mikuni/keihin jet sizes

[elliottles1]

Good day folks,

does anyone out there have any idea about comparable jet sizes from mikuni to keihin? there is a conversion chart going around online that seems to be miles out, showing only around a 15percent difference.
from the jets that i have at home the hole in the keihin seems to be approx twice the size as mikuni for the same jet size, if that makes sense.
i am building bikes and replacing knackered old mikuni carbs with new oko's and would like a rough start point.
anyone have any solid information?

cheers
les.

[teazer]

Basically, Mikuni HEX jets are rated in flow, where Keihin are the diameter.  At least that's true on older carbs.

Mikuni Round jets are close to diameters as keihin jets, so if you increase a Hex jet from say 100 to 200 that doubles flow but with round mikuni or keihin it basically quadruples flow.  a #200 Keihin is 2.0mm and roughly the same flow as as a #400 Hex jet.

I have no idea what size system OKO used.  I thought that they were a Mikuni carb body copy but used Keihin type and sized jets, but that's what I think I remember I read somewhere, so you need to ask what jets are in OKO carbs.

[m in sc]

okos should use the keihin protocols. if they are 'real' okos, they should use keihin long hex. just like a pwk. there are knockoff okos believe it or not.

[elliottles1]

hi,
yup, i am using OKO with keihin long hex jets.
So we could say that similar marked keihin jets are roughly double the flow of hex mikuni.
cheers
les.

[m in sc]

thats not how it works.

use the same series jet.  reason being that the length of the drilling and where its beveled also dictates flow series to series.
  Also, theres a correlation between vacuum signal and speed in the circuit and how much volume will be carried thru the jet. for example, if you have a carb thats running a 220 main jet, and stuff a 440 in it, that doesnt not nec mean you will get 2x the flow. there is a relation where as the 'bleed' on the venturi signal (the jet) gets larger, the flow rate of that jet compared to the same vacuum signal will decrease percentage wise.

I assume you are trying to use.. round keihin jets? dont.

[irk_miller]

thats not how it works.

use the same series jet.  reason being that the length of the drilling and where its beveled also dictates flow series to series.
  Also, theres a correlation between vacuum signal and speed in the circuit and how much volume will be carried thru the jet. for example, if you have a carb thats running a 220 main jet, and stuff a 440 in it, that doesnt not nec mean you will get 2x the flow. there is a relation where as the 'bleed' on the venturi signal (the jet) gets larger, the flow rate of that jet compared to the same vacuum signal will decrease percentage wise.

I assume you are trying to use.. round keihin jets? dont.

You forgot the part about weight of fuel and atmospheric pressure.  Fuel weight equals jet area x the square root of head pressure times fuel density.  LOL.  The simplified descriptions above work for the purpose of this thread, IMO. 

[m in sc]

I didn't feel it was nec to illustrate the point, but yes. Since fuel weight and atmospheric pressure are going to be a constant, for the most part, didn't think it was worth mentioning.

FWIW,  years ago when I 1st started playing with pwks/okos, i DID think the same thing, and actually sacrificed 2 long hex jets, tapped them to take round jets. I got the bike dialed in that way, but when i tried to do the 'conversion' it was way off, and just had to buy a stack of long hex jets.

this is from.. 2007? okos from CPW. they should def take genuine keihin brass.

[elliottles1]

Ive taken all that in. i will dial each bike in in the usual way, was just looking for a better stab at a "start point"  as i had begun to think that a similar stamped jet from one to the other might be waaay off. i know more  now.
thanks again
les.

[teazer]

Back to the "double" thing.  As someone mentioned, it's not like that.  at roughly #100, both types flow the same.  at #200, the Mikuni will flow twice as much as the #100, but the Keihin will flow closer to 4 times as much.  The difference in flow increase as numbers (sizes) increase.

Here's the old Round Jet to Hex jet Mikuni chart to illustrate how it works.  But those are not keihn jets, just similar characteristics.

On CB750 carbs we ran #120 with race gas and #195 with Methanol which was equivalent to roughly #400 or more in Amal main jets. Numbers are just a place to get roughly into the ballpark as you just posted.

[irk_miller]

Anytime you are dealing with exponents, as all these formulas have, it's never "double".  I just thought we were looking for a baseline, so that's why my smart ass comment.  You guys always provide the science. 

[m in sc]

love the comments! I tend to take this shit too serious sometimes