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Old 02-16-2015, 09:30 PM   #1
pistole OP
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Tuning a KTM Two Stroke For Less of a Hit

hi fellas.

I ride mainly single track and some pretty hardcore trails. My principal tool is a 2014 Husaberg FE450 (four stroke).

just added a 2015 KTM 250 EXC (two stroke).

the 450 is tame in comparison with 250 [ ] , and I think I may need to dial down the hit with the two stroke when she comes on her pipe ... frankly , the bike is scary.

so , did a bit of reading here and there was this post about ways to tune down a KTM two stroke , and his list was :

FMF Gnarley with the spark arrestor muffler.
Run the green spring.
Turn the spring carriage all the way in.
Disconnect the power wire.
Run a slightly richer main jet.
Steahly flywheel.
Extra base gasket.

Am keen to learn more about the power valve thing (I come from a karting background and adjustable power valves on two stroke engines are common).

the power valve on the KTM isn't readily adjustable (like a knob on a Rotax 125 Max karting engine) and apparently we have to mess with spring combinations.

any experience with how this changing of springs can be done , alternatives to buying KTM's springs , and how the different springs affect the was the power is delivered ?

thanks in advance.
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Old 02-16-2015, 09:49 PM   #2
pistole OP
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Old 02-16-2015, 09:56 PM   #3
jasonmt
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I am confused by "the power valve on the KTM isn't readily adjustable (like a knob on a Rotax 125 Max karting engine) and apparently we have to mess with spring combinations."






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Old 02-16-2015, 10:01 PM   #4
pistole OP
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thanks for that post.

I noticed that brass nut on the PV housing but in my owners manual , it only speaks of PV adjustment using different springs (the three springs , Red , Green and Yellow)

nothing is mentioned about turning that nut.
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Old 02-16-2015, 10:08 PM   #5
neepuk
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The adjuster nut will make very minimal adjustment. It takes about 5 minutes to pop in the green spring, and that will make an adjustment you can feel right away.
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Old 02-16-2015, 10:30 PM   #6
jasonmt
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Changing the PV aux springs will make a larger difference in PV operation and engine performance characteristics than adjustment of the aux springs preload will but you can definitely feel the difference between red full out, red flush and red full in. My bike has had a red spring in since before I rode the bike for the first time so I cannot comment on green / blue / yellow springs.

Red Spring- Screw Settings Low(in), Med, Hi(out)



Green Spring- Screw Settings Low(in), Med, Hi(out)

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Old 02-16-2015, 11:02 PM   #7
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You don't turn in the spring adjuster "all the way" There's a point where it will screw in too far and there's no threads to hold it. You'll be pulling the sidecover to fish it out.

A JD jetting kit will richen up the lower RPMs and help the ridability and control to an extent when compared to KTMs jetting. I wouldn't add a base gasket, yes it will raise the port timing but it will also increase the squish which will effect good combustion and result in a more difficult motor to jet. Your complaint is nothing new. Most riders take advantage of the bike's torque, ride a gear higher or drop a tooth on the rear. Also a throttle cam will help allot. I wouldn't suggest detuning the motor, just make it operate within your capabilities and that usally involves mods that make the motors power more controllable, not reduce it.
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Old 02-17-2015, 05:19 AM   #8
Grreatdog
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The first thing he listed is probably where to start. It certainly is before dicking around with base gaskets and jetting. Switching to a Gnarly expansion chamber will make an immediate huge difference. It completely changed my 200. The flywheel weight made no real difference and the various powervalve settings just moved the hard hit around.

The Gnarly pipe all but eliminated the hard hit and moved the power way down low - down to four stroke low. Too much so with the later opening powervalve settings. I ended up at the earliest opening to get some top end rush back. The Steahly went in the spares bin. Thing is though, I didn't care for the Gnarly pipe any place except tight trails.

So don't sell the stock pipe because you might want that hit back for some types of riding.
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Old 02-17-2015, 10:51 PM   #9
pistole OP
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Quote:
Originally Posted by Grreatdog View Post
The first thing he listed is probably where to start. It certainly is before dicking around with base gaskets and jetting. Switching to a Gnarly expansion chamber will make an immediate huge difference. It completely changed my 200. The flywheel weight made no real difference and the various powervalve settings just moved the hard hit around.

The Gnarly pipe all but eliminated the hard hit and moved the power way down low - down to four stroke low. Too much so with the later opening powervalve settings. I ended up at the earliest opening to get some top end rush back. The Steahly went in the spares bin. Thing is though, I didn't care for the Gnarly pipe any place except tight trails.

So don't sell the stock pipe because you might want that hit back for some types of riding.
thanks for reference on the Gnarly Pipe.

am reading up on it and seems a good (and relatively cheap) option (and tough for trail use).
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Old 02-18-2015, 06:07 AM   #10
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Everything you need to tame your smoker is right there. IT's called the clutch lever. YOu just need to put more seat time on your new smoker, and use the clutch lever to reign in your new thoroughbred.
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