Keihin needles diameter (PWK)

[David Trustrum]

Hi, I’m having a confusing time with tuning a bike specifying the needle reqd. Apart from the OEM Bike Manufacturers number & Sudco number the std Keihin numbering system seems fairly straight forward. Although more used to Mikuni stuff, all the Keihins I've used have been ok on needle so never had to worry much more than a taper or pos change.

Or so I thought.

The std number system is something like
DEK
The first being the taper, 2nd length where starts & 3rd being diameter.

Diameter being the most important to get right.

So some background. I am building a roadbike but with a Trinity Cheetah 496cc kit for a banshee (RZ350). I have new 35mm PWKs.

But it is very rich. I set the mains up on the dyno & the pilots until it would idle, but the needles are just too thin. My GasGas uses a PWK & the LTR jetting kit uses a thicker needle so you can control it better with the pilot cct from closed, ending up with bigger pilot jets & less large droplets being sucked through the needle. I'm down to 38 Pilots & that's pretty small.


OK so I need thicker needles so I go up a size with some OEM needles that are supposed to be leaner (thicker diameter according to conversion chart). No diff. Hmm, measure them & the DEK ones & they are both 2.68mm at the base diameter (closed throttle before taper starts).

So I order some BGNs (3 steps leaner & shallower taper on to of that). Still filthy rich. Measure those. 2.70mm. Hang on that’s virtually the same & a sight thinner than the chart says they should be. (N is supposed to be 2.725)

What’s going on? Aren’t these supposed to be thicker?

 

[fatcat216]

I don't have an answer so much as a question as to what you are measuring with and where.

K's are supposed to be 2.695 and N's 2.725. If you are getting 2.68 for a 2.695 and 2.70 for a 2.725 your measuring error variance is .015 to 0.025mm, easily captured if you are into the first degrees of taper.

The actual manufactured difference is supposed to be .03 mm. Your difference is .02, which is not virtually the same.

Are you sure you are on the L1 section when measuring?

 

[fatcat216]

Another thought is that by going with the B, you shortened the L1 length, which is the opposite of what you wanted to do, if you are running rich?

 

[David Trustrum]

Thanks for the reply.

There is a straight section before the taper starts I took several measurements along that area (all the same with dig calp) but mainly where there was a witness mark from the jet where it is when closed.

Actually the B (of BGN) is the taper not the L1 length, unless I'm wrong. The main issue is before the taper starts though as it is 'pants' 1/8 onwards. The Taper then is shallower so stays lean longer. That's how it measures & how the booklet sez.

But yes there is maybe more variance than I thought when look at the numbers (gee they don't change much), but they still seem too thin.

The OEM ones I tried were supposed to be 'L' diameter but were 2.68mm.

:think:

 

[fatcat216]

Yes, you are correct. Late night.
I'm sure this is the same chart as is in your manual? Is your DEK a Keihin needle? I don't see it listed.
http://www.carbparts.com/keihin/needles_tuning/pj.htm
or
http://www.allensperformance.co.uk/data-n427-48.html

Same info as above with some Canadian Dave flavoring thrown in.

So- what's your next step?

 

[David Trustrum]

The DEk came with the carb, it is on my list (that came with the carb). That list from CD's article has a cut & paste error, DEK is where the J column is above repeated.

The replacement BGN I got was from a distributor but the stamping isn't as neat as the first needle, so maybe a Pirate part.

Erm what's my next step?, erm, buy some brass nails & see if I can make needles on my 1930s lathe :ohmy: No.