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CR 125 Tuning where to start and what to do


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Hello all,

 

I was asked to setup a new topic regarding CR125 Tuning.

The history: 

 

I found a CR125 04 last Summer at Craigslist the bike was completely beat up and I started the rebuild.

Unfortunately I didn't took many pictures during the rebuild, however I made some changes when I rebuild the engine.

 

Everybody I know told me the CR125 04 has nor power at all and only big bore will help.

I started investigating and was anything else than satisfied.

 

Long story short:

I found many Webpages and companies who would provide the service but non of them told me to send the Case its not needed.
When I did racing in the late 80s and 90s I raced mainly 125cc bikes. At the beginning bone stock as I got faster I got my bikes tuned.
Don't want to use the word porting because this is just 1/3 of the entire process.

 

While reviewing the offerings and talking to some "Tuners" I got the impression its all about commercial. And hey that's my impression anybody who has a different experience please share with us.

 

When I was racing and my engine where tuned the guys I was working with requested the entire engine (Engine, Carb, exhaust) and NOT only the cylinder and head!

 

The fact that nobody was offering it right from my point of view I started investigating different 125 engines especially KTM, RM and YZ to be known having the best engine in this class.

The KTM is to much different to the CR so I just looked at the RM and YZ engine and skipped KTM it for now.

 

When I looked at the RM and YZ Case I found huge differences compared to the CR125 04 not so much to the 05 Case.

 

What did I decide?

With looking at different engines I decided to start working on the engine by myself.

 

Starting with clear up imperfection in Case matching, Casting flaws.

I will show some of the changes I did in the next postings, once I rebuild my engine again I will make pictures from the case changes I made.

 

 

 

 

 
 

 

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When I start comparing the different engines I found that the YZ Case is a lot different to the CR125 especially 04 case.

 

I start looking at Ebay to find a defect YZ case and I found one which was cracked but the intake and crank shaft area was ok. 

Paid 20$ for it so worth the investment.

 

In the below pictures you will see some red and green arrows:

Red means imperfection

Green might be interesting to look at.

 

The Cr125 04 Case from my point of view is a disaster and they made so many failures.  

 

1. The intake is like a step instead of a ramp to accelerate the airflow. Usually the intake should act like a funnel

2. The who knows why blocking wall to the crankshaft needs to go

3. The very far left edge is like a wall not a ramp compared to a YZ125.

 

CR125-04-1_zpsbd11afb3.jpg

 

What did I do:

1. I applied epoxy to the intake and removed the step

2. Removed the crazy wall towards the crankshaft

3. Applied epoxy to create a ramp style like you can see on the YZ125 picture.

4. aligned the Cylinder to the crank case no overlay anymore.

 

Based on the fact that I want to build a CR125 05 engine I had a look on a 05 as well.

From my point of view this is way better than the 04.

If you want to make some changes on the 04 copy the 05 and you should see a huge improvement already.

I have although a CR125 top end and I could compare the ports looks almost identical

.

Now the CR125 05:

1. To me the intake looks good its smooth and act like a funnel

2. The left edge to the transfer port can be smoothened a little bit.

I would only smoothen the left edge in the picture on both sides fill it with epoxy and make it smooth.

 

CR125-05-1_zpsaa2a22ec.jpg

 

 

Now the YZ125 05 and up

 

If you can see the intake is smooth and nice.

the left edge to the transfer ports are like a ramp not a wall

But what is interesting is the entire cutout from Cylinder downwards is smaller about 6-9mm less than with the CR125.

  

yz125-05-1_zps97a333e8.jpg

 

Just in Case somebody wants to ask what I am using for Case stuffing?

 

I use:

Devcon 10610 Aluminum Epoxy Putty F

 

and Cleaner Blend 300 to clean the parts.

 

It would be great if we can start a good discussion to make our beloved CR125 a bit better and faster.

 

Since I made those changes and some others the bike ripps!

I will add more information what I did to my Engine as well soon.

                       

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Ok here is what I would recommend to do.

 

1. Match Case with cylinder:

 

Mount the Cylinder to either the left or right Case no gasket and see if the Cylinder aligns with the Case.

If not here is what to do:

Generally you can say removing material from the cylinder will increase top end but be careful to much volume might not be good in the case.

Adding material to the case will increase the low to mid power.

What I did was just removed the material either cylinder or Case had both to make it smooth.

Once that is done do the same with the other half of the case.

 

2. Smooth the intake: 

 

This is a bit more complicated but can be done I applied a bit epoxy right in the edge of the step see picture  "apply epoxy" and use a Dremel or what ever to make it smooth.

It should be formed like a little ramp but be carful don't build a freestyle ramp. ;-)

 

3. remove imperfection to the reed case I use VForce 3 but had used the standard Block at the beginning.

Vforce 3:

If you mount the rubber boot (Insulator) to the Vforce Reed cage you can feel an imperfection of about 1mm all around.

Means the rubber boot is to small and could be widened will increase the amount of Air into the engine not much but its known that the 04 likes more air.

 

Standard reed Block:

When I started with the stock reed block I removed l to make the air flow smoother into the reed cage.

I will add a picture later.

 

4. Remove the little Wall towards the crankshaft (see picture remove wall)

 

I cut away the entire wall and smoothened it out. I you have never done it be carful not to cut or sand the Area were the crankshaft  spins!

 

5. Improve Airflow into transfer Port especially the one close to the exhaust. 

 

I applied epoxy to the case like in the picture (epoxy transfer port) and created a ramp like you can see on the YZ125 Case in a previous post.

This will allow more air into the transfer port. But be care full both sites should be equal

 

CR125-04-2_zps02b43bc4.jpg

 

 

YZ125-05-2_zps8c73bf32.jpg

 

6. Select the right size Piston:

 

Measure the cylinder in the top, mid and lower section and use the right measurement. 

Select the Piston of your choice and select one that you get a piston-cylinder clearance of at least 0,035 mm but not more than 0,05 mm.

Do the same with the piston ring and adjust the clearance according to the manual. CR125 04 is 0,4mm but not larger than 0,55 mm

 

7. Spend more time in jetting:

 

Especially the Mikuni Carburetor needs a lot of attention but once you got a base setting use it as a starting point. However the CR125 with the Mikuni reacts strong on different temperature and humidity.

 

What parts do I use:

Intake:
VFORCE 3 reeds with the above modifications

Carburetor:
PWK 38mm with a #7 slide will make some test with a #5 and #6 slide soon.

That’s my base setting:
Main: 182
Pilot 48
Airscrew: 1 ½
Needle: A715/289R it’s the stock CR250 99 needle
Slide: #7

Airboot:
 

Airboot from the CR125 2005 and up gives some air the difference of this boot compared to the 2004 is minor but the result is good.

Piston:

Don’t kill me I use Namura and have good experience if you choose the right size.
Change rings about every 10-12 hours of riding
Change of Piston about every 20-25 hours
The reason I use Namura is pricing I can get a complete top end for 80$ including gaskets.
I change the piston more often but that’s ok for me.

Gaskets:
 
Speaking of gaskets I use all gaskets from Namura except the base Gasket I order the high compression from Cometic is about 0,3mm thinner.
Porting will be change slightly but compression is way better.

Exhaust:

FMF Fatty
ProCircuit shorty (sorry Guys I like the sound and give a little bit more power down low.
 

Drive:

Prior I made the changes I used 12/52 now I am back with 13/52 and I am happy with the gearing.
I don’t use O-Ring Chain this eats up to much power.

 

That’s pretty much it worked on it for some month and made one change at a time.
The CR125 04 is not the best engine but if you do your homework this engine will work mine does.

If anybody has questions just let me know I am happy to answer.

Edited by pepe001
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A very interesting topic indeed. Keep pictures and info coming!

 

BUT- When i see you using 12/52 or now 13/52 gearing it is obvious your engine still isn't that powerful!

 

I personally have a Mugen kit on my HPP engines and when i still raced used 13/49. Now i'm still on 13/50 but anyway - longer than stock. If you have a strong engine it will pull those gears. If you need that short gearing your powerband has to be very short.

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Hi Nino

 

gearing is all about the tracks I never tried a longer gearing yet, but it might work.

As what I see on your bikes the 04 engine will never keep up with your parts they made to many mistakes with that engine. 

 

The 12/52 was when I got the bike bone stock it was a turd. You couldn't even lift the front wheel on a straight.

My Sons KX80 is more powerful but the battle is on.

 

I am following your AF conversion quit a time already cool bike, and nothing could beat a Mugen Kit.

You are blessed with a nice base on parts. I was stupid when I quit racing 15 years ago sold everything.

 

But the last summer it catches me again sitting on a bike. but its cool with 49 years of age and going back on a 125, I love it.

 

The tracks I am riding are really short if I would go lower than 52 I might never see the 5th gear.

As of now I barely make the 4th gear the power band is pretty good and the tracks I am riding on are pretty short.

 

I know your tracks in Switzerland rode some although in Italy I am from Germany but living in Georgia.

 

Mostly I am riding at Highland park right now but even Bremen is only short straights. 

Sometimes I miss those tracks in Europe with a longer layout although I miss the deep sand when I raced in the Netherlands. 

Was grown up close to the Dutch Border.

 

Based on the feeling I think the bike would do good in deep sand.

 

However my journey is not over in making the bike faster or better performing.

I started with the obvious parts where you can see that it doesn't work or never could. 

So far I am pleased with the result rode last week in Highlandpark and was asked by a thumper is that a 144 big bore.

Had a little smile on my face. :ride:

 

I am taking it not that serious anymore just riding for fun no races anymore. But cant keep my fingers from making thinks better.

Recently started to teach my Son how to ride, got him a KX80 in descent shape but solid base. 

 

I will still work on my bike but will focus more on him because he is doing good after 4-5 hours of riding.

I will not push him at all but he has talent which can be uncovered.

 

The next step on my bike will be I am going to bring a VHM Head from Europe with different inserts.

But when I looked at the cylinder I saw some parts what could be changed.

 

What did you do on your bike beside the Mugen Kit any special modifications? 

Maybe there is something in there what can be copied.

 

Unfortunately I just rebuild the top end again but in about beginning of march I will have the engine open again.

 

It would be great if we can share some tips and tricks here.

Thought about getting a big bore kit at the beginning but when I realized that they only focusing on the cylinder and head I walked away. 

 

Looking forward to an active discussion.

 

A very interesting topic indeed. Keep pictures and info coming!

 

BUT- When i see you using 12/52 or now 13/52 gearing it is obvious your engine still isn't that powerful!

 

I personally have a Mugen kit on my HPP engines and when i still raced used 13/49. Now i'm still on 13/50 but anyway - longer than stock. If you have a strong engine it will pull those gears. If you need that short gearing your powerband has to be very short.

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Was intrigued from your statement with the transmission just checked the standard gearing:

I guess you are using a 6speed?

 

Standard drive is:

CR 125 04 is 13/53 5 speed transmission

CR125 97 is 12/49  6 speed transmission

 

I did some calculation based on the actual drive:

 

As it looks we are not that fare away due to the fact that you are riding a 6 speed transmission right?

The difference 6 speed to 5 speed if stock gear box is as follows.

1st gear 97 is shorter 

2nd gear the same

3rd gear 97 shorter

4th gear 97 shorter

5th gear  the same

6th gear don't have that one yet  🙄

 

Taking the entire transmission including crankshaft and clutch we are even closer!

If I did the math correct it looks as follows:

 

Lets say we have 10000 RPM engine

6 speed with 13/50

5 speed with 13/52

 

Clutch basket is 64 teeth on both engines

Primary is 20 teeth on both bikes

 

We will see on the rear wheel following RPM:

 

Gear   rear wheel 97  rear wheel 04

1st      344                  338

2nd    435                   418

3rd     476                   510

4th     571                   603

5th     718                   691

6th     812                  no 6th gear

 

Means

1st, 2nd and 5th has a longer transmission on your bike

3rd and 4th has a longer transmission on my bike

 

But would be cool to see how my engine does with the 97 6 speed transmission that's my next project.

 

I was thinking of going lower with the rear sprocket for testing.

Seeing those numbers that will not work the gap would be to large between 2nd and 3rd gear. 

 

That will reflect my feeling on the track, 1st and 2nd felt to short 3rd and 4th were spot on cant use 5th gear that often to judge if I shift to 5th I have to get on the brakes the next turn will be there. 

My sweet spot on the Track I am riding is 3rd and 4th Gear, corners 2nd and 3rd in really tight I might go back to 1st gear when I am riding.

 

Thanks starting that discussion Nino showed me I have to go to the 6 speed transmission will be better, more shifting but hey its a 125. 

 

A very interesting topic indeed. Keep pictures and info coming!

 

BUT- When i see you using 12/52 or now 13/52 gearing it is obvious your engine still isn't that powerful!

 

I personally have a Mugen kit on my HPP engines and when i still raced used 13/49. Now i'm still on 13/50 but anyway - longer than stock. If you have a strong engine it will pull those gears. If you need that short gearing your powerband has to be very short.

Edited by pepe001
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standard drive on 2004: 13/53 (man i can't believe they went this short on final gearing!!)

standard drive on the 1998 5 speed engine most used in my AF: 13/51

 

5s vs 6s: the difference is so small you don't even notice it riding them face to face! I have three 125cc bikes and two 5s engines and two 6s engines and they ride the same. You obviously loose the 6th gear but beeing a motocrosser i never ever rode 6th anyway. Me too i'm 47 years old and in 15 years of doing championships and total riding of 28 years only once did i use 5th gear pinned on a MX-track and that was just last year 😉

 

I see many guys using smaller gearing to compensate for lack of horsepower or lack of ability to keep the revs up. But a powerful engine needs "room" to deliver it's ponies. If you have it so short it will run out of revs too soo. Me too i'm riding very small tracks these days. Often it's just some club races or practicing on small tracks. So what? I'm riding in 1-3rd gear. Why gearing your bike so short that you can reach 5th or 6th gear? It makes for lots of shifting but you actually don't move....

 

But for the rest of your topic we seem to be on the same level. Me too i'm addicted to 125s, still have a love and passion and just love it to rev the bike.

 

My engines are full of modifications. One which might be of interest is the modification of the HPP (powervalve) governor which makes for slightly later opening of the exhaust valves. I found out that by adding a bigger carburetor the bike had a little "bog" right where the powervalve opened. It seemed as gasflow would be interrupted when the valves open. It seemed that the velocity of the gasflow was hurt by the big hole of the exhaust so i tried to keep the valves closed a little longer and indeed that helped to make for a smoother transition. On my HPP engines it's done by adding shims which pre-load that spring (check the golden shims in the picture below). This makes for a ca. 400-600 rpm later opening of the exhaust valves:

8466428873_621a91112b_z.jpg

 

 

One of the next steps will be to make all the surfaces where you have gas flow along rough. But really rough !! In contrary to popular thinking this actually helps gasflow and makes for better airflow. It's the "Golfball"-principle.

Here's a carburetor done the way i will have treated my cases and ports in the cylinder as well. A good friend of mine and 7-time swiss champion has spent a lot of time and research and the engines treated this way already make much better power without any further modification/tuning! We are not talking about HP gains on top but rather how it hangs on the gas.It responds better and picks up sooner. Noticeable.

8882270774_cd7a510a2d_z.jpg

 

By the way - i personally prefer old-style Keihin PJ carburetors over the newer Keihin PWK Airstriker carbs. I tried all kind of carburetion setting but in the end none of the settings tried could match the power ade by my old and trusty PJ carb. I use the exact same carburetions since 21 years and it still goes so fast. So the 38mm PWK Air Striker is sitting in a box and collecting dust... 😉

 

This is my actual reedblock:

a tuned Boyesen RAD where i simply cut off the aluminium insert and replaced it with the stock rubber-thing. Check the inside as well! It's alll rough as hell too. Not smoothed or polished...no-quite the opposite. It runs awesome!!

8585720748_50b017144e_z.jpg

 

8584619703_6ea00d76cb_z.jpg

 

This is how the stock Boyesen looked like: it had a ugly border all around. I rode it before and replaced it with a Mototassinari VForce3 which i felt was a little better. But the modified Boyesen now beats the VForce. Better middle with still great ovverrev.

8573178813_cd420b4fba_z.jpg

 

I did a lot of exhaust testing and finally settled on my italian Messico pipes. I used them already when i was a active racer already but now i tried all kinds of pipes yet the Messico is the best. Second choice is the FMF Fatty which just comes a little short when the engine enters the poowerband.That's where the messico is so strong. Yet it still revs to the moon and pushes all the way through.

Pro Circuit for me is too slow. It makes a lot of revs but no torque. You need a lot of clutch. It revs to the moon but never has a really good pull.

 

http://www.messicoracing.it/pipes-125-144-200.html

 

15550130505_0e0b26d984_z.jpg

I have a bunch of pipes hanging on the wall...different stock ones, Messico, FMF, Pro Circuit, SPES, DEP....

11431462264_59abc29e9f_z.jpg

I even have a coned pipe but had it custom made to "Messico"-specs:

13464965535_e96d196973_z.jpg

 

You wrote you will buy a VHM head with removable dome...well - maybe try contacting german Tuner Helmut Tisberger as he is capable of converting your stock head to a domed head with removable domes. I think you will spend less money and get a better product.  Below you see a Mugen head which he converted to a domed head with changeable inserts. I now have 3 different shapes to choose from. I had to do it as the head i got with one of my Mugen kits was off and older Mugen with domed pistons. This head was not usable for me. Instead of selling it and waiting to find another one (which will not happen and if so would have cost me twice as much money...) i had it modified. This mod should be much less money than the VHM head!

 

Helmut Tisberger Tuning:

http://www.wz68835si.homepage.t-online.de/Bildergalerie

 

8419266543_c16991699d_z.jpg

 

8420364868_2e3cb1911a_z.jpg

 

Other things i learned are that newer CRs have much heavier transmission parts. The clutch has 1 aluminium and fibre disc more as well as a wider clutch hub and basket...why? Those engines aren't offering any more power than the older HPP engines before but they still beeded up the clutch and by doing so added weight!! When i did a waterpump part lately i discovered that even those parts are a lot heavier. What i learned from some tuners is that the later CRs suffered big time from overbuilt and heavy transmission parts where a lot of friction and weight eat away precious ponies.

 

Here's the old part on the left side (see all the marks that made for loss of coolant...) and the new one which is used in all newer CRs to its right. When i ordered from Partzilla they shipped me the new one whoch now replaces the other one with the plastic gear. Even ba holding it just in your hand you noticed a dramatic weight increase so i actually put it on my scale....needless to say i sold it right away as soon as i could get the "correct" part which i could find and buy somewhere in Germany.

15514109636_b9bba2a895_z.jpg

 

Old one:

15535062291_44b24c1923_z.jpg

New one - TWICE the weight !!!!

15514050426_18e431c57a_z.jpg

Edited by nino
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Great Info and I see that I did some changes what you did already. 

.

One thing the 04 based on the manual means 13-52 but never mind I will try it with either 51 or 50 have them here anyway.

Maybe I put it on today its nice in Georgia today about 15 C and sunny, I like it.

Going riding today.

 

Maybe that solve my concern with not being able to use 1st gear more often it is way to short. 

 

The Carb is it a 36mm and the coating was done as powder coating?

Will increase the airspeed you get a little less top end but who cares.

 

I absolutely agree with the discussion in polishing almost anything to smoothen the airflow.

That doesn't bring anything the air needs a bit roughness to get a better mixture. 

Polishing the exhaust is ok but although useless because you have to do it after each use.

 

Saw the Messico exhaust used them on my TM liked the pipe.

Maybe I need to order one taking back with me a little thinner wall dented them frequently but the power was awesome.

 

The governor is cool but I do have the crazy servo motor.

But at least it doesn't slow down the engine with moving parts.

 

I did almost the same with my reed block and rubber block.

Just put the VForce in about 2 weeks ago and what I saw the airflow into the reed block is not straight at all.

Once you hit the reed block there is an about 30-40  degree angle.

I think there is room for improvement, got a spare Reed block from ebay to mess around with.

 

Good idea about the head I am back in Germany next week for Christmas will contact him and see if I can get mine done.

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Nino, your attention to detail is amazing.  One thought though, the CRF250 is using the same clutch as the 125 in the later models.  I wonder if that is why they beefed it up, to handle the 250's power. 

well - than it seems obvious they share parts and saved money...but it hurts the 125s performance big time !!

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Maybe that solve my concern with not being able to use 1st gear more often it is way to short. 

 

The Carb is it a 36mm and the coating was done as powder coating?

Will increase the airspeed you get a little less top end but who cares.

 

Good idea about the head I am back in Germany next week for Christmas will contact him and see if I can get mine done.

A 53 chainring is way too short. On a stocker 52 might be needed but as mentioned a powerful engine should have no problem pulling 51 or even 50t chainrings.

 

I use bigger 38mm carburetors. 36 was stock but a bigger carb obviously makes for better mid-top. The rough inside is no coating but rather from sandblasting with a special "grain". You might ask Tisberger about this technique as well.He too is a big believer in this treatment. (He was the one doing my Boyesen RAD reedblock).

 

As mentioned i tried all sorts of jettings on a 38mm PWK Air striker (also JD Jetting kits) but the PWK never delivered the same top power my PJ would offer. So as of now i still use my 23 year old PJ carburetor 😉

 

head:

try and get your hands on a used cylinder head ! They can be found for "no" money. It wouldn't matter if the dome is shattered as well as it gets taken out anyway and replaced by machined domes later on. And you will have to measure the height of your piston though! Otherwise he won't be able to get you a machined dome without knowing exact measurements. Maybe contact him now before you leave so you could still take some measurements on the bike.

Edited by nino
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Contacted him regarding the head already because I am flying back next Saturday.

I will take a used head with me and measure the engine.

 

I used a 53 rear sprocket for about 20 minutes but went back to stock immediately was useless.

I try the 50 today and see how it goes, didn't thought about it at all to use it so far.

 

Still have an old modified carb from my TM and GASGAS was the same engine.

I will give it a try.

 

Hey isn't it older is better sometimes 😉

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Contacted him regarding the head already because I am flying back next Saturday.

I will take a used head with me and measure the engine.

 

I used a 53 rear sprocket for about 20 minutes but went back to stock immediately was useless.

I try the 50 today and see how it goes, didn't thought about it at all to use it so far.

 

Still have an old modified carb from my TM and GASGAS was the same engine.

I will give it a try.

 

Hey isn't it older is better sometimes 😉

I'd do at least 2 or 3 different shaped domes. So you have the choice later on. For sure he will ask you to measure the height on your engine...He is a very precise guy. Old school but very good !! Let him know Nino sent you!!

Edited by nino
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Awesome info guys....thanks for sharing!

No Problem its a lot of fun thinking of improvements.

 

Hope more are joining with there ideas and what they did.

 

Finished some test with a longer drive 13-50 and it went well.

Had to use a little clutch but not as much as I though I had.

 

Have to get used to it can ride most of the sections 1 gear lower now. 

Before I was shifting between 2nd and 4th some 5th gear.

Now I can use the 1st gear up to 4th didn't hit 5th today.

 

Ok highland park doesn't have long straights though overall it was good. 

Seems the slow 04 is catching up some speed without big bore! 

 

Going to add a Head like Nino requested have ordered it already. :ride:

Will be the next step for optimization working along with on the ports a little bit..

 

This time I will make detailed pictures about all changes.

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