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C50_min_1014_2_1464587959 blackdiamond Quote
12/22/2015 9:01 PM

I am finally getting around to the final assembly of this project after a lot of research, collecting parts and a fair amount of machine work and mods. The last generation of Honda two strokes are my favorite bikes of all time so I decided to pull out all the stops and build the best CR I could with the latest tech and trick parts available (within reason)

I really wanted to think outside of the box and make mods to the bike that make it a better machine, not just bolt on a bunch of bling that does nothing to make the bike better. I did have a few pet peeves with the CR250 that I could fix and I wanted to address them as well.

Originally I just had a few simple requirements on my wish list:

1. Lots of horsepower
2. As lightweight as possible
3. No air forks
4. Reliable

Then my OCD took over and I ended up with quite a bit more of an exotic build than I had originally planned but I think it's turning out great so far. I will post more updates, details and pics over the next few weeks.

Frame prepped and ready for final assembly.

All of the brackets have been removed from left frame spar and all of the electrical components will be relocated towards the center of the bike and in the airbox.

I removed the coil brackets from the down tube of the frame and moved it near the rear shock tower. Anyone that has done a top-end on a 2002-2007 CR will know why. The location of the coil is a total pain in the ass when pulling the cylinder and this mod makes it much easier. The bonus is that the weight is moved to the center of the bike and that is never a bad thing.

I added a return spring to the rear master cylinder eliminating the need for the tab on the frame or the OEM spring. Added a billet brake clevis and Ti hardware to finish it up.

Because the ignition box will be relocated to the center of the bike I was able to utilize the old mount on the head tube to fabricate a mount for a Honda HPSD steering damper.

C50 kawi448 Quote
12/22/2015 9:20 PM

This looks sweet! What suspension are you gonna run? And what did you do to the motor? I can't wait to see it all done

C50_gd_1399323318 garagedog Quote
12/22/2015 9:24 PM

Following this thread. Keep the updates coming!

C50_gd_1399323318 garagedog Quote
12/22/2015 9:27 PM

What year frame?

C50_min_1014_2_1464587959 blackdiamond Quote
12/22/2015 9:58 PM
kawi448 wrote:

This looks sweet! What ...more

KYB kit suspension based off the latest Honda version of the SSS forks and a 2015 CRF style rear shock.

C50_min_1014_2_1464587959 blackdiamond Quote
12/22/2015 10:00 PM
garagedog wrote:

What year frame?


C50_s1200_1230013_430855073684781_330863819_n_1406526194 c_dub Quote
12/22/2015 10:23 PM

Wow! Nice work black! I love the ingenuity and attention to detail.

I also agree on the last generation frame of the Honda two strokes (had an '03 myself). To date, in my opinion, still the sexiest aluminum frame.

C50_holeshot zehn Quote
12/22/2015 10:32 PM

Honda KYB SSS? Huh?

C50_dsc02641_1450117084 Empty Pockets Quote
12/22/2015 11:07 PM

Looks beautiful, can't wait to see this done, where did you get the tank and dare i ask how much ?

Any info on the shock swap that you're willing to share, what did you do to get it on there, change the length, linkage etc ?

C50_min_1014_2_1464587959 blackdiamond Quote
12/22/2015 11:11 PM
kawi448 wrote:

This looks sweet! What ...more

The motor is still work in progress. It is being done by Terry Varner and will be a full race spec motor tailored to outdoor MX. It will have the usual head and cylinder mods along with a modified VForce reed cage and programmable ignition that will allow independent 3D mapping for the ignition, carburetor power-jet and exhaust RC valve. It will use a 39mm taper bored short body Keihin airstryker carb that is the same as the OEM carb found on the late model RM250's. Varner did all of the mods on that as well. The TPS and power-jet will be fully functional unlike the popular Keihin carb swap that most do.

We are still working on the details and tying up a few loose ends but the transmission will be a lightweight 3 speed version that saves about two pounds of rotating weight inside the engine. It would never work offroad or for trail riding but for MX I think it will be awesome. We will see, I haven't tested it yet.

C50_dsc02641_1450117084 Empty Pockets Quote
12/22/2015 11:16 PM

Wow, sounds serious, love the thought that's going into this, going to be a bit special if it all pans out, fair play to you.

C50_mxon_maggiora_nl697_saturday_1474847906 colintrax Quote
12/22/2015 11:22 PM
kawi448 wrote:

This looks sweet! What ...more

blackdiamond wrote:

The motor is still work in ...more

How are you controlling the power valve? And who's doing the programming? That'll take alot of testing.
Good luck none the less, I look forward to seeing the end result!

C50_min_1014_2_1464587959 blackdiamond Quote
12/22/2015 11:26 PM
zehn wrote:

Honda KYB SSS? Huh?

Basically, the same basic fork that the Yamahas use currently (and the best performing IMO) but in the Honda version which was last spec'd on the 2012 CRF450.

C50 Awesome Johnson Quote
12/22/2015 11:49 PM

Black, care to go into detail on relocating the ignition and the rear shock rear shock as far as mounting it. Any mods or a direct fit? This is gonna be really nice!

C50_min_1014_2_1464587959 blackdiamond Quote
12/22/2015 11:50 PM
Empty Pockets wrote:

Wow, sounds serious, love ...more

The transmission is a bit complicated for sure, definitely uncharted territory for me but I was inspired by Max Anstie's Dixon Racing Kawasaki 250F at the glen Helen GP this year and wanted to see what was possible. His bike was featured in Vital's world famous "Pit Bits" and they were reportedly running either a 2,3 or 4 speed transmission in the Cosworth built engine based on the track layout. The cases were CNC'd out of billet to facilitate the use of the special trans. Check out the pics below, it must have cost a fortune to make those.

After seeing that bike and also noting how KTM has redesigned their entire engine (raising the transmission shafts and clutch over an inch) to reduce the drag caused by the gears running through the oil I couldn't help but wonder what effect that would have on the good ol' fashioned 2 smoker? We shaved over TWO POUNDS of weight in the transmission alone and eliminated 4 cogs spinning through the oil.

Here is the pic of Max's Cosworth engine:

C50_min_1014_2_1464587959 blackdiamond Quote
12/23/2015 12:09 AM
Empty Pockets wrote:

Looks beautiful, can't ...more

I actually import the tanks from Italy myself. They retail for $900+ based on the model. Incredibly light and made from the best carbon I have ever seen but I will be the first to tell you that spending $900 on a fuel tank is crazy. These are the same parts used on some factory bikes and GP teams. The one for the CR250 saves about 300 grams and is .42 gal larger without looking like a pumpkin. Ironically, the CR250/CR125 tank outsells the other part numbers I offer 3 to1. I would have bet a million dollars US that would not be the case but apparently I am not the only maniac doing exotic Honda two stroke builds or restorations.

The shock was much more involved than I originally thought it would be. I will post more on that later (pics as well) but I will sum it by saying that I am glad it is finally finished but it took a ton of tweaks and mods to make it happen. I thought it was "close" to a bolt on. I couldn't have been more wrong, and truth be told I would not do it again. It was more work than I think it was worth in my opinion.

C50_min_1014_2_1464587959 blackdiamond Quote
12/23/2015 12:44 AM
Awesome Johnson wrote:

Black, care to go into ...more

I will post more on the shock in the next few days because it will take a bit to explain properly. Here is a pic of the coil mounting. The ignition box mounts opposite of the carb, under the shock tower.

If you look closely at the pic below taken of MC's bike in 2005 you can see where Team Honda mounted the box, right above the kickstarter. This bike has it mounted in the same place. I just fabricated a bracket out of carbon fiber plate retained by the head-stay hardware and mounted the OEM aluminum bracket to it.

C50_min_1014_2_1464587959 blackdiamond Quote
12/23/2015 1:15 AM
kawi448 wrote:

This looks sweet! What ...more

blackdiamond wrote:

The motor is still work in ...more

colintrax wrote:

How are you controlling ...more

I agree, it will take a lot of dyno time and laps to dial it in. We are going to start with a stock ignition map and the "old stand by" 2000 CR250 airstryker Keihin carb swap to use as a baseline and slowly work from there. I have two separate ignitions and two separate carbs to run comparisons on. The ignition is not a Vortex or a JD, and it is used extensively in karting and vintage two stroke road racing. The ignition is still relatively unproven and I don't want to really say that much until we have it dialed in completely. I will tell you that the ignition has the ability to control the PV with up to 5 different stored maps for the RC valve alone and 10 different stored maps for the ignition curve. When you add a 3D power-jet map to that it gets pretty deep unless you do this type of programming for a living (and I do not)

The good news is that compared to some of the magic that the top level tuners perform in regards to ignition mapping on the latest 4 strokes it is still relatively simple.

C50_mx_bear_1457753510 Bearuno Quote
12/23/2015 2:15 AM

One of the best modification article series on the 03 - 07 CR250 was on the Two Stroke Motocross.com site. Very extensive, going as far as reinforcement welds in the crank cases to cover known weak areas. With the site it appears now dead and gone, I don't know how the series would be found.

Fair enough removing gears, if light weight is the goal. Though, you need to consider that the 3 spd Bike It / Cosworths Yamahas ( no idea on the KXfs, but I assume they might have tried the mega revs / minimal gears concept) were engines that revved to 16,500 rpm, and perhaps more. One Pom mag that regularly tested KTMs Works Bikes ( they tested Ferris's Bike It 3speed YZf that he did so well on, too) each season had, one year, the 250fs as having 4 gears - once again, engines that rev to the moon. A 2502T - not so much - most have peak power at between 8 and 9k. Yes, you can tune it for much higher power at higher revs, but it won't be much of an MX engine.

You can always carry a variety of sprockets and chains. Hell, in (many, many) years gone by, in OZ SX , teams / riders would sometimes run no gear lever, to avoid missed shifts/ impact / center punch shifts, on our tiny little AX style SX tracks of the era.

That ignition from one of the former Czech / Slovak countries? If it's the one I'm thinking of, they also do new powervalve motors, which, I've heard, are difficult to get, ex Honda.

The gen3 CR 250 frame is still my favourite Honda AF chassis.

C50_skarmavbild_2017_02_17_kl._00.15.41_1487287428 Olson Quote
12/23/2015 2:54 AM


It's rude to talk about religion. You never know who you're gonna offend

C50_min_1014_2_1464587959 blackdiamond Quote
12/23/2015 3:22 AM

You nailed it on the ignition, nice work! That is the one, I am surprised someone hasn't figured this out before now and adapted this to an MX application (to my knowledge)

It's going to take some work to sort it all out but it seems like a fun challenge to me and I think it will pay off in the long run.

In regards to the transmission, I basically eliminated 1st and 5th and raised the overall final drive ratio ever so slightly. In my personal experience, I have found the CR250 has fairly wide powerband with a lot of over-rev once it is set up correctly.

My thinking on this 3 speed concept was really basic and simple:

The only time I have ever used 1st gear on 250cc two stroke was on my way to the starting gate, and more often than not I immediately shifted to second because I am so impatient. Very rarely do I ever use 5th gear when riding MX. I typically run stock or taller gearing to spread the power out and help with traction as well. When you combine that with a factory level modified engine that has a ton of over-rev you have a bike that is capable of jumping 100-120ft. obstacles WFO in 4th gear (?)

Considering I am at the +40 stage of my career I am good with that. Once I started paying attention to the gears I actually use on literally every track I ride I realized the only time I used 1st was in the pits and the only time I used 5th was when I took a trip to the dez once a year. Eliminating those two gears sounded crazy at first but made sense after I started paying attention to what was really happening on the track.

I know of several riders that have confirmed they ride an entire SX track in one gear, with the exception of shifting up for the whoops. Even better, AC222 defeated Alessi at the Glen Helen GP in 2010 while riding the KTM 350 after his shifter fell off in one of the the motos. He raced the entire moto in 3rd gear. At the end of the day I just do not feel a bike used exclusively for MX needs 5 speeds but I could be wrong.

C50_min_1014_2_1464587959 blackdiamond Quote
12/23/2015 3:28 AM

Here is a before and after pic of the transmission:

C50_mx_bear_1457753510 Bearuno Quote
12/23/2015 3:58 AM

Yes, as a purely MX engine, 3 gears is probably fine - especially if you know the tracks you are going to.

My 500, I think I could get away with just 2 gears, 3rd and 4th, that's if I ever again spent much time on an MX track......

And, it's not hard / expensive to have a couple of sprocket sets and dedicated chains for different tracks in your tool box. I made mention of the 4ts mind boggling rev range , well because of their mind boggling rev range...... And feared you might be shooting for crazy revs. It was either Dirt Bike Rider or the ( now E only) MOTO mag ( British mags) that did those works bike tests, and they regarded the Bike IT conventional / carburettor Yamaha engine 3 speeders level with, or perhaps higher than the Works KTMs they tested. Those 4ts sure can rev - well, that's how they ( even the 450s) get their power, as well as twice or 1.75 times the Capacity advantage......

That series of modification articles I mentioned - I think the engine work might have been done by Terry Varner. It's good to see someone Not automatically trying the earlier mechanical PVd engine. The RC engine has plenty of potential, it's just far few people tried / too many found the ( nowadays pathetically low level of) 'E' tech a bit intimidating. Quite Laughably simple, nowadays, as I interpreted you as implying.

I recall either Dirt Bike or MXA did an article on McGraths 250 he used just prior to going to a 450, just prior to retiring. Mid/ high 50hp levels I think, whilst being quite civilised - Varner might have been the tuner?

C50_s200x600gblk_1452002839 Reefer Sutherland Quote
12/23/2015 4:17 AM
blackdiamond wrote:

You nailed it on the ...more

I had that ignition. Just never bothered posting about it. Their basemap alone is worth more than any one bolt-on mod alone. 3-4hp peak.

C50_avatar9000_1_1436970212 Markopolo400 Quote
12/23/2015 7:20 AM

Awesome build!

C50_min_1014_2_1464587959 blackdiamond Quote
12/23/2015 7:28 AM
Bearuno wrote:

Yes, as a purely MX ...more

It's interesting that you noted the potential of the RC valve engine because I feel the same way. They make a ton of power if set up correctly. The guys that build those crazy "super karts" are getting ridiculous numbers out of these engines (although using a very specific setup that wouldn't work for MX)

I feel the RC valve engines are deceptively fast because they are more linear but in reality I think they will make 3-5 more HP than the mechanical PV engine in most cases.

C50_img_1291_1490419411 bvm111 Quote
12/23/2015 8:10 AM
blackdiamond wrote:

You nailed it on the ...more

Reefer Sutherland wrote:

I had that ignition. Just ...more

You guys have me interested in the ignition, could it work on a 2006 KX250? I have a Tom Morgan tuned engine and it rips... But I like the idea of trying new things as well, even though I really need to get my suspension set up better before I go looking for extra HP!

Never try to argue with idiots; they will only bring you down to their level.....and being more experienced, they will beat you at their own game!

2016 KX450f
2006 KX250

C50_image_1480875671 Redrcr34 Quote
12/23/2015 8:29 AM

Nice build, can't wait to see the finished product

C50_min_1014_2_1464587959 blackdiamond Quote
12/23/2015 9:11 AM
blackdiamond wrote:

You nailed it on the ...more

Reefer Sutherland wrote:

I had that ignition. Just ...more

bvm111 wrote:

You guys have me ...more

I am almost certain it is not offered for that bike, they are only offered for the RM125, CR250 and CR500 to my knowledge. They don't really focus on MX but you could always try to get in touch with them at www.zeeltronic.com

C50_profile_1424660203 ML512 Quote
12/23/2015 9:21 AM
blackdiamond wrote:

Here is a before and after ...more

RCR does a fantastic job... Glad to see other people using his fine work.

Vital MX's Do-All Minion...

Instagram: @Michael_Lindsay512

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