Reread my posts. I didn't "diss" anything. Merely offered my opinion.

Opinions are not facts and may be wrong, including yours!

 

On the KLR650 forums guys are retarding the exhaust valve and 1/2 the guys who have done it swear it's better, 1/2 put it back. These cams use a lot of degrees of crank rotation to gently lift and set the valves back down for durability. The .050" lift times (when things start to flow) are such they run with ZERO overlap. Phasing the cams to give a little overlap can help, but it'll hurt somewhere else working with short.050 durations. My experience was exactly as predicted. It gave the motor a little more pop down low from the increase in dynamic compression from the intake valve closing early. But it wasn't filling the cylinder as good at upper rpms. I put it back. Moving the lobe centers that far isn't something you can rejet around to get the power back.

 

Just found my original thoughts in post #448 if anyone is interested.

Once again - this is not intended to slag anyone off - it's just my opinion.
My bike is a totally standard EFI (except for the clutch switch mod), most bikes on here seem to use more open pipes and carbs.

 

Hey DT, Went back to #448 and re-read your analysis of the ramifications of the mod.. So, once again, I am impressed with your analytical abilities. Perhaps the mod is making less than a 10 and more closely to an 8 degree change. I was careful to gauge the change in performance, before and after MCM, on my totally stock 2012. I now believe, on a stock bike, you feel the increase in the low to mid range brought on by the increase in the effective cylinder pressure - but cannot feel the increase available with the larger overlap due to restrictive stock intake and exhaust systems - That the new overlap, and its' power production up higher in the RPM range, are only revealed with a slip-on, opened airbox, and proper carb setup..




PS: 8 degrees at the cam is 4 degrees at the crank - for those that didn't know crank speed is double the cam speed..

 

I dunno, I guess now I'll plead "ignorant" and fall back to the key points that some here have actually documented - including the OP. Increased power through all rpm, but peaking earlier, proven with dyno results. From there I will go to the past two stroke experience - a peaky engine is not as easy to ride fast as one with better mid-range. From there my KLX250 will spend most of it's engine life between 4000-7000 rpm, so being able to pin 9000-10,000 rpm doesn't mean much.

Funny part is that is the similarity of argument against the big bores of some bikes, my own KLX650 included - the big bore seems to make the power at a bit lower peak rpm, the higher horsepower peaks earlier which seems to be the issue... it won't rev as high as easily and perceived as a detriment - but with the KLX678 it will kick the stocker's butt all the way up to that point and be far enough ahead it just doesn't matter... of course I could gear it to pull higher speed, but I don't need it.

That's kind of how I see the MCM on the KLX250. If I want a bunch of revs stay stock, maybe get some cams, eventually maybe see 25 hp at 9500 rpm. Tractability off road might suck, in the mid-range, but boy does it put out the rpm. On the other hand I am using the bike off road, the MCM and even a big bore for any additional punch through the mid-range is welcome - especially if at no cost, like the MCM. If I want more speed I gear higher. It's pulling a 15/39 right now rather easily, but I wouldn't take that gearing off road - too high to be of any value. It takes good gearing and mid-range punch for tractability and quickness on trails.

That is what should be considered by anyone contemplating the MCM or big bore. Both will likely lower the rpm peak where hp peaks, both have been proven to increase torque across the range where they do excel and even have higher horsepower figures, but at the lower rpm. To take advantage on the road, gearing is necessary to make that increased power work beyond the stock set up. More power makes higher gearing possible with no sacrifice or it makes easier riding with the stock gearing. I am playing with the former right now, but will be gearing for the latter, shortly.

I guess that is my take away on the whole thing. Neither of my machines are going to live at 90% red line, rather more like 50-80%, so gimme more power there.



Side note, I do find it interesting that the timing thing has come up among some good tuners over the decades with motorcycles spinning similar rpm and the changes made were identical. Some gain hp, some not, but the ones I've read about all gained power over stock up to that peak hp - higher power at any given rpm, but at the original peak rpm. I also find it interesting that those tuners did both intake and exhaust, not just one or the other, where it appears the KLR guys are only doing one - the exhaust. Makes me wonder what the cam timing is on them, why not both? I would guess cam timing is probably similar to most, which are pretty much equal in degrees from TDC, but I don't know. I don't know if they should gain or not. I don't know if my 650 could benefit or not, maybe I'll look into it, but no rush. It's fun now.

 

KLX678, from your post above, I've got this understood - " From there my KLX250 will spend most of it's engine life between 4000-7000 rpm, so being able to pin 9000-10,000 rpm doesn't mean much." "Neither of my machines are going to live at 90% red line, rather more like 50-80%, so gimme more power there."
Also, from your other recent posts, you are still running the stock exhaust.

So you are experiencing the MCM the same way I did when I was running stock exhaust - and that is perfectly fine - the MCM makes a difference running the way you are running..

But there is another side to the MCM that you are not experiencing. It is the power production above 7k once you put on an exhaust, pull the airbox lid, and rejet/re-setup the carb. This has to be felt to be understood. It is profound. It is what prompted me to start dyno testing in order to see what I was feeling. It is power production that I am convinced is a product of not only the "free breathing" of the intake and exhaust, but also the increased overlap of the MCM.

I am unaware of any "before and after MCM" dyno charts of any configuration of KLX250.

Given your posting proclivity, it is ashamed you are limited to observations and determinations of such a limited subset of the data/experience of the MCM. Let me know next time your going to be in DFW and I'll get you "in the know" of the wild side above 7K.

 

Here some update on my bike. I bought EJK. Installed it. It increased a little power, however I still was not able to make wheelies.
Then I made mod to OEM exhaust muffler, i.e. I removed two sections and installed my custom stainless pipe, removed cat. Then I welded everything into one peace and here you go my bike started to make wheelies!
The muffler weights 1.1 kg now.

Some pictures and video:
http://1drv.ms/1Lxhz0a

Totol mods: RPM restriction off, lid off, KACR off, liamda off, cams mod by marcelino, ejk, and oem muffler mod. Also I lifted my bike up (custom raising links). The bike now looks like real enduro horse

Now, bike runs like my old XR 250

 

I understand your comment. Thing is I know the trails I ride. Not much wide open time, lots of on-off-on. Thus the reason I like the stronger mid-range.

I have a reverse cone megaphone Pro Circuit pipe in the garage along with a 1-3/8 head pipe, but haven't put them on yet. Just lazy and I want to do some riding as is just to play a bit. Then I'll do the other stuff. Besides, school's out with few set plans for the summer, so I can play a bit. I run 12 plates in the 650 so I'm thinking 6-8 would work with the 250 and keep the sound at a fair level.

Right now I find it interesting that a generally stock, jetted KLX250 geared with a 15/39 will easily get to and pull 70 mph at around 6000 rpm with ability to accelerate a bit more. Good oomph for a small bike.

 

Assuming that 1-3/8 header is a big jump in I.D., it will change everything.. The MCM surely retains more low rpm trq than otherwise with a large diameter header but still, the (HP, TRQ) loss below about 6-6.5k is significant - but the top end increase is also significant. Also, I'm having hell getting the carb setup correct with the larger I.D. Megabomb + PowerCore 4... I refuse to accept the apparent trade-off of either a too rich midrange or a too lean top end..
I miss summers off.. I'll semi-retire back into teaching some day..

 

Finally got around to doing the MCM last weekend, and removed the ACR while it was apart. Now trying to get the jetting right. For reference, before the MCM I was running the "stage II" dynojet recommendations; 128 Main, 35 Pilot (stock), 3N on 2152 needle, and fuel screw about two turns out. Was happy with the way the bike ran, no flat spots or surging, pulled all the way to redline.

While researching the MCM, it seemed like the consensus was that more fuel was required after doing the mod. So in anticipation of the changes, I installed a 132 Main and dropped to the 4N while I had the bike apart. That was probably my first mistake, I should have left the jetting where it was and tried it as-is.

However, now that it's back together, the jetting is off. The low and mid range is where it needs the most work. The bike starts fine but is almost unrideable until fully warmed up. It surges and struggles until transitioning to higher RPM's. The top end (>6.5K RPM) seems to pull pretty well. I've been keeping an eye on Klxster's jetting posts, and it looks like some larger jets (main and pilot) are going to be required.

My engine configuration:
<1000' ASL
Stock airbox, lid on, snorkel removed
132 Main, 35 Pilot, 2152 Needle @ 4N, 2152 spring, screw @ ~2.5 turns.
Stock header, FMF Q4 w/ smaller 1 1/8" insert.

I'd like to keep the airbox lid and smaller Q4 outlet, so I can't replicate Klxster's setups exactly. I'm sure I won't flow as much air, and therefor need slightly smaller jets, just not sure where to start. I think the 132 Main is close, maybe a 136? But the pilot I have no idea. 38, 40? Needle position? And I think a Kouba fuelscrew will be a good investment at this point.

So what say you, KLX jetting experts? What's a good starting point for the above mods? I realize it's a lot of trial and error, but I'd like to get in the ballpark first. Thanks!