When I first got my 2007 klx250s, I started it up for my dad and he LAUGHED. He laughed and laughed and through his laughter he choked out "That's the most feminine bike I've ever heard!" He was right, we named it, "the Princess bike". I got the klx because I have a friend who swears by the 300 and it felt way better ergonomically than my husband's DRZ400 (not to mention 50 lbs lighter). We ride mostly in deep deep sugar sand here in florida and I knew the stock 250s probably wasn't going to cut it, but I bought it cheap and planned on doing the 351 with the Barker Exhaust.

I got lucky on craigslist and found a 2014 fmf powerbomb header with a PC4, along with a dynojet kit (DJ2152) and 13 tooth sprocket all for $250. Everything was only a month old when the guy decided to trade in his bike and kept the upgrades to sell separately. WIN!

I put on all of the above, and with some help from Klxster, I jetted the carb. I used the DN0228 needle from the DJ kit, with the clip in the top knotch and two washers super glued on top. I am running with a #40 pilot jet (perfect for Florida elevation) and a #157.5 main jet. Lidless airbox. I took her out for a drive and she pulls HARD above 6k rpm. I couldn't tell a huge difference driving slowly around the neighborhood, but on the street... WOW. I no longer worried that I wouldn't be able to get out of the way in an emergency on the highway.

I was on the fence about doing the MCM mod. For me, it's a scary mod. Scary because it's a free mod, unless you do it wrong... then it becomes a very expensive problem. But I liked the idea of gaining the extra torque below 6k rpm. It's really not a difficult mod to do, just scary. I liked that klxster had dyno'd his bike and could prove that it made a difference and that his bike didn't have any negative effects.... so... I did it.

Once again, I couldn't feel a huge difference in the neighborhood. HOWEVER, I just tested all of my mods this weekend in the Ocala National forest. I couldn't be happier. I was so happy with the performance that I am no longer planning on putting the money into a 351. I had all of the torque and power that I needed for what I was doing. It is SUCH a different bike! So different, in fact, that it's name is now "Mr. Princess", aka "The Tranny Bike". He's no princess anymore. If you are on the fence, I highly recommend doing these mods FIRST. I also recommend contacting klxster for help. That guy has this down to an art. Also, the most recent info for doing the MCM mod is not clearly written in the forum that I know of. It's a lot of outdated information that is going to create a lot of unnecessary work. If I have time, I will do a new write up on it.

Either way, it was awesome! Do it!

Here is Mr. Princess. The black and white bike is what he will eventually look like. It belongs to MrDuhfactor who did a great series on youtube on a 351 build https://www.youtube.com/user/MrDuhfactor

 

Thanks for your write up. I was on the fence about doing the MCM mod after all of the other carb and exhaust mods were already done. You just made up my mind and I'll be doing it as soon as I do my valve clearance check.

 

Awesome. Let me know if you can tell a difference. I couldn't tell on the street, but in the dirt it was obvious. I used the write up from klx678 to do the mod, but there are some newer tips that need to be added to that. When it comes to the ACR, you no longer need to use a coat hanger or punch out the doo-da. You only need to remove the spring (but keep it just in case you change
your mind later). There has been a lot more research and experience since that write up was made.

Tips from my experience:
1. Undo the two lower bolts on the radiator fan and swing it up and out of the way on the top bolt. Just make your life easier from the get-go.

2. Losen the bolts on the cam sprockets before you find tdc because they are a bear to break free. Mine had red loctite on them. I used my hammer drill. It's a pain to find tdc on the compression stroke then realize you need to work a little more to get those suckers out.

3. Remove both the cam chain tensioner and the chain tension spring to give yourself plenty of slack. (The tensioner is above the spring)

4. After you have moved the bolts to the new locations in the sprockets and you are trying to set the timing, make sure you put the cam chain spring back in to make sure your alignment is still correct before your close everything up. Some people forget to check the timing after putting the tensioner and spring back in and that makes a lot of extra work.

5. When you think you have everything correct, before you put the cover back on (but everything else is back together), turn that motor by hand several times to make sure you don't hear any metallic clanking or slapping. Some people turn the wheel by hand while it is in gear, I used the bolt on the flywheel from the left case cover. Then, because I was paranoid, I put it in second gear (spark plug still removed) and took it for a jog down the street to make sure I still couldn't hear a problem. If that all sounds good, hit the starter a couple of short bursts to listen to it. If all sounds good, close her up and start it. Let it run at just an idle and listen. Then move the throttle (in neutral) to listen to it at different rpm's before you take it for a ride.


You probably already know all of this, but if any of it makes your life easier or prevents a problem then it's worth it. Good luck!

 

Bryan, Steff did all her mods in practically one sitting. She went from stock power to "state of the art" power and couldn't really make a proper assessment until she "got'r outta the hood"..

The MCM really should be the FIRST mod done to a stock KLX with one exception - that exception being the case whereby the owner intends to focus all performance mods toward top end power (7k-10.5) and doesn't care about losses below that. Such focus will require a full exhaust system (Header and matching slipon), running a lidless airbox, and a lidless CVK34 setup - and will make approx +1 hp around 9K rpm over same bike + MCM, but with a significant loss of TRQ below 6.5K.. This is from our current best data.

Full exhaust, lidless, with MCM will produce best overall power curves..

 

Thanks for all the info. My air box is lifeless already. I bought the bike used up in Maine. It came with a full fmf system, jet kit, and lidless. Was the loss in top end hp noticeable? Also, the pointers are always nice. Thanks!

 

BTW, Steff, a bit of probably useless info - your bike can now easily "pull" the 14 tooth stock primary. We have no information regarding the pro's and cons of using either with your current mod combo. I would assume the 13 creates a more "explosive" powerband offroad with the 14 creating a more "linear" acceleration/power delivery.. Top speed differences (if this is a concern) are completely unknown and not intuitive..

 

There is very little chance that your bike has a proper setup on the CVK34 for lidless running - most likely, you are very lean.

There has been two members, running "top end only" max power setups, that added the MCM and did not like the results - they noticed the loss of power up high and reverted back to stock timings.

Since you are already, no doubt, losing a lot of power up top from an improper carb setup, all you'll notice is more everywhere when you add MCM and proper lidless fueling to your bike..

 

Not sure of the jet sizes used from the DJ kit. The previous owner installed them. It doesn't show any signs of running lean. After 4 tanks of Sunoco race fuel (no ethanol), the plugs a are perfectly colored. Also, I don't have any stuttering or any other signs of running lean. The bike seems to pull pretty well all the way up to the top. It'll never win any trophies, but I think it's running optimally as of now. I just read my last post. My phone auto corrected me. Meant to put lidless, not lifeless. Huge difference.

 

Not showing signs of wrong fueling while wrongly fueled is a hallmark of our little KLX's - a little something I found out while dyno tuning my fueling curves .. It'll run just fine with 13.5-14 to one AFR, as well as 10:1 AFR.. I hate to say it but this fact about our KLX's is probably the one attribute that has caused decades of wrong fueling setups, and the attending loss or unrealization of power and TRQ. Instead of dyno testing a setup, it was just passed along as creating a "good running bike"..

I offer dyno proven setups that properly fuel lidless KLX's - these setups ended up being quite "outside the envelope" from the dogma of "follow me cause I followed them" fueling setups.. For instance, your bike, near sea level, requires a DJ144 or K158 main jet - not something your PO is likely to have known..

 

If I was too start with a stock KLX 250 the first thing I would do is the 351 BB
kit. imho