The folks in the title got my previous 250 spot on, thank you, but I'm going through a 125 right now and wanted to pick Vital people's brains.
I've seen the pictures and height wise and radius wise I'm happy with how much needs to come off, but how much deeper are folks going when grinding a 4.0 to 5.0 for their small bore KTMs Mikuni TMX?
Most, like Diggers pre-polished pic, look like they've had a tickle, but tough to gauge how much meat has been taken off the sloped face of the original cutout.
I know this is difficult/useless to give a legit numeric value as I'd have no way of applying these numbers to the sloped section, but is it just a light going over or is there proper material removal here?
I'm assuming the height of the arch and the radius of the arch are the important areas.
Furthermore, if I am overweight for a 125 would going up to say a 540 / 560 on the main be beneficial theoretically, as a lighter dude would in a sand situation with the bike working harder?
I appreciate everyone's work and I know it's just a base given different areas/conditions etc and some leg work will be needed, I just also know a lot of you guys are MUCH smarter than me on this front 👍
Thanks for any pointers
I try to avoid hand grinding the slides as it’s just easier to buy them. But I believe in the thread Digger says to simply blend the new, raised outer arch down into the slide going front.
As for the main, I am running a 540 almost all the time. Granted, that’s with AvGas. But it’s also at higher temps and up to 4,500ft elevation. I’ve sometimes run a 560 at lower temps and elevation.
1mm. Just blend the slope. Don’t make it wider. I use a round file. It’s actually easy to measure with a caliper.
Thanks man, appreciate that 👍
I scribed a line at the 7.6mm height from a flat bench to the top of the arch like the drawing FGR01 posted, but I may have made the radius a touch wider unfortunately, but if so it won't be a lot at all and I haven't and don't plan on notching the reed side as some have, I figured that's a big bore thing?
The sloped face 'into' the slide I just kind of blended it from where it was to where the new line is and I understand now that this is really a 'feel' thing more than anything, like old school head porting haha, although I've only ever done roughing out and had a boss/colleague do the final, tidy portion of the job.
FGR, interestingly this bike came with a 580 on it, although with an untouched 4.0 slide, stock needle jet, KTM optional needle and squashed and nasty OE jet block gasket haha, tempted to try the 580 in a cold/wet UK as the only other main I have here is a 520.
I have an untouched 5.75 here if you need it.
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Thanks, I appreciate the offer 👍
I'm in the UK though so I'd probably grab one from Frank's in Netherlands if I have balls'd this up... which is always a possibility 😂 Waiting on the needle jet and pilot to show up tomorrow before I can try it, so fingers crossed.
If I remember right one of the Japanese 125’s came with a 5 or 5.5 and you can score them for like $70. I think it was the kx?
I could be wrong, it’s been a couple years..
Good call man, thanks, I had a KX 125 5.5 slide for my 250 👍 I've gone the ghetto route on the 125 though haha
I floated mine to match how much I took off the face with a die grinder, then polished (cr250)
It’s crazy how little material you take off and how big of difference it makes.
I believe I’ve grinded it down 3-4 times now and still not even at a whole mm. Noticed a difference every time.
With the stock slide I was running a huge main and really lean on the needle. I cut it down until I was able to run the needle in the second/third position and i believe went down on the main a little
Hoping I haven't gone overboard and leaned it out too much 😂
Waiting on jet block gasket, I didn't notch the reed side at all, mind.
also when grinding 1mm off the arch do you blend it front to back following that slope as well or consistently take off 1mm along the entire length of the slope.
I'll attach a poorly drawn diagram to help convey my question the top will be a front view of the slide and the bottom drawings are a side view section view displaying the slope front to back.![]()
If I remember right, it's a a straight cylinder shape "bore" so you can use correct size socket for test fitting.
As rough example - if xx mm socket(OD) fits snug to 4.0 cutout, use xx +2mm for test fitting 5.0 shape. Also follow scraped 1mm line at the front.
Combine top right and bottom left of your drawing
So I do remove material from the sides of the arch then. I saw in a previous message someone said not to make it wider though. Opinions on this?
I originally had 4.0 and 5.5 Mikuni OEM slides but the 5.5 lost it's plating so I used it as reference for grinding(and initial test fitting with sockets) with and would say definitely it was also wider, but not deeper. So I kept the "edge of the barrel" about in the same place
Yea ok I think I got it. The depth of the cutaway basically how far into the slide it goes stays the same, like it ends on the same edge just the outer arch gets grinder larger and blended. Like you said, top right of my Pic and bottom left
I believe you have it all correct now. If you can wait until tomorrow, I will give you an exact measurement of the width of the cutout on a #5 slide.
On your question in the other thread... both the slides in that pic are #5 slides. I originally made that pic because at one point we had some worry that the #5 slides from JD had changed or they were sending out incorrectly labeled or boxed slides. That pic was to compare 2 slides from 3 years apart to show they were identical.
Pit Row
If you could measure the width that would be great!
Sorry for the delay. Made some measurements on Mikuni slides. Did them all while I was at it for convenience sake
4.0 6.15H 25.16W
5.0 7.60H 26.63W
5.5 8.35H 27.40W
5.75 8.64H 27.50W
This is pretty accurate, as well as I can get with a caliper. Going from a 4.0 to a 5.0 is roughly 1.5mm all around,
Interesting, and thank you for taking your time to measure! Just curious however that previous messages all said take 1mm off to get from a 4 to a 5. Im assuming your measurements are closer to what it should be?
I like to think mine are closer, since I sat her with a caliper doing it. But the truth is it's pretty hard to measure them exactly. I tried to be as exact and consistent slide to slide as possible. Anyone have a 3D scanner that can be super exact?
Also posted this in main thread because I need an answer rather soon, do I or do I not grind this highlighted portion of the slide to go from #4 to #5. My understanding is i don't touch this part and grind the out arch up to this point. However looks like this person did grind it.
When I ground mine I blended to that spot, I did not grind that spot.
Since this is easy to get out this is really something that you can work up to if you’re uncomfortable with the task. Grind a little and put it together and see how it runs. Still too rich grind a little more.
Ok thanks for the reply, my very close family friend is a machienest so im having him do it today after work. Ill follow the procedure outlined by others in this thread so hopefully all will go well. Probably will take of a hair under 1mm to play it safe, test then readjust if needed.
How does this look now that it's ground down and smoothed with cartridge.
I think it looks pretty good. On all the various sizes I have here, this distance of the red line looks the same, regardless of what size the slide is. So, I think you have it right... do not grind back into that part... blend to that edge.
That looks good. You don't cut into that circled area, you just change/blend that area at the front of it, like you have it.
Yup thats what I did ground up to that edge. Thanks for the help guys it looks like it turned out not too bad and am just waiting for needles now. Hope to test it soon.
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