needles/jets

kwackman12 · Post by **kwackman12** » Sat Sep 26, 2009 2:40 pm

Just taken my carbs to bits on my 300cc conversion, it has 145 mains pilot jet 40, & the needle is a n99e After checking the micfofich i believe the mains should be 135's & the pilot jet 38, & the needle n68x is this correct?
Also due to it having more (air) volume 300cc? passing through the carbs should I reduce the size of the jets/needles even more? say 130, 36, & as for the needle I havent got a clue, Any help would be gratefully received.
It has always "hesitated" all the way through the rev range until about 3/4 throttle

Charles · Post by **Charles** » Sat Sep 26, 2009 6:47 pm

Hi Gary,

isn't the n99 an improved needle also used in the C3 models? Got those fitted myself too. Bought those in an original kwak set together with those 'new' design pistons (those where the piston rings enclose the pin).

Why do you want smaller jets. 300cc sucks more air into the engine so I'd suspect you need larger jets than than the standard 250cc ones?!

Charles

JanBros · Post by **JanBros** » Sat Sep 26, 2009 7:05 pm

bigger displacement means more air is passing the carb's, but automatically the air sucks up more fuel. which then again doesn't mean jet's /needles should be the same as a 250 KR1

every engine is different, so one can't say what you need, only an idication off what direction to go . you have to setup your carb's yourself

the-elf · Post by **the-elf** » Sat Sep 26, 2009 9:18 pm

Only way to get the fueling set up right is on a dyno.

Bikemike · Post by **Bikemike** » Sat Sep 26, 2009 10:21 pm

It's probably no help (as several have pointed out, each engine is individual etc) but my quite heavily tuned (but still 250cc) Graham File engine runs 142.5 mains and 40 pilots; so yours doesn't sound far out to me.
As pointed out previously the n99 needle is the improved one fitted to the C3 model, it's generally considered the one to have.

Mike

kwackman12 · Post by **kwackman12** » Sun Sep 27, 2009 12:44 am

the-elf wrote:Only way to get the fueling set up right is on a dyno.

Are there any dyno stations that carry needle/jets as I dont want to buy new one's only to find out that they are wrong.
I did have it on a rolling road a couple of years ago but I think the guy only worked on what was already in the carbs!
Thanks for the info

the-elf · Post by **the-elf** » Sun Sep 27, 2009 1:55 am

kwackman12 wrote:
the-elf wrote:Only way to get the fueling set up right is on a dyno.
Are there any dyno stations that carry needle/jets as I dont want to buy new one's only to find out that they are wrong.

Nope, but a run on a good dyno with a decent operator will be able to point you in the right direction, after that you can then target the right jets etc. For road use jetting for max power is not that important as you will not be changing the main jet every time you ride unlike us racers. But a good dyno operator will be able to point you in the right direction.

the-elf · Post by **the-elf** » Sun Sep 27, 2009 1:58 am

forgot to say. You want them to target mid range etc for road use. Look at the post in the Kr archive about carbs and there is a load of info on what bit of the carb effects the engine at different revs.

kwackman12 · Post by **kwackman12** » Sun Sep 27, 2009 3:56 pm

the-elf wrote:forgot to say. You want them to target mid range etc for road use. Look at the post in the Kr archive about carbs and there is a load of info on what bit of the carb effects the engine at different revs.

Thanks for the info "Elf" just had a read of the archive, think I will try a set of 137 mains & 38 pilot's & keep the 99 needle. Does anyone have a set they would like to sell? if not I'll contact Allan's
Will keep everyone updated as I dont think there's much info on jetting the 300 cc conversion

KR-1R · Post by **KR-1R** » Sun Sep 27, 2009 9:22 pm

The numbers on main jets typically indicate how many cc or ml the jet will flow...
so a 130 main will flow 130cc or 130ml per minute?(this is minimum for KR-1 at 250cc)
if the 300 conversion is expected to make 20% more torque/power then
you probably need jetting upto 20% richer (20% more fuel).
I might start at 150 to 155 mains (certainly dont go lower than the 145 mains you have) - if the needle is longer (or taper) than stock the main jet may still be restricted at W.O.T.

The needle and pilots wont mean much on a Dyno thats run at Wide Open Throttle (where only the main jet is governing fuel flow)
If you intend to dyno, you do need to find a dyno setup that can run at partial throttle openings (to be able to test the other various jetting components).

you can see (chart below) why a clip position change is also the easiest alteration and in testing on the road, makes the biggest difference the average rider can feel.

the top chart is generic for Keihin and include the FCR etc as well as PWK... but it gives you the idea as to what functions at what throttle opening

kwackman12 · Post by **kwackman12** » Sun Sep 27, 2009 9:52 pm

KR-1R wrote:the needle and pilots wont mean much on a Dyno thats run at Wide Open Throttle

you do need to find a dyno setup that can run at partial throttle openings.

Think this is where I went wrong on the last dyno run, it was set up for "full throttle & max power" I need to spend a bit of time gettting this right as it rides crap compared to my track bike & the RS 250 I own, been lucky with these as I havent had to touch the carburation. I want to get it sorted as I have a trip planed to the Nurburgring in October & dont want to get left behind

,

ScottaKR · Post by **ScottaKR** » Mon Sep 28, 2009 7:23 am

If it was set up for "full throttle & max power" as you say, then you definately dont want to be playing with your mains, as you may end up with it being way too lean @ WOT.
I'd try playing with the clip position on the needles as KR-1R suggested, as it has a big effect up to 3/4 throttle which is where you said it clears up.

TwoStroke Institute · Mon Sep 28, 2009 10:24 am

Keihin jets are NOT flow rated they are a size jet.Mikuni jets are flow rated ie a 200 main is 200cc per min.
The relationship between one jet size to the next on Keihin is non-linear so a 200 to 210 is more than a 5% increase in fuel.
I'm afraid the only real way to set the jetting(unless your very experienced) is with the dyno and perferably an EGT gauge.

KR-1R · Post by **KR-1R** » Mon Sep 28, 2009 9:31 pm

my bad advice then...I didnt mention or have knowledge of the static head (pressure) or procedures each factory might use to calibrate their jets to.

for a range of jets (from same manufacturer)...
If you plot Flow vs Diameter (arbitrary jetting number system) you get an upward (logarithmic) curve - non linear but demonstrates some form of relationship / formula
If you now plot the same Flow vs the calculated Area (previous Diameter converted into bore area) you get a straight line - now linear, youve resolved the relationship.
from this you could dedeuce Mikuni might be numbered according to Area (coinciding with flowrate), and Keihin might be to a diameter system

Take two orifices (A) and (B)
Make (B) 20% bigger (gross area and flow) than (A), stuff each with the same sized partial blockage eg a needle inserted (or further upsteam).
Orifice (B) may in fact now have a 30% or more real (net) area/flow advantage over (A) BUT its only rated at 20% bigger?? shows other influences.

Im sure if you plug your rated mikuni jet into a reservioir 2m above the outlet youll flow More fluid than the Same jet with a reservoir at 1m head.

I will take your advice on non linearity for sizing Keihin mains [insert bow down emoticon]

Thats all maths and an different ways of twisting information (I dont have a degree)
Education wont make one person smarter than someone with out one.
As with all information it must be tried and tested by your own belief system before you should accept it.

Lets just assume if you DONT try to plug your Mikuni jets into your Keihin carbs youll be fine.

wheres my Tampons??

Sheik Yerbouti · Post by **Sheik Yerbouti** » Tue Sep 29, 2009 10:58 am

You go girlfriend!! Is there any more coming? I'll grab some pop corn.