Jetting crossover chart
Banned
MotoGP
Joined: Aug 2007
Posts: 5,942
From: Lake View Terrace, CA
Also there are a couple of terms that need to be understood before any assumptions on flow rate can be made due to the tapers in a jet.
They would be:
Bernoulli's principle - Wikipedia, the free encyclopedia
Venturi effect - Wikipedia, the free encyclopedia
Orifice plate - Wikipedia, the free encyclopedia
These show how a taper (or lack of one in the case of an orifice plate) changes the flow through a restriction.
They would be:
Bernoulli's principle - Wikipedia, the free encyclopedia
Venturi effect - Wikipedia, the free encyclopedia
Orifice plate - Wikipedia, the free encyclopedia
These show how a taper (or lack of one in the case of an orifice plate) changes the flow through a restriction.
Slacker
Superstock
Superstock
Joined: Jun 2005
Posts: 489
From: San Antonio, TX
In that picture, the needles #2 and #4 were tilted slightly away from the camera, sorry for that. Even with the tilted needles, the pics came out much more readable than I had expected.
With the taking something at face value comment, well that is what brought me back to this thread. The face value of the chart has been questioned.
I thought so at first also, but after really analyzing the setup it became clear. You must look at the BASE of the jets to compare the different sizes. You are looking at the wrong end.
I did my best to eliminate any inconsistency so that the illustration could be read accurately. The needle holder I made is phenolic plate, it is as smooth as plate glass and can be cut and drilled like wood. The needle holes were drilled along a straight line and 3/8"apart. Holes were reamed first, then I lightly smoothed the surface across all 4 holes with 320 grit emery paper. The camera was on a flat surface for all pics and centered on the middle 2 needles. What I should have done differently is used a slightly smaller drill bit from my tap & die kit and could have gotten a much tighter fit on the needles. This would have eliminated the needle tilt.
If anyone is so inclined to measure other jets, I certainly welcome the input! I do not want you all to just believe one guy. Testable repeatable is the game.
Seasoned tech
SuperSport
Joined: Apr 2012
Posts: 576
From: Issaquah, WA
LOL! I was figuring the difference for the appearance in the needle length was 2 and 4 were slightly tilted away. That's kind of what it looked like.
Actually no I wasn't. I was looking at the shoulder where it tightens down to the carb. That is going to be the reference point. The only other factor we cannot see which is if the metering point in the jet is higher or lower in relation to that surface. It should be the same so that the jets flow the same however that could also be a controlling factor.
Bernoulli's effect and the Venturi effect, essentially the same thing, are constant for any jet you put in there so they are not a factor. The taper, which we've already talked about, would be a controling factor in how the jet flows. This is why when the old school guys would say to just drill the jet out if you needed a bigger size I'd cringe! Yeah it may get more fuel but now if you ever remove it you have a worthless jet as it's markings are inaccurate and it's not going to flow as designed.
Bernoulli's effect and the Venturi effect, essentially the same thing, are constant for any jet you put in there so they are not a factor. The taper, which we've already talked about, would be a controling factor in how the jet flows. This is why when the old school guys would say to just drill the jet out if you needed a bigger size I'd cringe! Yeah it may get more fuel but now if you ever remove it you have a worthless jet as it's markings are inaccurate and it's not going to flow as designed.
Senior Member
MotoGP
Joined: Jul 2010
Posts: 3,871
From: Phoenix, AZ
I would say initially it's consistent, but that doesn't mean it's not a factor...
Seasoned tech
SuperSport
Joined: Apr 2012
Posts: 576
From: Issaquah, WA
Well it's going to be the same velocity of air going through the carb under the same circumstances. So weather you've got a D or K jet in it the pressures acting on either jet will be the same. The difference is how that jet deals with those pressures by having varying orfice sizes and tapers.
Banned
MotoGP
Joined: Aug 2007
Posts: 5,942
From: Lake View Terrace, CA
It kind of sucks when you have someone on your ignore list and then they get quoted....
As for the principles I listed, if you do know anything about them you will also know that changing the angle of the taper into and out of them changes the flow rate.
Also another thing for you guys to ponder.....
Factory Pro: Producers of the EC997 Low Inertia Eddy Current Dynamometer Series and Quality MC performance products
" Unlike other jet companies (dj, rd, keyster), Factory Pro Replica jets, Keihin and Mikuni, follow the original manufacturer's size scale so the Factory Pro Replica jets are interchangeable with original jets - saving a lot of time and improving accuracy for a good tuner."
Though I guess you guys can say Marc doesn't know what he is talking about either....
As for the principles I listed, if you do know anything about them you will also know that changing the angle of the taper into and out of them changes the flow rate.
Also another thing for you guys to ponder.....
Factory Pro: Producers of the EC997 Low Inertia Eddy Current Dynamometer Series and Quality MC performance products
" Unlike other jet companies (dj, rd, keyster), Factory Pro Replica jets, Keihin and Mikuni, follow the original manufacturer's size scale so the Factory Pro Replica jets are interchangeable with original jets - saving a lot of time and improving accuracy for a good tuner."
Though I guess you guys can say Marc doesn't know what he is talking about either....
Seasoned tech
SuperSport
Joined: Apr 2012
Posts: 576
From: Issaquah, WA
It kind of sucks when you have someone on your ignore list and then they get quoted....
As for the principles I listed, if you do know anything about them you will also know that changing the angle of the taper into and out of them changes the flow rate.
Also another thing for you guys to ponder.....
Factory Pro: Producers of the EC997 Low Inertia Eddy Current Dynamometer Series and Quality MC performance products
" Unlike other jet companies (dj, rd, keyster), Factory Pro Replica jets, Keihin and Mikuni, follow the original manufacturer's size scale so the Factory Pro Replica jets are interchangeable with original jets - saving a lot of time and improving accuracy for a good tuner."
Though I guess you guys can say Marc doesn't know what he is talking about either....
As for the principles I listed, if you do know anything about them you will also know that changing the angle of the taper into and out of them changes the flow rate.
Also another thing for you guys to ponder.....
Factory Pro: Producers of the EC997 Low Inertia Eddy Current Dynamometer Series and Quality MC performance products
" Unlike other jet companies (dj, rd, keyster), Factory Pro Replica jets, Keihin and Mikuni, follow the original manufacturer's size scale so the Factory Pro Replica jets are interchangeable with original jets - saving a lot of time and improving accuracy for a good tuner."
Though I guess you guys can say Marc doesn't know what he is talking about either....
Yeah it kind of makes you look silly when you just keep repeating what he just said too. Such as we've talked about the taper of the orifice changing the flow rate several times now but you keep bringing it up like it's something new and hasn't been discussed. Also I've said that DJ makes it harder to tune unless you have all DJ jets which is why the FP jets are easier to use to tune with because they follow the factory jet sizing which you seem to think is a new concept in this thread as well.
Slacker
Superstock
Superstock
Joined: Jun 2005
Posts: 489
From: San Antonio, TX
LOL! I was figuring the difference for the appearance in the needle length was 2 and 4 were slightly tilted away. That's kind of what it looked like.
Actually no I wasn't. I was looking at the shoulder where it tightens down to the carb. That is going to be the reference point. The only other factor we cannot see which is if the metering point in the jet is higher or lower in relation to that surface. It should be the same so that the jets flow the same however that could also be a controlling factor.
Bernoulli's effect and the Venturi effect, essentially the same thing, are constant for any jet you put in there so they are not a factor. The taper, which we've already talked about, would be a controling factor in how the jet flows. This is why when the old school guys would say to just drill the jet out if you needed a bigger size I'd cringe! Yeah it may get more fuel but now if you ever remove it you have a worthless jet as it's markings are inaccurate and it's not going to flow as designed.
Actually no I wasn't. I was looking at the shoulder where it tightens down to the carb. That is going to be the reference point. The only other factor we cannot see which is if the metering point in the jet is higher or lower in relation to that surface. It should be the same so that the jets flow the same however that could also be a controlling factor.
Bernoulli's effect and the Venturi effect, essentially the same thing, are constant for any jet you put in there so they are not a factor. The taper, which we've already talked about, would be a controling factor in how the jet flows. This is why when the old school guys would say to just drill the jet out if you needed a bigger size I'd cringe! Yeah it may get more fuel but now if you ever remove it you have a worthless jet as it's markings are inaccurate and it's not going to flow as designed.
Here is a modified picture. The YELLOW lines are valid points of interest. The RED line is not valid because of the different overall jet lengths.
I cannot attest to flow rate since I am not smart enough at this point. The chart shows: Hole Size - Keihin # = DJ # = Mikuni #.
I make no claim as to flow rate, and neither does the chart. It states a direct comparison based on orifice diameter ALONE.
Mark at FactoryPro seems to be a stand-up guy. Many in the SuperHawk community have praised his kit since the beginning. His methods are scientific and totally transparent. He does not attempt to hide anything. I have not heard of anyone question his replica jet sizing, so I am not sure what you are getting at.
If you choose to cover your ears simply because you do not like certain people, then you will never be able to benefit from their contributions. You will, in time, become ignorant and will only hurt yourself.
That said, do you have any information that accurately depicts the differing flow rates of various jets? Simply stating that the effects are different is not helpful. Which flows more? And by how much??
Senior Member
MotoGP
Joined: Jul 2010
Posts: 3,871
From: Phoenix, AZ
What's needed is a micrometer or calipers to measure from the base of the threads to the shoulder of the jet that rests against the carb body, like GTS said. The lines drawn from a picture that aren't even really lined up or in perspective (leaning forward and backward) will do more harm than good IMO...
What information are we after here? Or are we just talking... cause it seems we're going in circles...?
What information are we after here? Or are we just talking... cause it seems we're going in circles...?
Seasoned tech
SuperSport
Joined: Apr 2012
Posts: 576
From: Issaquah, WA
OK so you've said something here that I think is worth noting. For starters neither of the lines on the jets you drew are valid. The line needs to be at the shoulder of the jets where they seat against the carb. That is the one point that is constant as far as the location of the jets in the carb. OK now that being said you said that the depth of the orfice was the same from the inlet side. However the inlet side is taller on the DJ jets so that tells me that the jet orfice is going to be lower in the fuel than the F or K jets. This will certainly make some difference in the amount of fuel that will flow through the jet even if they have the same diameter orfice.
Slacker
Superstock
Superstock
Joined: Jun 2005
Posts: 489
From: San Antonio, TX
I apologize for the confusion. My terminology is incorrect. When saying 'inlet side' I was referring to the slot end that is immersed in fuel, and where the fuel enters the main jet from the float bowl.
Seasoned tech
SuperSport
Joined: Apr 2012
Posts: 576
From: Issaquah, WA
Exactly as I interpreted it. So if the flange where the jet seats against the carb to the end where the screw driver goes into is longer then the end of the jet where the screwdriver sits is deeper in the fuel. Now if the orfice is the same depth when measured from that end where the screw driver goes that means the orfice is going to be lower in the fuel as well.
Seasoned tech
SuperSport
Joined: Apr 2012
Posts: 576
From: Issaquah, WA
And actually now that I think about it if the orfice is the same distance in relation to the end of the jet where the screwdriver goes between all the jets then that should be the point of reference on the needles as far as height goes on the needle for determining the orfice size.
Slacker
Superstock
Superstock
Joined: Jun 2005
Posts: 489
From: San Antonio, TX
And actually now that I think about it if the orfice is the same distance in relation to the end of the jet where the screwdriver goes between all the jets then that should be the point of reference on the needles as far as height goes on the needle for determining the orfice size.
The needle thickness at the angled yellow line measures 2.09, 2.04, 1.99, 1.94 mm
Orifice inlet depth measured again is 2.65mm.
The line in this picture approximates the orifice inlet.
Jet head is: D 4.36mm K 3.08 mm high.
Jet Overall: D 9.37mm K 8.04 mm
Which would leave the threaded portion about equal between the two.
The outlet diameter of D 185 is 2.00 mm and is the smallest bore size other than the orifices. The inlet is a bit larger on the DJ, and on the K both ends are significantly larger.
And, like you said, the DJ orifice opening is deeper in the float bowl so this could affect flow rate along with all the other variables.
Flow rate? I looked into calculating this but there are at least 2 variables which I really do not know. I am thinking more along the lines of an actual physical flow rate measurement. For this I would probably need 2 jets of the same number from both makers with another pair of a lower number to calculate the flow curve (D185 vs D175 and K185 vs K175).
Am I rambling?
Senior Member
Superstock
Joined: May 2012
Posts: 397
From: UK
LOL
At the end of the day it does not really matter what markings are on the jet whether it's a Dynojet or Keihin. When tunning our bikes we should stick to the same style (brand) and not mix. We will either go by the seat of our pants and judge if we need to go richer or leaner and go up or down a size accordingly or go by whatever the dynochart reads.
It makes sense for the likes of Dynojet to keep their scales in comparison to other brands a closely guarded secret or even change them from time to time in an attempt to stop them being copied by another company. And besides as has been previously mentioned and from what we are led to believe both style of jets behave in a totally different way. The Keihin being a more straight through bore and the Dynojets being tapered to give venturi effect on the flow rate.
The only reason I can see for wanting a conversion is if someone says they have so and so size Dynojets in thier bike and you want to convert that to the Keihin equivalent to try in ones own bike. But that would not work as they behave in a totally different way and everyone's tuning requirements will be different due to wear and tear, altitude etc.
Saying that it is very interesting trying to work out the differences and how they behave compared to one another. And as for measuring them you would need specialist tools and equiptment for getting any accurate measurements especially when they are tapered. Don't forget even a 1000th of an inch can make a difference when it come to tuning.
(:-})
At the end of the day it does not really matter what markings are on the jet whether it's a Dynojet or Keihin. When tunning our bikes we should stick to the same style (brand) and not mix. We will either go by the seat of our pants and judge if we need to go richer or leaner and go up or down a size accordingly or go by whatever the dynochart reads.
It makes sense for the likes of Dynojet to keep their scales in comparison to other brands a closely guarded secret or even change them from time to time in an attempt to stop them being copied by another company. And besides as has been previously mentioned and from what we are led to believe both style of jets behave in a totally different way. The Keihin being a more straight through bore and the Dynojets being tapered to give venturi effect on the flow rate.
The only reason I can see for wanting a conversion is if someone says they have so and so size Dynojets in thier bike and you want to convert that to the Keihin equivalent to try in ones own bike. But that would not work as they behave in a totally different way and everyone's tuning requirements will be different due to wear and tear, altitude etc.
Saying that it is very interesting trying to work out the differences and how they behave compared to one another. And as for measuring them you would need specialist tools and equiptment for getting any accurate measurements especially when they are tapered. Don't forget even a 1000th of an inch can make a difference when it come to tuning.
(:-})
Seasoned tech
SuperSport
Joined: Apr 2012
Posts: 576
From: Issaquah, WA
Dynojet, Keihin, and Factory Pro all use a taper in the orificeof their jets.
There is no secret algorithm to making a jet. It’s an orifice that allows a given amount offuel or air through to properly mix fuel. To try to make them different in an effort to keep some kind of secretmakes no sense. And to change it fromtime to time would be a kiss of death as those who buy their product and thenneed to re-jet for some reason would be lost and no one would buy their stuffknowing it was going to be even more of a hassle than it already is.
The conversion is 100% relevant as if someone has mistakenlypurchased a Dynojet kit and then needs to re-jet and the local shop doesn’tcarry an assortment of Dynojet jets they are forced to use factory jets andneed to have a clue where to start.
There is no secret algorithm to making a jet. It’s an orifice that allows a given amount offuel or air through to properly mix fuel. To try to make them different in an effort to keep some kind of secretmakes no sense. And to change it fromtime to time would be a kiss of death as those who buy their product and thenneed to re-jet for some reason would be lost and no one would buy their stuffknowing it was going to be even more of a hassle than it already is.
The conversion is 100% relevant as if someone has mistakenlypurchased a Dynojet kit and then needs to re-jet and the local shop doesn’tcarry an assortment of Dynojet jets they are forced to use factory jets andneed to have a clue where to start.
Junior Member
Squid
Joined: Feb 2025
Posts: 1
Jetting
I had a short discussion earlier in which main jet orifices were brought up. I had previously (several years ago) held several jets up to light to try and gauge the different sizes by looking through each one and *thought* that the Keihin vs DJ looked very very close in relation to the numbering listed on each jet.
Today I became curious so went around looking for various things to use as a jet gauge. Nails, hex keys, pins, needles, paperclips, precision screwdrivers, plastic dart tips, and torch tip cleaners. The jets available were DynoJet 190 and 185, and Keihin 180.
I was able to pass one particular screwdriver shank through the DJ190, but it would NOT go through either of the other 2. Then used a 1/16" hex key that the K 180 fit onto easily with room to spare and a SLIGHT wobble. The DJ 185 also fit the hex key but had a tiny bit more wobble. The DJ 185 is apparently larger than the K 180.
Micrometer shows screwdriver shank 1.89mm and the hex key 1.74mm at widest point.
So, for those of you who have installed DJ mains on a stock setup in place of the Keihin mains with similar numbers and have good spark plug coloring, it seems to be because they ARE THE SAME ******* SIZE.
I cannot measure the flow rate except by putting the mains in and seeing how the engine behaves. So the DJ could flow more at higher speed or more at lower speed, but judging from my experience this is not the case. Well, maybe they do flow more at low speed. The dyno charts usually show a rich flat area in the torque curve around 4-6k when DynoJet kits are installed and set for best allover performance.
So all those charts on various websites showing jet sizing cross reference, which were mostly written long ago, seem to have some seriously erroneous data.
Any thoughts?
Today I became curious so went around looking for various things to use as a jet gauge. Nails, hex keys, pins, needles, paperclips, precision screwdrivers, plastic dart tips, and torch tip cleaners. The jets available were DynoJet 190 and 185, and Keihin 180.
I was able to pass one particular screwdriver shank through the DJ190, but it would NOT go through either of the other 2. Then used a 1/16" hex key that the K 180 fit onto easily with room to spare and a SLIGHT wobble. The DJ 185 also fit the hex key but had a tiny bit more wobble. The DJ 185 is apparently larger than the K 180.
Micrometer shows screwdriver shank 1.89mm and the hex key 1.74mm at widest point.
So, for those of you who have installed DJ mains on a stock setup in place of the Keihin mains with similar numbers and have good spark plug coloring, it seems to be because they ARE THE SAME ******* SIZE.
I cannot measure the flow rate except by putting the mains in and seeing how the engine behaves. So the DJ could flow more at higher speed or more at lower speed, but judging from my experience this is not the case. Well, maybe they do flow more at low speed. The dyno charts usually show a rich flat area in the torque curve around 4-6k when DynoJet kits are installed and set for best allover performance.
So all those charts on various websites showing jet sizing cross reference, which were mostly written long ago, seem to have some seriously erroneous data.
Any thoughts?
well, I won’t say what I was gonna say just because I don’t want to offend anyone and if you have ever ridden one with a performance package that hammer puts out you will not call it that anymore there are not many girls out there who couldride this thing and even come close to pathway pushing it because the torque is off the charge, it has become a very dangerous bike for someone who does not know how to ride to get on, but for the professional and absolute blessing when someone pulls up on a big boy or my R1 and compatible, because the expression the person riding the other bike gets after that little sporty create some thunder in the scene and don’t get left out pulls off from most of the pack. Only a couple can hang with it until you hit the quarters. It’s hilarious what a funny funny situation did it create I used to think it was a sissy bike not no more be careful you run up on somebody with one of them. They’ll embarrass you no matter how good you are that bike is something that doesn’t get talked about and doesn’t get seen much, but it will definitely separate them in from the boys national sleeper have a great dayThis is a serious beast man totally changed my outlook on Harley.
Last edited by Papaklutch; Feb 4, 2025 at 02:19 PM.
Thread
Thread Starter
Forum
Replies
Last Post
