a mate of mine from the nsw coast wrote this up the other day. he has a 300 and is actually a combustion engineer. hope this sheds some light for anyone needing light shedded...
KTM 250/300 >2008 Jetting and Engine Tuning
by >moto< on Thu May 19, 2011 12:43 pm
.A massive part of getting your KTM 2 Banger to work and work well is based around the jetting. I have played extensively with this since owning my 300 and have also researched a lot to broaden my own knowledge on how to jet the bike.
There are heaps of jetting charts, tables and descriptions on the net that tell you how the relationship between the different circuits react so I won't go into that detail here. It should also be noted that there other factors that effect how you bike feels such as gearing, map switch setting and power valve settings/springs not to mention personal preference.
Here is my guide to what I have found, researched and tested. This will act as a crib sheet to help me as much as anyone else!
Needle Types
The standard needles are dual taper and have a longer initial taper due to it's steep angle making it much richer in 1/8th to 1/4 throttle area. The second taper on both needles are the same angle but the taper starts earlier on the NExx series needles. The N2xx series needles second taper then continues to the end of the needle, were as the NExx series needles have an even steeper third taper to bring more fuel in the mid to full throttle positions.
The result of these differing tapers means that the N3xx/N8xx/N2xx series needles are richer 1/8th to 1/4 throttle and leaner from about 1/3rd throttle onwards than NExx series.
This tends to make people lower the N3xx/N8xx/N2xx series needle clip positions too far to gain a clean low opening throttle response. Worse still leaning the pilot out to compensate which will kill proper throttle response.
Running the needle in a lean clip position to clean up the low throttle will lean out the rest of the throttle making the bike more difficult to ride in all but flat open terrain. As I know from riding my 450 4 banger traction and controllability is how you need your power to feed in. Super snappy 2t power is fun but will is much harder to hook-up.
So that it is easy to understand the stanadard needles and they're relationship to each other I have listed them in order. Standard needle codes in order from lean to rich in 1/2 clip increments. The exception is the N3CJ which is a full clip leaner than the N8RJ (from 1/4 to full throttle the needles are all the same):
N3CJ
N8RJ
N2ZJ
N8RW
N2ZW
N8RH
So, as mentioned above it is widely regarding that limited success can be had with the standard needles unless the slide is changed to compensate for the disparity in the standard needle tapers. Cue to the Suzuki needles!
There has been a lot of talk and testing happening in the states where triple taper Suzuki needles are being used with the stock Keihin PWK 36mm slide. The taper of the needles are such that we are able to provide a richer initial taper meaning no off idle lean bog or 1/8th throttle stutter whilst still achieving proper fueling over the rest of the needle. The result is super smooth torque and power right off to full throttle.
The specific needles being tried are from a 2008 Suzuki RM250. Again for ease of reference and understanding of which needle is rich or lean they are listed below from rich to lean in 1/2 clip increments:
NECF
NEDF
NECG
NEDG
NECH
NEDH
NECW ('W' is substituted for 'I')
NEDW ('W' is substituted for 'I')
NECJ Part Number 13383-37FE0
NEDJ
NECK
NEDK
NECL
NEDL
Needle Diameters, again, rich to lean.
NECF/NEDF 2.705
NECG/NEDG 2.715
NECH/NEDH 2.725
NECW/NEDW 2.735
NEDJ 2.745
NECJ 2.745
NECK/NEDK 2.755
NECL/NEDL 2.765
NECJ and NEDJ have been reported to be a big improvement over any of the standard needles by a number of people both in the US and and Aus. I have these needles on order an will test with them soon and provide feedback.
For a 250 the NECW and NEDW needles are the ones to try. Always start from clip 3 and with your air screw 1.5 turns out and go up if needed. If you have to go to 1/2 turn out on the air screw you need a bigger pilot jet, more than 2.5 turns out and you need a smaller jet.
As a result of the different profile of the Suzuki type triple taper needle it is likely required that for best results you will need to drop one size on the main jet (due the third taper not present on the stock dual taper needles). The current set of testers have also preferred a 38 pilot jet over the stock 35 as it feels more 'torquey'.
Power Valve Springs
The other are that can dramatically effect how your engine delivers it's power is playing with the power valve springs provided and the brass, square drive, preload screw. Contrary to popular belief the PV springs and even the preload adjuster do not alter the point at which the PV starts to open. This always occurs at 5,500RPM.
The colored spring supplied provide different rates of tension to provide an resisting force to the power valve opening. What they do control is the rev range that the PV starts to open until the point it's fully open. Below I have listed the spring from fastest to slowest opening and the RPM in which they are effective:
Red 5500 RPM - 7400 RPM
Blue (optional)
Yellow 5500 RPM - 7800 RPM
Green 5500 RPM - 8300 RPM
So you can see that the Red spring allows the bike to be at full power sooner in the RPM range than the other springs. This is why it's often referred to as the 'aggressive' spring. I disagree with this and find it to be the most linear as you have full power sooner. Through my own testing I have found that it can be aggressive when running a lean stock needle as this really saps the low end power/torque of the bike so when the the PV does come in the 'hit' feels much more pronounced.
Also the hit comes from the PV allowing full power when fully open so the Yellow and Green springs keep the bike tamer for longer but then transition quickly to where all the power is providing a decent hit of power. Again, all personal preference but give it a try, you might like it!.
