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Thanks
Having trouble jetting my bike aswell, especially since I run some more technical stuff in cold Sweden atm. Stock jetting has worked good on faster motocross tracks earler this season but it loads up bad in the woods, Right now in on JD kit with larger main jet and its more crisp but makes tthe bike hit a lot harder. Making it also harder to ride, feels almost lean in the midrange.
The Shop
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Luxon 4-Post Bar Mounts
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Anyway, gains felt "similar" to what I got out of the YZ 250 cylinder/head mod which was 4.5 peak with extended front and back side curve. Can't say for sure it was quite that much, but my point is any decent motor guy should be able to get a reasonable gain out of that motor.
For now I am going to carry on with the untouched head. I am going to look for other issues as well. Once I have my baseline established I will bring the bike to Chad for a dyno run to see how well I got it to run and THEN I'll start throwing stuff at it.
I'l be at Pala testing tomorrow if anyone would like to come by and shoot the shit.
The head can be milled leaving the combustion chamber untouched. KTM's do have a slightly different squish profile on the later model 250SX compared to earlier models or Japanese bikes. See attached pic (courtesy of TT).
On my bike, if there were any loss up it was minimal. Going to very high compression (race gas territory) with no other changes will impact top end for sure, though. The crispness down low was also very worth the trade off to me. Further, after adding a PC exhaust I have significantly more top end and overrev than ever.
Another jetting reference point: was 50 degrees Fahr yesterday and had to tweak jetting after adding the PC pipe/silencer. Ended up with pilot at 27.5, JD blue needle on clip 4.5, and main at 450. Air screw at 2.0 was not bad, but think I could go out a turn or two to clean up off idle a bit more.
Edit: forgot to mention that I'm now running 14/48 gearing.
https://youtu.be/_-4as7XQKCw?start=8
First ride on my 17 today. 50 degrees, sea level. Ran it with the stock jetting, felt pretty good maybe a little blubbery when rolling on the throttle. Went up 1 position on the needle (second from the top) and it was crisp throughout. Running Sunoco 112 50/50 with 91. Motorex synthetic at 50:1. Vforce 4 and a turbine core 2.1 slip on. So far I’m happy with it as is, plenty of power pulled hard and smooth off the bottom.
Pit Row
If you use this carb, be sure to block off the powerjet circuit since the stock KTM ECU can not operate it. The powerjet circuit is operated by an electric soleniod which closes the circuit when the solenoid is energized, and opens the circuit when the solenoid is not powered. This means that since the circuit will never be powered closed on the KTM, it will be continuously flowing fuel unless it is blocked off and will mess up your overall jetting. A simple way to block the circuit is to remove the actual brass powerjet jet and either solder or epoxy it closed and re-install, or just remove the jet entirely and install a threaded plug.
The same procedure is also required if you're retrofitting a carb from a YZ or KX which also use a Powerjet.
http://www.rb-designs.com/rb_designs_llc_017.htm
A good number for 250's is 0.045"
4000ft
75F
6.0 slide
27.5 Pilot 3 turns out
450 Main
43-73 in POS #3
S-1 NJ
With this setup and location I could drop pilot to 25 (didn't have one but) but the bike was as smooth as it's ever been. One thing I noticed and didn't quite have time to sort out was that if I cracked the throttle from idle to WOT with the bike in neutral on the stand it didn't rev up right away. Had a momentary Waaa sound. It went waaaREEEEENG. On the track I didn't notice it though. I'm thinking the 42-75 needle will fix that. I just didn't have time to make the swap by the time I got to that point. There's certainly more that could be done and today's test should be taken with a grain of salt given the changes. I do feel the 6.0 is a step in the right direction BUT a 5.5 might be worth a shot depending on where you ride. I think we can also validate Diggers recommendation for a starting point as well, although I didn't use the S-4 NJ yet.
Are you going to Cauhilla on Monday?
EDIT: I had posted that the 4.0 slide is much better...I meant the 6.0.
RB's instructions even tell you which NAPA part numbers to use when you purchase the test solder.
MJ started with 450, but it was to lean and went to 460
PJ 32.5
AS 2.0 turns out
NJ S-4
Needle 42-75 3rd clip
Slide 5.75
Straight C12 mixed 60:1 with amsoil dominator
40*F and 650'-700'
The bike is a rocket now. I have noticed less over rev. I think I need a bigger main jet, but I didn't have one with me so I can't confirm. I'm sure once its warmer out that this jetting will be right on. No bog anywhere, no hanging idle, just crisp power at the touch of the throttle.
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