Jump to content

Calm down a 250sx

Recommended Posts

Hello. 

Got myself a 2015 250sx. Ive owned a 2013 with aftermarket exhaust, red spring flush etc and it was just too wild. I know its "smooth" but that beast would just get up and GO with a twist. 

Now I want to try and calm this thing down and make it smoother and "weaker"/"slow".

I will use a flywheel weight, the smoother ignition, dont have have stock exhaust but a fatty with stock silencer. 

Now Im gonna adjust the powervalve and if Ive understand it right the coloured spring decides how fast the powervalves open and the preload decides at what rpm it begins to open. 

To me this means that green spring all the way in is gonna mean it starts to open as late as possible and as slowly as possible. 

Correct? What other mods is affordable and will tame this beast? 

Ive seen dynos with the red spring and different preloads and more preload seems to delay when the hit comes so my thinking should be right?

Other than that it gets new tires, fresh linkage and revalved suspension. 

Share this post


Link to post
Share on other sites

I agree with the PV spring swap. Be careful about turning the adjuster "all the way in". The adjuster screw threads in only run as deep as the cover is thick. Meaning, you can screw it in deep enough that it comes unthreaded from the cover. You don't want to go deeper than about 4 threads showing.

Does the SX have the selectable map switch wire?

Jetting and tuning it a little richer works too.

The Fatty is a mid-to-top end pipe.

What kind of riding and terrain do you do? What throttle range do you find yourself in most of the time?

  • Like 1

Share this post


Link to post
Share on other sites

I just turned down my exc 300.  The green spring really did the trick.  I thought it was smoother around 1.5-2 turns in from flush than it was further out or further in.   I'm not really sure but the power coming on seemed most natural that way.  If you turn it in all the way to delay the hit as long as possible you can wind up with a flat spot before the valve opens and it will make it seem abrupt when the PV finally opens.  

I wasn't really looking for the least power at any rpm, I was looking for the most manageable rate of torque increase.   There really is no magic formula, and it is so simple to change, just go out and try it.  

I also went to a safe, flat, and open place to try out the settings without any distractions.  It's amazing what the PV spring can do.  Really, coming from four strokes, for less than five bucks and minutes of work to be able to change the character of a bike like that is incredible. 

 

  • Like 2

Share this post


Link to post
Share on other sites

Check out the cross training and enduro videos on you tube by Barry Morris aka OZ DRZ they just recently done what you are talking about . One of the first mods should be carb tuning ( they used Suzuki needles) and flywheel weight 

Share this post


Link to post
Share on other sites
Buy and XC cylinder, I'd imagine the porting is a little more gentle.

The head will be different the cylinder will be the same.
  • Like 3

Share this post


Link to post
Share on other sites

Throttle control. Sounds simple enough but is sometimes hard to do. You can add a flywheel weight, adjust the power valve, jet it rich, change the head and ignition . You don't  have to mod you're bike just your riding style.  learn to be gentle on the gas. Maybe try a g2 throttle tamer.

Share this post


Link to post
Share on other sites

I have a 250sx that I have calmed down very well for woods racing.  It's still stupid fast, but at least it does not try to kill you in the woods.  

First, I have to disagree with some of the above regarding PV springs.  The green spring will hold the PV shut longer, but it will open, and when it does, the bike will come on with a vengeance.  I see PV springs and preload as a very effective way to tune your bike to specific race/ride conditions, but not a way to tame the overall power.

In my experience, there are two cost-effective ways to make the overall power a little more friendly.  A throttle cam system (about $100) does wonders.  The other very effective mod is a XCW head.  The lower compression head (it's like a smooth dome compared to the SX, which has a distinct squish band) seems to make the power very smooth and linear.  You should be able to find one for about $50 as nobody wants them.  All the XCW guys want to run an SX head, so there is a ton XCW heads laying around.  My bike did not need to be re-jetted or anything.  Just bolt on the head and you are good to go.  

Another common mod is a FWW.  I always run FWWs on 250cc and above bikes, but the FWW on my SX seemed to have very little effect compared to my results on other bikes.  I would do the throttle cam and XCW head first.

Good luck with the SX.  Mine has been a very reliable and fun bike.  

  • Like 1

Share this post


Link to post
Share on other sites

 Lower the compression a little by enlarging the dome. Top end will still rip but it'll mellow out the hit. Jetting will be more tolerant too. 

 

 I like a heavy fww on a 250 mx bike. 

Share this post


Link to post
Share on other sites

I've never found opening the pv late helps if the jetting is right.

Share this post


Link to post
Share on other sites

Interesting seeing total opposite opinions based on exact same mods, guess I have to make them myself and decide. 

I got my stock exhaust being shipped back to me. 

I watched the enduroizing video and its very informal and the mods seem to really make a drastic difference and he rides the bike after every couple of mods. Very noticeable on a video, close to same power but much more manageable. 

Red spring flush is a wild, jerky and wheelspinning ride and green spring/fww/throttle is really turning it around, almost like its not the same bike. 

Next up seems to be the xcw head as it was really cheap used as you said, even cheaper than throttle. 

Hopefully I can get the bike manageable and somewhat confidence inspiring like a 4 stroke. Well, coming from a 2013 with red spring flush and top end exhaust I imagine its only gonna be alot better. 

Share this post


Link to post
Share on other sites

Well well well. What a piece of shit bike I got from the trade, completly rejected maintaince other than oil in gas and transmission. 

Ive busted my ass off in the garage for a week, put over 30hrs in taking every bearing apart. Whole linkage and swing has rusted together, even the bolts had oxidization on them to the point I almost couldnt use them again. No wonder the wasnt any side play, it was stuck. 

Got that shit fixed and bolted on stock silencer and it was completly flat above mid range, just gurgle when I open the throttle. I guessed it was to big main jet because fmf exhaust and I was almost right - the whole main jet FELL OUT of the carb when I opened the bottom and it was a 155 together with "i" needle at second clip. What a bloody joke, no wonder it sounded "crisp", it was starved. 1500m over the ocean and 40-45 celcius? At super warm summer is 25-30 degrees and ride at sea level, glad I checked but not happy previous owner didnt. 

Took off his air filter after riding some on the field and not to my suprise I found small sand and grits in the boots. Wiped it down but couldnt bare myself to ride it so I took it apart, took the carb out and cleaned it. Thankfully, as there was sand/grits in the carb and in intake for the reeds. Cleaned it out, put in the G needle at 4th clip, 40 pilot and 160 main - ran like a boss. Waiting on a NECJ as it got a small hickup when I crack the throttle but pulls really nice. 

Finally got myself to try it at the track and its such a beast even tuned down. Thinking of the XCW head but not sure if its too small change for 100 bucks. 

As I expected it was kinda scary ride as it jerked and wanted to wheelie but I felt better later on the day when I had better pace and made sure to hit the band well before a jump to have the right speed. 

Need to buy myself a compression tester and see how the piston/cylinder looks. My only hope it that it was so dirty and oily that most debri stuck before entering the motor. 

Sigh.

Share this post


Link to post
Share on other sites
Hello. 
Got myself a 2015 250sx. Ive owned a 2013 with aftermarket exhaust, red spring flush etc and it was just too wild. I know its "smooth" but that beast would just get up and GO with a twist. 
Now I want to try and calm this thing down and make it smoother and "weaker"/"slow".
I will use a flywheel weight, the smoother ignition, dont have have stock exhaust but a fatty with stock silencer. 
Now Im gonna adjust the powervalve and if Ive understand it right the coloured spring decides how fast the powervalves open and the preload decides at what rpm it begins to open. 
To me this means that green spring all the way in is gonna mean it starts to open as late as possible and as slowly as possible. 
Correct? What other mods is affordable and will tame this beast? 
Ive seen dynos with the red spring and different preloads and more preload seems to delay when the hit comes so my thinking should be right?
Other than that it gets new tires, fresh linkage and revalved suspension. 

I figured out many moons ago that the so called weaker/slower is actually faster when it comes to faster/consistent lap times for most non pro's.

Share this post


Link to post
Share on other sites

gday from Australia 

I have a 2013 ktm 250sx, to tame mine down I used a boyesen reed block green spring screwed 2 in from level at the moment, fmf platinum expansion and shorty muff them richened up the jetting on main to a 168 I think or 165 will have to look at my notes...

the joy of the boyesen is ya can change reeds for the purpose from full carbon (race top end) or carbon and normal reed which I'm using at the moment..

I'm a beginner rider but used to race mountain bike and am catchin up to my ridin buddies even with a fully worn out rear!! the set up I'm at now seems to work, its  almost like traction control it seems to give the same amount of wheel spin on most surfaces... this w.end will be the last ride on this tyre so I cant vouch on new tyre set up for ya.. 

and if ya want to know it still wants to stand up on the rear at times if I'm not carefull with the throttle!!

damn it cant attach a pic ah got it!!

20170710_082739.jpg

Share this post


Link to post
Share on other sites

Lowering the ports is a great option to boost low RPM torque and slow down the high rpm hit and improve fuel economy. A thinner base gasket is the easy way. Else machine the cylinder base a fraction of a mm. Worked great on my son's ktm50, kx65 and my yz250. Just check your squish clearance doesn't go below 1.2mm for a 250 and compression not too high for easy starting.

Share this post


Link to post
Share on other sites

I've built a couple SX's and TC's into smooth machines, they are hard hitting tire spinners out of the box which is fun for about 20 minutes and then it just starts to tire you out. 

My favorite mods are:

XCW head(someone will be willing to trade you so freebie)
XCW ignition(can be found used for under $100)
Green PV spring
14/48 sometimes 49 or 50 but always a 14t
grind the throttle tube to so it grabs less cable and ramps up later, basically a free throttle cam. 

On my 17's i'm running a 7oz clutch weight but i'm my 15 i didnt feel that it needed a FWW.

RM250 or YZ250 needle, both run much smoother then the KTM needles. 

 

Share this post


Link to post
Share on other sites

Havent tried to change any gear and dont know if I shall. 

Ive installed the green spring and a few turns on, showing like 2-3 threads and its gotten alot better. 

Happy with this setup but intrigued about xcw mods.

Installed NECJ 4th clip, 40 and 170 but seemed to lean. Went up to 42/172 and it pulls alot longer at top. Will have to see if I move the clip to 3rd but rather abit rich than lean. No spooge. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×