Does needle straight diameter affect idle mixture?
- Thread starter Davehuge
- Start date
Jakobi
Super Moderator
Well, its a loaded question Dave.
The answer should be YES!
But it depends on where the idle screw is (height of the slide) and also the length of the straight section on the particular needle you are using.
Ideally at idle the straight section will be sitting in the nozzle/needle jet, restricting fuel pickup into the body. As you open the throttle and lift the slide the needle also rises in the nozzle and moves on to the tapers and increases fuel supply.
The pilot and airscrew are a totally independent circuit, however the combined effect still influences the idle mixture; therefore the diameter does affect this.
Times that it might not though, are usually when using a needle such as an N1EF which has a very short straight section before it leads into its taper. With this needle you can easily wind the idle screw in and lift the needle high enough to end up on the taper.
The answer should be YES!
But it depends on where the idle screw is (height of the slide) and also the length of the straight section on the particular needle you are using.
Ideally at idle the straight section will be sitting in the nozzle/needle jet, restricting fuel pickup into the body. As you open the throttle and lift the slide the needle also rises in the nozzle and moves on to the tapers and increases fuel supply.
The pilot and airscrew are a totally independent circuit, however the combined effect still influences the idle mixture; therefore the diameter does affect this.
Times that it might not though, are usually when using a needle such as an N1EF which has a very short straight section before it leads into its taper. With this needle you can easily wind the idle screw in and lift the needle high enough to end up on the taper.
(F5)
Gold Level Site Supporter
As above of course and ideally there is only flow through the relatively high velocity closed throttle so all through the pilot jet. But some will of course come past the needle. If the needle is too thin it will be fat wet drops that don't atomise very well in limited volume flow.
Thus a rich wet mixture you can't cure with progressively smaller pilot jets the change needle you may need to go up in pilot if you see what I mean
Thus a rich wet mixture you can't cure with progressively smaller pilot jets the change needle you may need to go up in pilot if you see what I mean
uawelder
New member
If you want a visual representation, check out this thread from ThumperTalk in 2010. I have this saved to my favorites, I wish that they had included the effects of the air screw in the first chart.
Enjoy
Enjoy
HTML:
https://thumpertalk.com/forums/topic/812931-jetting-a-little-closer-look/
Davehuge
Member
Wow, thanks for sharing that uawelder, just spent 30 minutes reading it and getting my head around it. Very interesting.
Whoever hallsy is, he's definitely put some work into that post.
So assuming that info is all correct...
I can see that by fitting a larger diameter/leaner needle with the same clip position, and all other components remaining the same...the only effect it has is to lean off the first 1/16 of throttle opening! Displayed by the red dotted line.
To be honest, I was expecting a larger/leaner needle to lean off the mixture over most of the throttle positions, obviously I was mistaken.
Whoever hallsy is, he's definitely put some work into that post.
So assuming that info is all correct...
I can see that by fitting a larger diameter/leaner needle with the same clip position, and all other components remaining the same...the only effect it has is to lean off the first 1/16 of throttle opening! Displayed by the red dotted line.
To be honest, I was expecting a larger/leaner needle to lean off the mixture over most of the throttle positions, obviously I was mistaken.
uawelder
New member
Your welcome.
The first chart was written by James Dean from JDjetting. I think that the needle diameter has more effect than that chart shows (should be at second mark -4 and extend to 1/8 throttle not 1/16). Look at drawings he made later in the thread.
I try to research first before I experiment. This usually results in saving time and money for me. Not always though (lectron was a very expensive mistake).
I will look for smart/knowledgeable people and then search their past posts. This seems to work better for me than searching by subject.
Jakobi being a perfect example on this forum and hallsy on ThumperTalk. There are many others, but half the fun is in finding them.
Good luck in your research.
The first chart was written by James Dean from JDjetting. I think that the needle diameter has more effect than that chart shows (should be at second mark -4 and extend to 1/8 throttle not 1/16). Look at drawings he made later in the thread.
I try to research first before I experiment. This usually results in saving time and money for me. Not always though (lectron was a very expensive mistake).
I will look for smart/knowledgeable people and then search their past posts. This seems to work better for me than searching by subject.
Jakobi being a perfect example on this forum and hallsy on ThumperTalk. There are many others, but half the fun is in finding them.
Good luck in your research.
Jakobi
Super Moderator
Thanks for the kind words.
I agree re the graph you attached. I feel the change of the diameter much more than that small window.. A few needle changes will highlight the effect quickly in real terms.
The length of that particular needles straight section will also influence it, and depending where you have it clip wise also changes the timing in which it moves off the straight into the taper to a certain degree as well.
Another good experiment is to use a bit of marker paint or the likes and index the throttle against the throttle body to give a visual of how much you actually turn the throttle when riding. Can help pinpoint certain circuits that may feel a bit off.
I agree re the graph you attached. I feel the change of the diameter much more than that small window.. A few needle changes will highlight the effect quickly in real terms.
The length of that particular needles straight section will also influence it, and depending where you have it clip wise also changes the timing in which it moves off the straight into the taper to a certain degree as well.
Another good experiment is to use a bit of marker paint or the likes and index the throttle against the throttle body to give a visual of how much you actually turn the throttle when riding. Can help pinpoint certain circuits that may feel a bit off.
I totally agree with uawelder and Jakobi. Graphs dont show it all. I had a similar understanding problem, but James (Dean) could clarify it. Its great that he's close to the base and you can talk to him though he is very busy.
First is, you dont know what a 1/16 or a 1/8 throttle opening definitely is. So we talk about what we feel, but thats estimated.
Therefore it is indeed a very good idea to mark the throttle grip to see what the values really are. You will immediately notice how little 1/16 and 1/8 is.
Though that sounds very simple it helps to better understand what throttle openings you feel vs how much the throttle is opened in reality.
That is because what you see is not what you get
Especially when comparing graphs. The straight diameter is very short and even it it seems the needle is straight further down its length it isnt. But as most people you can feel that - as one of the guys already said - the straight diameter effects the jetting further than 1/16, probably up to 1/8 throttle opening.Just as a side note concerninj JD:
One of the major improvements of JD's needles is, that you can quickly get rid of a possible low down burble, caused by a too rich mixture due to a too short straight portion of the needle.
Slower riders, doing lots of tricky tech stuff have more problems with this effect than fast riders or racers never being at 1/8 throttle opening longer than half a second.
Second thing is, that JD leans the mixture over 70% of the rev range by simply using thicker needles. [As far as it concerns JD kit for GG 2018 EC/GP, it may be different on other bikes/carbs]
This lets the engine react crisp and makes it feel more responsive. And it is the reason why he recommends a bigger main jet than standard. JD178 vs 170/172 standard, depending on the country the bike is delivered to.
