amount of emulsified air/fuel mixture allowed into the carb's throat from the pilot circuit. It goes like this: Air enters the pilot circuit through the pilot air jet (a fixed orifice on many carbs, a replaceable jet on others). That air mixes with the fuel being drawn through the pilot jet. That a/f mixture then passes by the fuel screw's tip on its way to entering the carbs throat. The fuel screw controls the volume of a/f mixture that's delivered while the pilot jet (working in tandem with the pilot air jet) determine the strength of that mixture.

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  • Huffa 2

Posted July 13, 2007 - 05:08 PM

#7

Not that it really matters, but for the sake of accuracy....a fuel screw doesn't actually adjust the amount of fuel entering the pilot jet. Fuel screws adjust the amount of emulsified air/fuel mixture allowed into the carb's throat from the pilot circuit. It goes like this: Air enters the pilot circuit through the pilot air jet (a fixed orifice on many carbs, a replaceable jet on others). That air mixes with the fuel being drawn through the pilot jet. That a/f mixture then passes by the fuel screw's tip on its way to entering the carbs throat. The fuel screw controls the volume of a/f mixture that's delivered while the pilot jet (working in tandem with the pilot air jet) determine the strength of that mixture.


OHHH, I learned something new again. So it does not change the A/F mixture at all, here I always thought I was actually changing the ratio of air & fuel, evidently not!

Thanks! :thumbsup:

  • HawkGT

Posted July 13, 2007 - 05:39 PM

#8

OHHH, I learned something new again. So it does not change the A/F mixture at all, here I always thought I was actually changing the ratio of air & fuel, evidently not!

Thanks! :thumbsup:


You ARE changing the leanness or richness of the mixture being sucked into the engine when you adjust the fuel screw. My wording might have been a misleading because we generally don't think of emulsified mixtures as being a/f mixtures (even though they really are).

What the pilot circuit delivers is emulsified fuel into the carb's throat (emulsified fuel = liquid fuel broken up into little tiny droplets and mixed with air). You control the delivered volume of that emulsified fuel with the fuel screw. You control the "strength" of the emulsified mixture (i.e. how much fuel and how much air is in the emulsified mix) with the pilot jet.

But once that mixture enters the carb's throat it combines with additional fresh air thats being sucked in under the slide and past the butterfly. So by changing the volume of the a/f mixture being delivered by the pilot circuit you're changing the richness/leanness of the TOTAL mixture being sucked into the engine.

Is that more clear or even more confusing?? :thumbsup:

  • HawkGT

Posted July 13, 2007 - 05:52 PM

#9

Maybe this will help.

Here's a typical fuels screw arrangement:
Posted Image

1) pilot air jet orifice
5) fuel screw
6) pilot jet
the dark area is emulsified fuel


Here's a typical air screw arrangement:
Posted Image

1) air screw
5) pilot jet
the dark area is emulsified fuel

  • martinfan30

Posted July 13, 2007 - 10:35 PM

#10

thanks for the correction hawk, but basically what was said is turning the mixture screw clockwise leans the low end mixture while c.clockwise will enrichen it. correct? i did not mean to mislead i just want to be sure the basic part of the conversation was correct.

  • Huffa 2

Posted July 14, 2007 - 02:20 AM

#11

Thanks for be so concerned in explaining it right :thumbsup: way ta go Hawk :thumbsup:

Now I actually know how it's somewhat being channeled through the carb "halls".

  • HawkGT

Posted July 14, 2007 - 08:38 AM

#12

thanks for the correction hawk, but basically what was said is turning the mixture screw clockwise leans the low end mixture while c.clockwise will enrichen it. correct? i did not mean to mislead i just want to be sure the basic part of the conversation was correct.


Yes, that's right....

Fuel screws: IN (clockwise) = leaner. OUT (counterclockwise) = richer.
Air screws works the opposite way: IN (clockwise) = richer. OUT (counterclockwise) = leaner.


Four stroke carbs almost always have fuel screws while two stroke carbs almost always have air screws. If in doubt you can distinguish the two types by their location on the carb. A fuel screw will generally be somewhere in front of the slide while an air screw will be behind the slide. Actually, it's in front or behind the pilot jet that is the distinguishing characteristic but using the slide as the reference point is easier and accurate in most cases.

  • XR650L_Dave

Posted July 14, 2007 - 06:29 PM

#13

Clockwise looking from the top or the bottom?

(ignore! just being a smart-sss)

Dave





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