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View Full Version : How do I mellow out a 250SX?


Team200
23-03-2012, 08:42 PM
I've got a mates KTM 250SX at my house, I think it's an '04 and holy crap the thing is an animal. It has a fresh motor and since he's just spent so much on the rebuild, he wants to keep the bike. I had chad do the suspension and it's a lot better, springs are still pretty heavy though.
So the real question is how do I make the motor more like an EXC, the owner is a similar age to me and he doesn't ride much so the thing is just too much.
I was thinking something along the lines of:

1. 1/2 tonne of fly wheel weight (how heavy is too heavy? is 12 ounces enough?)
2. more base gaskets (how far can I go before I loose too much bottom end due to the raised exhaust port and lack of comp)
3. Retard the ignition, I've heard of people doing this to YZ's, do the Kato's have enough adjustment for it to work? How far do you recommend retarding it?

Any other suggestions on how I can make it a more bush-friendly bike?

Owen

diZco
23-03-2012, 08:45 PM
Flywheel weight won't do anything... i'd look at getting it jetted properly so it's at least linear. Maybe look at taller gearing, ignition timing.. other option is the porting. Dunno bout the base gaskets.. never tried it.

Or stick an oily rag in the muffler. :)

Mine was an animal.. but not alot I did changed it.

You can adjust the powervalve spring tension, but you'll just delay the inneviatable hit.

Just undo the engine mount bolts, take out the motor, put in a 250sxf motor, and you should find it alot more mellow.

TonyH
23-03-2012, 08:51 PM
Is the expansion chamber different between the two? ^dunno

Maybe shoehorn a header pipe from a 250 exc-f :laughing-

dmcca
23-03-2012, 09:01 PM
Where is it hitting too hard? Off idle or as it comes on the pipe in the midrange?

Adding base gaskets will make it worse cause it'll reduce bottom end and you'll get a bigger top end hit as the pv opens. Adding fww is a good thing. Retarding ignition will reduce bottom and add to top again, possibly making it worse if he's struggling in the midrange. Usually it'll be easier to ride by adding bottom end and taking from top end, reducing the hit as the pipe starts to work properly.

insomniac_jc
23-03-2012, 09:02 PM
2. more base gaskets (how far can I go before I loose too much bottom end due to the raised exhaust port and lack of comp)
3. Retard the ignition, I've heard of people doing this to YZ's, do the Kato's have enough adjustment for it to work? How far do you recommend retarding it?

Owen

G'day Owen

I'd probably advise against adding base gaskets. Yes it will lower the static compression but it will also advance the port timing which will do the opposite of what you want. I'd try LOWERING the cylinder as much as possible. Run as few base gaskets as you can. Get the head modified to correct the compression. Retarding timing will also soften the delivery.

brash
23-03-2012, 09:31 PM
Were the powervalves adjustable in these? On my 150sx it's a couple of turns of a nut and changes the power delivery completely, I think the 250/300 are the 3 different coloured springs. Or does this bike pre-date that.

diZco
23-03-2012, 09:38 PM
You could adjust the spring tension with the screw.. I think... wasn't it 2005 they started using different coloured springs?

Best thing for em was tall gearing I found, they are just super responsive and power and delaying or moving that power around just pronounced the hit somewhere else. So I just ran really clean jetting with standard pipe. Flywheel weight on mine did nothing to tame the responsiveness or sheer power.

Bront450
23-03-2012, 09:50 PM
You should be able to pick up a EXC or XC-W head for bugger all as everyone puts SX heads on them. 03-06 will fit. The barrel is the same on a SX as a EXC so its in the head, light crank and spark. Try to fatten up the jetting a bit like Disco said. My 09 is a pussy cat and will lug up anything, I've got the YZ neddle(forget what it is but its allover ktmtalk) and a gnarly pipe with the yellow spring on the power valve. Tried the green spring but found I was waiting for the power to come on all the time when the going opened up a bit.

diZco
23-03-2012, 09:56 PM
yeah the n3ej yz250 needle worked well in mine, spooged up a little more than the JD needle, but I thought it was nicer to ride..

Tom68
23-03-2012, 10:13 PM
Rig the power valve so it stays in the down position.

AD_KTM
23-03-2012, 10:14 PM
I have an 04 and it is a bit of an animal too and has hurt me before..but i"m a sucker for punishment and like the snappiness. Is it running stock gearing? (13/50) if so try a 14 tooth sprocket on the front. You could try changing the powervalve spring color, mine came with a red one fitted and i haven't bothered trying any others to see what difference it makes.

ggg
23-03-2012, 10:27 PM
taller gearing is the cheapest why to reduce the hit so what if tops out at 160kph

dalboy
23-03-2012, 10:33 PM
exc cdi, powervalve adjusted to hit down low. (i accidentaly put a pv spring from a 150sx in my 200 once and it went like a 250 4t, pv opened at idle and there was NO hit, maybe research/try that), fww.
i dont like the idea of changing gearing, yes it will make it less snappy, but then you'll be clutching it heaps in the tight stuff. ride a gear higher instead

oldboy
23-03-2012, 10:41 PM
I have an 04 and it is a bit of an animal too and has hurt me before..but i"m a sucker for punishment and like the snappiness. Is it running stock gearing? (13/50) if so try a 14 tooth sprocket on the front. You could try changing the powervalve spring color, mine came with a red one fitted and i haven't bothered trying any others to see what difference it makes.

Ole mate is onto it. 14 tooth front sprocket and red pv spring to make the awesome power linear. n3ej yammy yz250 needle or 2007 rm250 needle nedw. Does it have a no.7 or 6.5 slide in it ??

diZco
23-03-2012, 10:53 PM
the 14 tooth really helped with the economy too

-RinGo-
23-03-2012, 11:23 PM
You could adjust the spring tension with the screw.. I think... wasn't it 2005 they started using different coloured springs?

Best thing for em was tall gearing I found, they are just super responsive and power and delaying or moving that power around just pronounced the hit somewhere else. So I just ran really clean jetting with standard pipe. Flywheel weight on mine did nothing to tame the responsiveness or sheer power.

I agree with this - add a 2 - 3 teeth of the rear end or add 1 to the front. Makes a big difference.

You can adjust the powervalve springs from 2003 onwards. So you should be able to play around with red/yellow/green varieties on the 2004 model. Stick to the red one though, especially if you make the gearing taller.

I enjoyed playing around with YZ spec needles in my 250 SXs. I just found they smoothed the delivery of the power into band. I don't have a massive technical base for this though, its just how it felt to me. Can't remember the specs from my bikes for the life of me though!

Good luck! They are a rewarding bike to ride fast ^tng

Nutty
23-03-2012, 11:34 PM
Is the expansion chamber different between the two? ^dunno

No, single Vs double skinned, same spec.

This may sound weird....BUT....set it up to make full torque. Red PV spring, 49mm PV height, +0.10mm deck height, BR8EG-6.5 plug, 1.2mm squish clearance, 12.9:1 comp.

Then gear it up, they'll pull 14:45 without a blink. Making as much torque as possible down low negates the hit. This is how I set up Pro FMX bikes and it works perfect. Maybe a FWW but it won't be needed if the gearing is good, depending on intended use.

diZco
23-03-2012, 11:36 PM
Most of what you say sounds weird, but that sounds spot on :)

SoCalledFreeman
24-03-2012, 03:36 AM
No, single Vs double skinned, same spec.

This may sound weird....BUT....set it up to make full torque. Red PV spring, 49mm PV height, +0.10mm deck height, BR8EG-6.5 plug, 1.2mm squish clearance, 12.9:1 comp.

Then gear it up, they'll pull 14:45 without a blink. Making as much torque as possible down low negates the hit. This is how I set up Pro FMX bikes and it works perfect. Maybe a FWW but it won't be needed if the gearing is good, depending on intended use.

Sounds about right ^duh

What about the mental EXC300 you built nutty?

Was that just geared down?

ktm400
24-03-2012, 11:07 AM
Put a seat belt on it for him owen ^ton

Nutty
24-03-2012, 03:36 PM
Sounds about right ^duh

What about the mental EXC300 you built nutty?

Was that just geared down?

Nope :laughing- The Big Banger runs stock gearing but I'm probably going to settle on 14:48 so I don't have to be so careful on the throttle.

With 13:50 it will idle itself up out of ditches with no rider :)

>moto<
26-03-2012, 11:59 AM
I'm going to look at an 03 tonight with a mate. He'll buy it if the bike is in good nick so I'll be interested in how you get on Owen :)

Nutty
26-03-2012, 12:21 PM
I'm going to look at an 03 tonight with a mate. He'll buy it if the bike is in good nick so I'll be interested in how you get on Owen :)

The 03 was even more powerful, it was the scariest 250 MX ever.

rabskyline
26-03-2012, 12:23 PM
The 03 was even more powerful, it was the scariest 250 MX ever.

are they all the SX 250's pretty mental bikes ? seem really cheap(2005 locally to me for about 2 grand) !

>moto<
26-03-2012, 12:30 PM
The 03 was even more powerful, it was the scariest 250 MX ever.

^lol^roost

How's the suspension and frame geometry Nutty. A nightmare or half reasonable?

AD_KTM
27-03-2012, 03:44 AM
are they all the SX 250's pretty mental bikes ? seem really cheap(2005 locally to me for about 2 grand) !

One things for certain about the 250SX they have plenty of motor..which is why i bought mine :laughing- but the older models had some nasty traits..vague handling in the front end..headshake under power, very abrupt power delivery which did get smoother from 05 onwards. The newer models are light years ahead in frame and suspension design but KTM does make one hell of a 2T motor.

bluerider
27-03-2012, 07:43 AM
^lol^roost

How's the suspension and frame geometry Nutty. A nightmare or half reasonable?

Just as scary as the motor :speechles

Nutty
27-03-2012, 07:53 AM
^lol^roost

How's the suspension and frame geometry Nutty. A nightmare or half reasonable?

Just as scary as the motor :speechles

The 03 at the time was hailed as a significant improvement, chassis wise, on the previous models. The Yanks at the time rated the chassis improvements but cried like babies about the motor.

The 03 SX was the first of the short intake upright motors and it came off the bottom like a top fuel car.

I'm with Bluerider on the chassis, I spannered a bunch of them for various pro FMX riders and recall them being a frightening device. Every one of the pro FMX guys wanted the motor mellowed out, they couldn't get up a dry ramp without spinning up

Team200
27-03-2012, 10:33 AM
Thanks for the feedback gents.

So playing with base gaskets is out -I always thought it sounded a bit counter-intuitive but I'd heard it worked on YZ's so I thought I'd sound it out.

I'll try playing with the gearing, but I'm suss on this as well because most of the riding will be done in tight wombat ST and 1st is pretty tall...

Have ordered a FWW, the steahly site says the 12 ounce is equivalent to an EXC so I'll give that a try.

I'll try the power valve springs out of my '12 300 as well, I'd imagine they will fitt?? Nutty?

I also sumbled accross an old 6-day muffler from an EXC. It's pretty long, anyone had any expereience with fitting exc mufflers on SX's?

Someone mentioned that retarding the ignition will kill bottom and add top -wtf? I thought advancing the timing wouild have this effect, can anyone enlighten me on how retarding the ignition increases top end?

Owen.

Tom68
27-03-2012, 10:41 AM
Someone mentioned that retarding the ignition will kill bottom and add top -wtf? I thought advancing the timing wouild have this effect, can anyone enlighten me on how retarding the ignition increases top end?

Owen.

Think it's something to do with the pressure waves in the exhaust, retarded timing will make the pipe run hotter.

Nutty
27-03-2012, 10:46 AM
I'll try the power valve springs out of my '12 300 as well, I'd imagine they will fit?? Nutty? Yep

I also sumbled accross an old 6-day muffler from an EXC. It's pretty long, anyone had any expereience with fitting exc mufflers on SX's? Yep, they fit AND mellow the hit

Someone mentioned that retarding the ignition will kill bottom and add top -wtf? I thought advancing the timing wouild have this effect, can anyone enlighten me on how retarding the ignition increases top end?

Owen.

Retarding the timing CAN increase top end if the head is set up correctly. What happens is that the fuel charge is ignited later in the compression stroke, reducing pumping losses.

Low compression, strong spark, low density fuel and quick flame front speeds are also needed.

As a general rule it doesn't work, each motor will have a timing 'sweet spot' which you can dial in by riding feel if you're good enough or by dyno if you're not.

Team200
27-03-2012, 10:47 AM
The 03 was even more powerful, it was the scariest 250 MX ever.

Ah huh. Just found out that the one I have is definately an 03.^BGHD

Do you reckon running an ignition from a later model mellow it out at all or is it in the mechanicals?

Owen.

Nutty
27-03-2012, 10:56 AM
Ah huh. Just found out that the one I have is definately an 03.^BGHD

Do you reckon running an ignition from a later model mellow it out at all or is it in the mechanicals?

Owen.

Nope, the magic's in the port layout, head and intake runners.

Tom68
27-03-2012, 11:52 AM
http://www.szocimotoros.hu/download/Basic2strokeTunning.doc

The doc downloads.

AFFECTS OF THE IGNITION TIMING
Here is how changes in the static ignition timing affects the power band of a Japanese dirt bike. Advancing the timing will make the power band hit harder in the mid range but fall flat on top end. Advancing the timing gives the flame front in the combustion chamber, adequate time to travel across the chamber to form a great pressure rise. The rapid pressure rise contributes to a power band's "Hit". In some cases the pressure rise can be so great that it causes an audible pinging noise from the engine. As the engine rpm increases, the pressure in the cylinder becomes so great that pumping losses occur to the piston. That is why engines with too much spark advance or too high of a compression ratio, run flat at high rpm.
Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of retard at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm, but the biggest benefit is how the heat affects the tuning in the pipe. When the temperature rises, the velocity of the waves in the pipe increases. At high rpm this can cause a closer synchronization between the returning compression wave and the piston speed. This effectively extends the rpm peak of the pipe.

>moto<
27-03-2012, 12:09 PM
Well my mate was wrapped with the power of the bike and is sold, we pick it up tomorrow. It has a FMF pipe and Vforce reeds already, lofts the wheel in every gear down our private sealed road test track and sounds like it will eat humans if they get within 200 metres.

First thing will be to check jetting, sort out the PV spring, re-pack the muffler, do the oils and see where we are at and how Owen is getting on with flywheel weights etc. :D Gearing will be right up there too as it rips through the gears so damn fast. Will stick a 14 up front.

I was going to advise him to get the 1999 YZ250 needle and reed petals but having the V force reeds has thrown a spanner in the works there. Longer term will be to do the top end, set squish and drop the cylinder a smidge to beef up compression.

Nutty
27-03-2012, 12:20 PM
Retarding the timing CAN increase top end if the head is set up correctly. What happens is that the fuel charge is ignited later in the compression stroke, reducing pumping losses.


AFFECTS OF THE IGNITION TIMING
Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of retard at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm, but the biggest benefit is how the heat affects the tuning in the pipe. When the temperature rises, the velocity of the waves in the pipe increases. At high rpm this can cause a closer synchronization between the returning compression wave and the piston speed. This effectively extends the rpm peak of the pipe.

As I said it was...what has to be considered is that the pipe is not a variable to be adjusted to suit, the timing is. Once a motor is set-up only the timing and jetting are variables, hence the 'sweet spot' for timing 2T motors. Optimum timing increases the broadness of the power peak on the P curve, which is the very 'sweet spot' I posted about.

Also consider that an enduro motor has wildly variable pipe temps and moderately variable Cyl head temps, the post "Affects(sic) of the Ignition Timing" relates mainly to circuit applications.

rabskyline
27-03-2012, 01:01 PM
Well my mate was wrapped with the power of the bike and is sold, we pick it up tomorrow. It has a FMF pipe and Vforce reeds already, lofts the wheel in every gear down our private sealed road test track and sounds like it will eat humans if they get within 200 metres.



Sounds like a hoon law candidate ! !psm

Tom68
27-03-2012, 01:25 PM
Also consider that an enduro motor has wildly variable pipe temps and moderately variable Cyl head temps,

Very aware of that when riding.

ktm400
27-03-2012, 05:57 PM
The 03 was even more powerful, it was the scariest 250 MX ever.

Well my mate was wrapped with the power of the bike and is sold, we pick it up tomorrow. It has a FMF pipe and Vforce reeds already, lofts the wheel in every gear down our private sealed road test track and sounds like it will eat humans if they get within 200 metres.

First thing will be to check jetting, sort out the PV spring, re-pack the muffler, do the oils and see where we are at and how Owen is getting on with flywheel weights etc. :D Gearing will be right up there too as it rips through the gears so damn fast. Will stick a 14 up front.

I was going to advise him to get the 1999 YZ250 needle and reed petals but having the V force reeds has thrown a spanner in the works there. Longer term will be to do the top end, set squish and drop the cylinder a smidge to beef up compression.

I gotta get me one of these fo sure... !psm

>moto<
27-03-2012, 06:10 PM
I gotta get me one of these fo sure... !psm

It's fun to ride.....but in a stupid I can't ride it properly sort of way :D

diZco
27-03-2012, 06:39 PM
I gotta get me one of these fo sure... !psm

I think your 380 would still have to trump it (youve got that a bit detuned dont you)? i dragged my 250sx against the 01 300exc which was a fairly angry one from memory ?and we were dead even. I reckon against an 08/09 300EXC the SX would trump it no worries. The real killer with the 250sx is it's responsiveness. It's reactive as all buggery. Probably be pretty nice in the sand actually.

>moto<
27-03-2012, 09:16 PM
I have a modded 10 300EXC and with the same gearing the SX would show my bike a pair of clean heels I reckon.

>moto<
31-03-2012, 10:00 AM
Well, unbelievably someone has geared the bike DOWN from stock, it was running 13/52! Swapped to 14/52 and it's still a beast. I'll throw him a 48t I have lying around but it looks like NP's suggestion of 14/45 is the way to go. If he likes the 48t we can buy a 45 and test that.

The bike has a FMF Fatty pipe (near new) and Pro Circuit shorty muffler. The pipe doesn't fit the head perfectly so we'll try my Pro Circuit Platinum pipe I have lying around.

Itis running a NOZE needle in clip 3 with a 48PJ and MJ is a 170. I'll play with jetting and throw in a suzuki triple taper needle to smooth it up a bit.

Interestingly it has a new 2009 250SX ignition, didn't notice this before so we called the PO and asked him. He said the steering lock bolt was removed and the fork ended up crushing the CDI. So, I found the map wire and disconnected for the 'soft' map and it was hardly noticeable. Maybe better when gearing etc is sorted.

Now that it runs the newer ignition we can swap the CDI for stock EXC one that I have lying around which will help also. Just as well I have so much stuff lying around :D

Still need to check PV set-up though I will report back with findings later as I'm busy this weekend.

Owen - thought I'd post it here to collate all info, if you want me to post elsewhere let me know :)