CBR125 Modding Help.

[TooSpecial]

Hi All.

I have some questions about modding a CBR125.

I need to achieve 17HP if it is at all possible.
The bike is going to be raced next season, so I need to get it as beefy as possible.

The general plans will of course be to have a race fairing kit with no unnecessary weight. The sprockets will be changed each race depending on the tracks needs for maximum acceleration or top end.

I would like some guidance from others who might have worked any of the CBR125s for performance.

Could you possibly list things in the order of what you expect might give the most gains to power.

And lastly, what other mods would you recommend I look into that would just make the bike feel better. EG: I have been flipping my gear linkage for ages. I prefer to press down to change up a gear. It makes me feel more comfortable when the bike is leaning over. Stops me from having to get my foot under the peg when its near the track surface.

The rules i need to stick with:

I cant change the rims to any size other than what was standard from the factory. And I cant do things like putting in a big bore kit. I can work on the head. And i can change internals if it will help.

I just have one side question.
Are aftermarket filters worth it. I feel a small increase what so ever would be as it is a race bike. But would I not get better performance by simply removing the filter and using a small piece of piping aimed straight forward with a stocking on it to filter the big things out, sort of like a ram kit. The engine only has to last one season. which is about 4 hours of racing.

Thanks for the help guys. I appreciate any advice.

[Rogerborg]

Sorry, nothing useful to offer, but thanks for asking an actually interesting question about tuning a 125.

Presumably you've got a noise limit on the exhaust, but can you otherwise do what you like with it?

What's your budget?
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[lihp]

What series are you racing it in?

And have you checked the regulations of the series to see what extent of modifications are permitted?
____________________
covent.gardens: lihp is my most favourite member ever

[Teflon-Mike]

To finish first.. first you must finish....

Being first past the post, starts long before you start the bike, and IME more attention you pay to making it LAST rather than FAST more often gets you over the line, with a chance of being the first...

"What can I fit to my bike to make it faster?"
"Have you tried a better Rider?"
Bit of aged paddock humour of much truth... The 'better rider' isn't just faster, but they tend to be a lot easier on the machinery; they don't fall off as often, which saves a lot of swearing trying to find all the bits, and bending stuff straight.. but also tends to mean that they are on the grid, at every meeting, with a chance of crossing the line at all of them, and taking championship placings, rather than just one-off placings or fastest laps.

Lets get statistical... you have a 'field' turn up to the first round of the series, of perhaps 30 entries.. you will have a fairly normal distribution in there of good riders, and bad, good bikes and bad, ad a skewed distribution of rich and poor.

Those with most funding, stand the best chance of being at every round, on a bike, with the capability to win... There might be ten of them... and they'll be hard to beat. BUT if you want to stand a chance, the sums that will do it have £-signs not BHP ratings.

Those with least funding? Well, odds is they will run out of money before the end of the series; sooner if they strike a calamity, like crashing out or blowing up.... there'll be anther ten of them.. and you don't really need to worry about beating them; you JUST need to be sure not to be one; watch the pennies, and keep the bike 'whole' and make sure its on the grid at every round.

Good Riders? There will be ten, that are better than average, ten who are abut average, and ten who aren't all that good.

BUT, there's also the bikes; and again, there'll be about eight that are a little quicker than most, and about twelve that are barely able to scrape through scrutineering, and tan some-where in between, tending towards the gaffer-tape specials as the season progresses.

SO, out of a field of 30 entries, only half are real 'competition', likely to be at every round. Five of them, will be better riders on better bikes, likely to be hard to beat. Leaves a 'real' competition of just ten, who wont be on the best bikes, or be the best riders, or the best funded.. they will be good rides eeking out the budget on an average bike, or average ridrs chukng their cash at a better bike, or other perms, that add up to a 'sum' thats ot far off 'average'.

Means that REALLY you don't need to be much more than an average rider, or have more than 'average' funding, or have a much more than 'average' bike... to be in contention, in that ten, and have at least even-odds of a top-ten series placing at season end, AND be chasing the 'top-five'...

Which leads to an equation that says that what wins races, more championships, isn't 'maximum' but 'maximisation'.. putting together the best over-all 'package bike, rider, funding... and getting as much out of it as you can, not puttig all your hopes one one element, a great engine, or a great chassis, or a great rider or a great budget, but 'enough' of it all.

Which takes up back to top. Look at the big picture...

Within the rules.. of which the first is, it's only cheating of you get caught... that begs consideration.... you can waste a huge chunk of budget building a motor 'to regs'.. which is all rather pointless, if some-one's slapped on a 180-Big-Bore and not got caught..... or has skimmed the head to 14:1 and is running on Avgas.... THAT is a topic all on it's own, but keeping to the spirit of the regs...

FWIW, I believe that there's an HRC cam kit, for the CBR125 series, and that without that, you don't stand much chance of getting more than the book-power from one; like most small displacement 4T's, that's where the major impediment to making much more power is. If you can lay your hands on one of them, the great; but about the only place I'd spend money on 'tuning'.. and I'd fit as per kit instructions and leave the rest of the motor alone, for reliability.

Decent tyres would get money; decent brakes would get money; decent shock would get money; the the rest, making the bike as light, tight and 'tidy' as I could.. WITH peculiar regard to the ACU Standing-Regs, vis lock-wire and number-decals and mudguards covering approved % of tyre etc..... another bit of paddock lore... you can have the best bike in the paddock, but wont win a thing unless the scrutineers let it on the fucking track!

Spirit of the rules is for a close, low-cost series...

So, bigger picture; Keep it Simple, Keep it Standard..ish.

Prep the bike for reliability - Prep the bike for crashability

And save your money, for entries and practice sessions, and making sure you can put it on the grid at every round.

Staying Power, not Brake-Horse-Power... be at every meet... and if you don't win, remember, there's always another race.....
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Rogerborg]

Incredible advice, Tef as always.

How many race seasons have you done?

On your modern supersports 600.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[TooSpecial]

Thanks guys for responding so quick.

The noise is regulated. To a point. Nobody gets tested on their noise levels unless another team has a complaint about it.

I can do pretty much whatever I want other than an over bore or changing the size of the rims. Of course there is no NOS or Turbos/Chargers allowed. And I'm not certain right now. But To my knowledge we have to race on regular pump gas.

Work on the head is allowed. I have asked if we can modify the crank, if i ever felt the need. Which i wouldn't imagine I would. But its nice to know if we can. But I haven't had a response on that yet.

So weight saving by any means necessary. I was thinking about going without a battery. But one rule is the electric start has to work at any time. So I might just go with a really small battery.

I don't have a set budget. I cant afford to go crazy, and it kills me, as you say Teflon there are the big money guys. The ones who's parents sold the child who cant race to pay for he kit for the one who can. Haha. But luckily I have access to a machine shop and a dyno and such race prepy and engine buildy type things. I do agree with you on the build a reliable bike front. And that's one of the reasons I don't want to mess with the bottom end. A side note, we are allowed to drill the stock pistons skirt to lighten it.

The series is pretty much just a club series. I plan on going to the 250s in two years. And I will still be racing the 125s when I do. That way I will get a lot more track time than the guys that just race 250s. And I will be learning more about the mechanical side of things building and maintaining my 125s.

I want to be competitive. So I have to put some money into the bike. I do know that a lot of the race is in the one riding, so i have also set aside some cash for lots of private practice sessions. And I have some classes with guys who used to perform top of the pack when they were racing. So working on my technique is a big priority in the first season. And if I finish the season in a respectable place I will be able to judge if the bike needs more money or the race lessons.

I have played around on some 600s. But I spent most of my riding time on 400s. I can keep up with the guys who lead the middle of the pack with the knowledge I picked up messing around. So hopefully with some real race specific work and a bike with a little grunt I can be at the back of the leading pack by the end of the first season. And kill it the following season.

This will be my first race season I enter.

[TooSpecial]

I have noticed the top 5 guys have the cash and put in some track practice leading up to a race. I honestly don't think any practice racing technique.

The next batch of guys are there spending some cash and just showing up for races. So unless I just am not meant to race things should go ok.

Here is something I was thinking about, that will be allowed. And that nobody has done. I assume due to price and no gains return. But like I said I have access to an engineering shop so I can do it for free.
I know the guys like to fill their tanks for the day and go racing. Because having a half empty tank with fuel sloshing around makes the bike uncomfortable to ride. So what I want to do is cut the tank down to hold a bit more than enough fuel for a heat. This will remove a lot of metal, dropping the weight of the bike some more. And with a tighter tank the fuel wont slosh as much. Plus I won't have the weight of all that fuel in a full sized tank. Haha. I will just have to remember to top up before each heat.

Any thoughts on that. Or other odd things that may help.

[Rogerborg]

Give it a while, some folk who actually have a clue what they're on about will show up and help.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[Old Git Racing]

The normal route for 4 stroke tuning would be cam, gas flowed/bigger valve head/valve timing, higher compression piston, bigger carbs, matched exhaust and programmable ignition. I suspect your problem would be sourcing the bits for that particular bike.
Would this race series be in Thailand? Where do others in the series get bits, which tuners do they use?
The only real advice I can offer is lose as much weight as possible and have bigger balls than everyone else. .

OGR
____________________
2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Robby]

To stop fuel sloshing around you just need some foam in the fuel tank. Google it.

Weight loss - you can probably ditch the charging system (alternator and reg/rec), and switch to a lithium-ion battery. You'l be getting rid of all the lights and changing the fairings. Can you cut down the subframe? Anything to do with taking a passenger can be removed.

You might be able to squeeze some more power out of the engine, but it will be a lot of cost for very small returns. I would put that time and money into learning to race.

[TooSpecial]

The racing is in South Africa. The guys who really put money into their rides are very tight lipped about the whole thing. If I want any real parts I'm going to have to order them offline. All the bits i need to build a race head will be ordered. And I would like to get an ECU that is programmable. With the ECU though I have no idea what brand is any good, that isn't over priced. I don't mind spending a little on the ECU if it is something I will be able to put onto my 250 when I get there.

There will be foam/a plate in the tank to help with the sloshing. The main reason I want to cut down the tank and see how it goes is, one I'd be interested to see the effect the weight saving would have, and two I love to cut and engineer. So its a bit of a fun thing to do. Plus a replacement tank wont cost much if i wreck the thing.

Gas flow, Porting, Polishing and anything else that is just time and labor won't cost me anything. So what I would be spending on that will go towards getting the bits. Thanks for all the help guys. I haven't built a race bike before. I really thought there would be more "tricks of the trade" about it. But if it is just building the head, weight saving, remap, bigger carb and an exhaust, I will look into the most affordable way of doing it.

I can cut the back end off the bike and put a shortened tail on.

I am excited to actually get going with my race lessons. Hopefully I will be able to get something out of all this and I will post some pics and specs when I'm done.

[Ariel Badger]

I would suggest that you buy a copy of Tuning for speed it may be almost 50 years old but it is the bible of tuning and relevant today.
A quick look online shows that there are plenty of off the shelf parts for racing like high lift cams and high comp pistons but do not go much above 10-1 if you have to run petrol rather than dope. The book i recommended gives a lot of info about head porting, carb and exhaust balancing etc as well as lightening every nut and bolt on the bike. do you need first gear? better to slip the clutch in second IMHO and you can then take the first gears out of the box and dump a kilo of steel. The other gears will be far too heavy and you will be able to spark erode a lot of metal out of them. I even use titanium tube for clutch push rods as it save weight.
Racing is a very expensive addiction, heroin will be a cheaper option.
____________________
Bikers make great organ donors, get 115 on your licence today.

[G]

Best if you can put a link up to the regs.
Can you supercharge it? If so, probably a good start. I might consider an electric one.

[Old Git Racing]

For the ignition check out Zeeltronic, don't know if they do one for yours but you can use it with a battery to keep the starter, i.e. not self generating like a Femsa, but programmable with any curve you want.

OGR
____________________
2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Teflon-Mike]

We used to run the ignition total loss; We'd turn the magneto rotor down in the lathe so we only had the points cam, and then take the windings of the stator, so we just had the points to trigger sparks from a battery.

It doesn't find you any power, but ditching the rotor is a heck of a lot of fly-wheel mass to remove; so the pick-up rate is like having an extra pony or two, and you do gain more throttle response...

Suspect it may be a bit more involved to circumvent a fully computerised engine management system, depending what sensors there are and how they are triggered... but principle remains...

But, to return to the 'package'... having been the stupid numpty running up and down the sodding padock, wheezing, out of breath "WHY the FUCK cant you buy a set of rollers".. tryig to bump the bludy thing into action; and discreetly going to find a coffee shack, somewhere on the other side of the course a VERY long way from the van, after pushing it back there, and two batteries are sat on the inner wing NOT charging... and he swapped a flat one for a flat one.... or I'm being crushed under the chuffing bike chucked off the back of the sweeper truck at the end of the third heat, cos he fitted the battery half flattened in the first after the second....

It's back to the top; and to finish first, first you must finish, and maximising the entire package...

And the best bit of advice I can offer is dial-down the imagination!

Of all the racers I have known or spannered for.. getting "ideas" is where the problems ALWAYS seem to start....

Trying to weld shock-mounts to a swing-arm at three-o'clock in the morning?!.. in the garage behind a terraced house, with the neighbours yelling out the windows when you crank up the angle grinder, while the rider screams, "But I've already paid for the entry!" and apologetically mumbles the line guaranteed to make you want to garotte them with a piece of lock-wire... "Well! I thought it was a good Idea at the time!"

Keep it Simple, Keep it standard... give ourself the best chance of being on the grid at every event, and IF and I really mean IF you change anything; you change ONE thing at a time... a LITTLE at a time.

There is absolutely nothing worse than trying to untangle the conflicting problems of a whole pack of large-scale mods, dreamed up and implemented with large quantities of enthusiasm and optimism, in a short time scale!

The Devil is in the Detail, always in the detail, and an inversion of the thinking.

Think NOT, "What do I need to do to win?" - Think, "So what do I need to remember to do, so I don't loose!"

Like make sure batteries are numbered, and charged in rota, and that the connectors ARE connected; Or, you don't get all eager to ft the new High-Compression pistons that arrived on Friday morning, on the Saturday before the meet on Sunday.. and find yourself trying to find, or make a head gasket, after all the shops have shut Saturday afternoon; Or you waste your practice session, trying to set up the jetting for a new exhaust, rather than learning the course.... Being thorough in your prep; cleaning the bike down properly after every event; making a bug-sheet of things to attend to; and sticking to the list, NOT doing stuff that's not on the list, because of ideas, or forgetting stuff that was on the list.

Its all about not wasting time.. either round the track, but more importantly, before you get there; and more you try and do, more time it will take, and the more problems, that will beg more time still, its like to cause.

I had a very wizened old boy for a wood-work teacher at secondary school; looked like a tortoise, and only time we ever heard him swear, he offered this nugget of advice; "Power-Tools let fools, fuck-up faster"..

Now, expand a simple took-box, to a fully kitted tool-room.. like where I did my workshop training, making the tools for a factory....everything you could ever want, lathes, mills, shapers, flame cutters, electro-erosion, hardening forge, surface grinders, rotary grinders, welding equipment of all types, presses, gawd knows... a veritable sweetie shop for mechl-heads!

And between six of us 'trainees', all with more than one, we were lucky to have two working vehicles between us at any one time, cadging lifts into work of the oppos on the works-floor!

Saturday Morning, we all came in for four hours of time and a quarter, and an afternoon, doing our 'foreigners' skimming cylinder-heads, milling out sheared oil-pump screws and the like.... and trying out our 'ideas'....

And more and better facilitates you have to do stuff... more ideas you get to utilise them.... more problems you MAKE trying...

Then you have a vehicle that, more often than not, is not in any shape to even move under its own power, let alone set a lap-record!

BE WARNED.. Keep It Simple -Keep It Standard, and put your time, attention and money into making sure it gets to the start line..

Do that, and the only place you can go is the finish.. but worry more about getting it over the finish line, and you probably wont even make the start!

Oh, & Borg? Grow up.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[smegballs]

Well at least thread hasn't degenerated into powerband colors....

I reckon 125 would be a good place to start out. If/when I ever get a trackbike, I was thinking a RS125 would be a good bike to learn how to ride fast on. The maintenence is fine if just doing track milage and the rubber has sticky sizes available.

[P.]

Is this a late FI one or carbed.

I query as the early carb ones have really weak cam bearings and knacker quite quickly. Would suggest u grading that as part of your awsum-power search

[MCN.]

Mike is back with his word salads and all is right with the world...

Seriously ignore this guy, despite the weapons grade autism length posts his practical experience is very limited.

On topic - best mod you can make is an aftermarket ignition system.

[Vincent]

[Rogerborg]

[mentalboy]

[G]

[Robby]

There was a mention of having to keep the electric start intact - hence me recommending a li-on battery, but not down to the size of an RC size one. Dumping the charging system means removing some rotating mass as someone else picked up, which should be a good thing.

You won't save much weight by cutting down the fuel tank, the weight comes from the fuel inside it. Only carry the fuel you need for the race.

In fact, if the whole season is about 4 hours of racing on a 4 stroke 125, you'll probably do the season on a tank of fuel.

Thing is, you're not going to win. You're a novice, other teams have more money and experience. This is about learning to race and enjoying yourself. This is why I would go down the route of easy gains (unbolting things you don't need) and then riding. As soon as you get into anything more serious - such as head work or replacing steel bits of lighter metal - you're into a very small gain for a large expense.

If you want to mess about with programmable ignition, then I would look into a megajolt system. The gains will be tiny, and if you cock it up you'll lose power and/or melt your piston.

[MCN.]

[Rogerborg]