cutting a #6 slide to #7 tips..?

I ride a 99 ktm 250exc and im trying to clean up the jetting some as i know these bikes are rich off idle. ive already ordered a single taper needle, and i also want a #7 slide as ive heard those help alot too and right now i have a #6, i just dont want to pay the price of a # 7. Some people have recommended that I can just file mine to a #7..? is this something that is possible, and how would I go about filing it, is there a way to measure it (for example, 6mm to 7mm, or is that not how its determined?)  any tips are much appreciated. thanks.

once you do it there is no going back, maybe better to find a slide to try 1st,  or try to jet around the area with other circuits. have you checked your reeds

well my bike still runs good, i have just heard that these bikes run rich with the #6 slide and triple taper needle that they have. the bike does seem rich some at the low end and i get quite a bit of spooge but its not to the point of being bad reeds or anything. im trying a single taper needle but i want to try a #7 slide as well but they are pricey for what they are

This is an excellent discussion on how to measure the slide cut. If you aren't already a member you may need to join KTMTalk in order to read.

For example a 7 slide will actually measure 7.6 MM, your 6 should measure 6.6 MM. It's almost impossible to get the factory spec. Your interested in the bottom edge of the cut, not the top as many seem to think. I would do a little at a time and test. Maybe even 6.5 MM which would measure 7.1 MM. As mentioned there is no going back if you go to far. It's common to cut the slide, but it is a precise measurement and should only be attempted if your quite handy with tools and measurements. 


The most common way is to use the sanding drum accessory on a Dremel.

If your uncomfortable cutting the slide I'd look for a used one on Ebay or elsewhere. 

You can also buy a new one from JD Jetting for around $60.


I agree your #6 is quite rich and a 7 will go a long way to clean up your low throttle. In the early days it was common to use rich slides and rich diameter needles. It's great for launch out of corners and aggressive riding but is a headache for anything less.

A tip, change one thing and only one thing at a time with jetting. Change the slide first before changing needles. If your current needle happens to be NOZx or N85x, then maybe you should just change the needle first. These series needles are known to be very rich at low throttle. The single taper Sudco needles are very easy to understand and work with and also cheap, so they won't break the bank if you need to order some leaner diameters.

thanks for the great info jersey_devil! and reguarding the needle, my bike currently has an n85d needle which i know is very rich, i just ordered a sudco cek needle that i want to try. do you know if that would clean up the rich jetting alot, or would i need the #7 slide to go with it? i dont need the bike to be as clean as possible that i could probably get from a slide and needle change, i just want it better then it is now. would i be able to accomplish that with just a needle change? thanks.


also, if i decided to cut my slide, what exactly on the slide do i measure. from which point to which point is supposed to measure 6.6mm, ect?

and does anyone know if a local fabrication/performance shop would be able to cut it to a #7, and what a ballpark price would be?

Buy one!

well to buy one its between 50-60 bucks for one, so im just seeing if i can get something done to my current one for cheaper.

RB Designs will do a pro job for you, but after popping for that and 2x shipping you might wish you had just bought another - hell it's less than a tire anymore.

Sometimes a fresh top end cleans the jetting right up?  not saying the OP's  '99 bike needs it.

the top end was just done last season and has great compression, just about have to stand on the kick starter and starts first kick cold so i know thats good

The info you requested on cutting the slide is contained in the link I provided. A picture is worth a thousand words and there is still misunderstanding.

In the photograph you need to be concerned with B to E, bottom of the cut to bottom of the slide.  I place the slide on a smooth flat table edge and use the external jaws of a digital vernier caliper. This is not the most precise way as you must "eyeball" the center which is curved, but you'll be close.

In the picture D & E appear different but in reality are the same. Even pictures can be misleading.




Look for a used slide on Ebay, or a bike website's classified section. A "fair" price for one that is in good shape should be between $25 to $35. 

You can send it off, but as mentioned with the shipping both ways you might as well add another $15 or so and buy a new one from JD Jetting. Most shops will charge about $25.


Your issue is 2 fold. First that series needle, it's the older brother to the NOZx series, has a very rich taper right off idle. Second the 6 slide. To really clean up the off idle you really need to do both. The Sudco CEK or CEL will go about half way. It may be enough to satisfy you, but the leaner slide completes the transition. You can change either or though. When you use the Sudco C series, bump up the mj about 2 sizes. The C series have a lean tip diameter and you need a richer mj to compensate. You'll see the difference if you hold your N85 against the CEK.

Edited by jersey_devil

thanks for the great information! one question tho out of curiosity, whats the difference between the CEK and CEL needle, thanks.

last letter

k- richer just off idle towards main jet

L- leaner just off idle than K

BUT I'M NO EXPERT lol   it could be that the last letter indicates needle's top diameter A being richest and Z being leanest, top diameter and pilot jet functions are a little related

thanks for the great information! one question tho out of curiosity, whats the difference between the CEK and CEL needle, thanks.

As mentioned the last letter denotes needle starting diameter, the higher in the alphebet the fatter or leaner. With the Sudco single taper needles, needle diameter affects idle to just short of 1/4 throttle only. CEL is one step leaner diameter than CEK. 

Low throttle is the most difficult to tune as so many parts interact. 

The pj and a/s affect 0 to 1/4 throttle, needle diameter from 0 to 1/4, slide, mainly from 0 to 1/4+, clip position from about 1/8 to 1/2.

so guys i just got my parts in the mail today. I got a sudco CEK needle along with a 7.0 slide and a 175 main and 45 pilot jet. One question though is what do you guys recommend for a clip position for the CEK needle (im thinking 3 or 4)? im running at sea level, in around 60-80 degrees and running 44:1.

start in the mid position then experiment down or up see what you like

i have mine in the 3rd clip (from the top) for now and ill see how that works out. does anyone have any input on what they run theres at?

does that sound too lean to you guys, and maybe i should try the 4th clip, or is 3 okay? thanks

I have one of these bikes and I was able to clean up the jetting with a 45/175 #6 CEK 3rd or 4th position. I'd be interested in hearing how you like the #7 slide since I always wondered how it runs with a #7. But the CEK cleaned up the spooge so I stopped there.

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