Main jet sizes

CBX Steve · Post by **CBX Steve** » Tue Sep 05, 2006 5:05 am

Someone may have the answer.

If a main jet has a size quoted as #95, what is the actual diameter of that jet size? It does not match to a #95 drill size.

EMS · Post by **EMS** » Tue Sep 05, 2006 7:53 am

0.95mm??

alimey4u2 · Post by **alimey4u2** » Tue Sep 05, 2006 5:34 pm

Actually a measurement of flow capacity, not diameter.......

Sorry Mike.......

EMS · Post by **EMS** » Tue Sep 05, 2006 9:02 pm

So what does the 95 stand for? 95 of what per what?

415ZR1 · Post by **415ZR1** » Tue Sep 05, 2006 10:48 pm

Been watching

alimey4u2 · Post by **alimey4u2** » Wed Sep 06, 2006 4:00 am

I would be very interested in this too 415ZR1 , comparing theoretical to reality.....

Mike Nixon · Post by **Mike Nixon** » Sun Sep 10, 2006 12:24 am

alimey4u2 wrote:Actually a measurement of flow capacity, not diameter.......

Sorry Mike.......

Well, I guess i'll highjack also... Keihin street bike jets are indeed diameter sized. A 100 is 1.00mm. A 95 is 0.95mm. There are of course flow rated jets, and they are used mainly in racing carburetors, or in many cases carbs that are just designed to be fiddled with, racing or not. The reason there are these two systems is because when you start playing with changes to a diameter rated jet, desired changes to its flow do not directly correspond to changes in its size. There's that pesky old rule about the flow changing some 4 plus times with each square of the diameter, if I remember it right. With the flow rated jet however, a desired change in flow is closely matched mathematically to a change in size. As far as I know, these are the only two jet sizing systems that exist. As for Dynojet, nominally at least, they are identical to Keihin. Dynojet likes to insist however that since they use a different entrance and exit chamfer spec, that their jets, though sized identically to Honda's Keihin street bike (mm) system, that they in fact flow differently. Can't prove it by me, but that is what they claim. :) For all intents and purposes however, they're the same (Honda Keihin mm jets and Dynojet replacement jets). The subject of jet drills is another big one, but I'll save that for later. Oh, one more thing. Among the two systems, the diameter (whether mm or inch) and flow, there are unfortunately at least one sample of each radically different type that have very nearly the same thread spec -- i.e. diameter and pitch. Really unfortunately, these two samples that are close are so close that many folks have succeeded in cross-threading them. They're just alike to be honestly mixed up, and enough different to be heart-breaking.

CBX Steve · Post by **CBX Steve** » Sun Sep 10, 2006 1:03 am

Mike, thanks for the explaination.

alimey4u2 · Post by **alimey4u2** » Sun Sep 10, 2006 3:12 pm

Not trusting my memory, I contacted "Sudco" & they confirmed it was a flow measurement for Keihins. However, knowing Mike N's reputation as the guru of carbs, I would go with his answer.........

Mike Nixon · Post by **Mike Nixon** » Sun Sep 10, 2006 5:47 pm

alimey4u2 wrote:Not trusting my memory, I contacted "Sudco" & they confirmed it was a flow measurement for Keihins. However, knowing Mike N's reputation as the guru of carbs, I would go with his answer.........

Appreciate the vote of confidence. :) Not to make things any more complicated, bu actually, Keihin carbs have both systems, that is, both diameter and flow. Most of the road bikes use the diameter jetting, but offroad bikes are usually built with flow system jetting. Mikuni on the other hand, as I recall, uses the flow rate system for 90% of their carbs, no matter what they go on. Used to be able to divide the two systems easily by the outside shape of the jet. Back in the 60s, everyone "knew" that round jets were diameter system and hex were flow. But that wasn't really true because Mikuni had two different round jets styles. The really small round jet was dia based and the larger bodied round jet was flow. So the only real division that is consistent is street and offroad, at least for Keihin. Most of Mikuni's carbs, whether on or offroad, are flow based. Most, but not all. Confused yet? This is why I *always* check jets for size physically when working with carburetors. The only thing we can say relative to our bikes that is constant is the all of Honda's big road bike carbs use diameter based Keihin jets.

alimey4u2 · Post by **alimey4u2** » Sun Sep 10, 2006 6:36 pm

That therefore explains the confusion Mike, thanks for the input, I have learned something else tonight......

EMS · Post by **EMS** » Tue Sep 12, 2006 4:18 am

Feeling a little better about my original post in this thread now, I would still be interested in knowing what a "flow" rating on a jet is expressing...

Is it grams per second? Milliliters per minute? Or is it just a refernce number not meaning anything measurable?

EMS · Post by **EMS** » Tue Sep 12, 2006 4:18 am

Feeling a little better about my original post in this thread now, I would still be interested in knowing what a "flow" rating on a jet is expressing...

Is it grams per second? Milliliters per minute? Or is it just a refernce number not meaning anything measurable?

alimey4u2 · Post by **alimey4u2** » Sun Sep 17, 2006 5:33 pm

Mike, My original question to Sudco was questioning orifice diameter or flow in milliliters/min but their response was kind off short & sweet, it being: "flow".....

However for "flow" to be of any mathematical consequence it "should" be in mass per unit time, or am I being too methodical here.......

All that being said, I would "guestimate" it as milliliters/min.

Mike Nixon · Post by **Mike Nixon** » Sun Sep 17, 2006 6:44 pm

alimey4u2 wrote:Mike, My original question to Sudco was questioning orifice diameter or flow in milliliters/min but their response was kind off short & sweet, it being: "flow".....

However for "flow" to be of any mathematical consequence it "should" be in mass per unit time, or am I being too methodical here.......

All that being said, I would "guestimate" it as milliliters/min.

I have heard it is ml/min also. But I have not had it confirmed by an authoritative source. And like EMS, I would suspect that the number stamped on the jet is in that case derivitive, not directly referenced to said millilitres. :)