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Author Topic: Raising a needle jet on a Keihin CV Carb? (1/4 TO 3/4 Throttle)  (Read 6104 times)

Offline SrgtBear

  • Posts: 692
  • The arsenic has oddly shaped feet...
Does anyone know how to raise the needle jet on a Keihin CV Carb.  I looked online a few places and all I can really find is definitions on what it does "1/4-3/4 throttle."  It's for my 75 XS 500.
Quote
1/4 TO 3/4 Throttle
The JET NEEDLE is the most effective component in the range. Changing the STRAIGHT DIAMETER (D) will change the calibration in the transition range from the SLOW circuit to the MAIN circuit (1/8 to 1/4) throttle. A smaller diameter will make this range richer and a larger diameter will lean this range. TAPER (A) changes are only made if there is a problem balancing the calibration between 1/4 and 3/4 throttle. If the mixture is rich at 1/4 throttle and lean at 3/4 throttle, a JET NEEDLE with a larger taper is needed. If mixture is lean at 1/4 throttle and rich at 3/4 throttle, change to smaller taper. If the calibration is lean from 1/4 to 3/4 throttle, raise the JET NEEDLE by lowering clip position, or use JET NEEDLE with shorter length (L1). If the calibration is rich, lower the JET NEEDLE with a longer (L1).

 Anyways last time I checked my needle doesn't have notch and I was able to find something about using shims or washers.  Even then I am not really sure where they are placed and how it's done.

Anyone have experience with raising a needle jet on a Keihin Carb?  Maybe some images?
Ted

Kawasaki Kaiju (怪獣) - 79 KZ650 - In Progress - http://www.dotheton.com/forum/index.php?topic=39748.0
75 Yamaha XS500B Cafe - Completed - http://www.dotheton.com/forum/index.php?topic=14392.0
74 Honda XL 125 - Original - http://www.dotheton.com/forum/index.php?topic=25706.0

Offline SrgtBear

  • Posts: 692
  • The arsenic has oddly shaped feet...
I may have answered my own question but I just found this.  Is there a way to do it just in case I don't have clip positions?
« Last Edit: May 18, 2011, 01:36:44 by SrgtBear »
Ted

Kawasaki Kaiju (怪獣) - 79 KZ650 - In Progress - http://www.dotheton.com/forum/index.php?topic=39748.0
75 Yamaha XS500B Cafe - Completed - http://www.dotheton.com/forum/index.php?topic=14392.0
74 Honda XL 125 - Original - http://www.dotheton.com/forum/index.php?topic=25706.0

Offline teazer

  • Site Supporter
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  • Posts: 6589
Almost!  Many CV carbs do not have adjustable needles, so it's possible to but shims which are just thin washers. They go on the needle under the clip to raise the needle slightly. Remove the carb top and take out the slide.  Remove the needle and slip one of the shims over the needle and replace it in the slide, then reassemble it all. Raising the needle allows more fuel through the needle/needle jet and that has most impact from 1/4 to 3/4 throttle on a normal slide carb.  On a CV carb it impacts full throttle midrange revs.

Because the slide rises and falls with pressure differential in the bore, there's no way to define 1/4 throttle as it relates to needle position.

Have you changed the main jets yet? CV carbs are adjusted a little differently to normal slide carbs.  Changes in mani jet have an effect all teh way across teh rev range on a CV carbs where they impact only the top end (WOT) on a slide carb.

There's a good description on the Factory Pro web site
http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html 
or
http://www.factorypro.com/tech/carbtun.html for the short version

Shims are available in several places including here:
http://exhaustgas.com/ProductDetail.asp?ProductID=175&DepartmentID=1&CategoryID=61&RepID=&BasketID=
« Last Edit: May 18, 2011, 01:56:02 by teazer »

Offline SrgtBear

  • Posts: 692
  • The arsenic has oddly shaped feet...
This helps and yes.  Main jets have been changed.  I am going to work on fine tuning it soon and with the main jets changed i noticed a loss of 1/4 to 3/4 throttle.  It will rev low with a role of the throttle up to 2,500 rpms then it likes to bounce up to 5,000 and above.  So I am assuming the needle is where my problem lies.  When not in gear there is no slow throttle response that can gradually rev from 2500 to 5000 rpms.
Ted

Kawasaki Kaiju (怪獣) - 79 KZ650 - In Progress - http://www.dotheton.com/forum/index.php?topic=39748.0
75 Yamaha XS500B Cafe - Completed - http://www.dotheton.com/forum/index.php?topic=14392.0
74 Honda XL 125 - Original - http://www.dotheton.com/forum/index.php?topic=25706.0

Offline crazypj

  • Posts: 10555
  • Split personality, I fake being smart
It's probably too rich not too lean in mid range.
 common problem when you fit pods to CV carbs.
 I make air correctors for CB360 but no idea what you'll need for 500 Yam?
'you can take my word for it or argue until you find out I'm right'
I gave my girlfriend an orgasm the other night, but, she spat it back at me
Some people die at 25 and arenít buried until 75. óBenjamin Franklin
CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline teazer

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  • Posts: 6589
How much did you increase the main jets and what other modifications were done to the bike - and how is the bike flat out? I also agree with PJ that it sounds as if it's gone rich through teh lower mid range.

If/when you are 110% sure the very top end at high revs is correct (main jet) then I'd try to lower the fuel level (float will be further from the base of the carb) by 1mm and see how that changes things. You may also end up drilling the bleed hole in the slide to the next size to improve pick up.

If you take the filters off and run the bike, when you blip the throttle do both slides rise reasonably quickly in synch with the throttle or are one or both sluggish?

Offline SrgtBear

  • Posts: 692
  • The arsenic has oddly shaped feet...
I am running 135 main jets (two sizes larger than stock).  I went with those jets based on a similar TX500 setup that was running pods and long strait pipes.  My setup consist of pods and full separate headers into shorty reverse cone megaphones.  It feels good.  It pulls great full open.  I just hadn't had time to ride it hard, clutch and and cut the end to check the color of the plugs.  Carbs are synced btw.

« Last Edit: May 18, 2011, 12:24:35 by SrgtBear »
Ted

Kawasaki Kaiju (怪獣) - 79 KZ650 - In Progress - http://www.dotheton.com/forum/index.php?topic=39748.0
75 Yamaha XS500B Cafe - Completed - http://www.dotheton.com/forum/index.php?topic=14392.0
74 Honda XL 125 - Original - http://www.dotheton.com/forum/index.php?topic=25706.0