Hello friends!
Well finnaly i bought my Honda CBR 125 R, 2005 i really like this perticular model, before that i had Honda rebel also 125. Well - my bike is getting max 100km/h and i'm wondering to play with the main jet i carburator. Well i did some research, and it seems that its not so simple to just give bigger jet and go on road. I found that i should have 118 size of the main jet, and also found that every 4 size of main jet you should add 1 size to pilot jet (that info was on this forum), also, with the bigger jet size, there might be a need for chainging exhaust (i like original ones becouse they are really silent). Does anyone have some checked parameters that fits and works well? What do you guys think about just changing main jet to like 140 and try to manipulate fuel air mixture?

 

You need to get on dyno 1st and do some baseline runs with AFR plotted. Then you’d know which way to adjust mixtures.

What generates power is OXYGEN! Not fuel! The more air you can get into engine, the more power it will make. Then you select jets to meter extra fuel to match extra OXYGEN! So, you need to make physical changes to engine to flow more air 1st. Best bang-for-buck hot-rod mods always starts from inside out (most HP gained/dollar spent):

- big bore/high compression pistons, nothing else makes as much power increase for so little spent as this

- larger valves, to flow more air for bigger pistons, best to have fully-radiused valve seats

- port & polish intake & exhaust tracts to move air through faster

- larger carbs with wider throat and butterfly to flow more air

- larger air-box with free-flow air-filtre, remove any snakes and snorkels intakes

- custom exhaust with larger diameter to flow more air

- finally, custom cams to tie all above mods together

- programmable 3D ignition box for extra advance with high-octane petrol

- final dyno-tuning to optimize AFR and ignition


Now you might hit 110kph. My wife’s 1991 CB125TT comes with 16.4hp from factory and tops out at 116kph. Huh? How?? Because it has TWO cylinders. Meaning more valves than single and more valves flow more air.

 

For example, here’s dyno-chart with AFR for my bone-stock Ninja 250. Provided by previous owner since I wanted to verify it was bone-stock because I was going to race it in 250 Production class. Gets exact 27whp as majority of stock Ninja 250s.



You can see that it has way, way too much petrol under WOT. For convenience, dyno shoppe’s software will typically plot dashed line at 13.5:1 AFR since that’s where NA engines makes most power (boosted engines need to be richer 12.5-12.0:1). This AFR plot is very common for stock bikes because every major manufacturer sets up carbs/EFi with too much fuel under WOT for safety. In event you get bad petrol out in boonies on super-hot day. Or fuel-pump starts failing, fuel-filtre starts clogging, carb/injectors gets dirty, etc. Extra fuel gives extra safety margin so engine doesn’t blow up when these things happen to reduce fuel flow. But… all that extra fuel robs power…

After coming in 2nd overall in 250 championship my 1st season, I decide to move up to 250 Superbike class where I’m allowed to modify engine. First step is cheap stuff other racers have discovered that gives best bang-for-buck gains (saves money for race tyres and food):

- Tyga full race exhaust
- Tyga CF muffler
- snorkel removal and 2x 20mm holes drilled in airbox
- K&N free-flow air filtre

I shipped bike to my sponsor’s dyno to be safe since I had no idea what changes to AFR these mods did. He asked me what upgrades I had done, thought about it for 5-sec, and immediately downsized my main-jets from factory 98 to 96 before even putting it on dyno. Rememeber that ‘60s hot-rod saying? i get to see it in action!! Due to my sponsorship agreement, I can’t share his dyno charts, but here’s another Ninja 250 racer with exact same mods. He started out with similar factory power and gained +5.7hp (+21%) with no carb adjustments! It’s extra air that generates extra power!



This means there’s 21% more oxygen molecules going into engine under WOT than before. It used the extra petrol set by factory to combust with that extra air. But was still too rich in 11.5:1 AFR. My bike had slightly higher stock power than this, along with smaller mains, leaned out mixtures to more powerful 12.0:1 AFR. But still too rich. I gained +6.1hp to 33.2whp.

He downsized main jets once more to 94 and it picked up another 0.3hp to 33.5whp at 12.5:1 AFR. He said this was good enough, we won’t get that much more by leaning it out more but will run much too close to edge of self-destruction for little gains. Overall, I gained +6.4hp (+24%) more from 24% more oxygen getting into engine along with reducing mains by 4 sizes!!!

My top-speed went from 156kph to 169kph! WOOHOO!!!

I also change carb jetting at noon lunch break. Use larger 96 mains in morning with colder denser air (more oxygen). Then smaller 94 mains in afternoon with hotter less dense air.

 

Here’s another example of EU Ninja 250 with EFI in stock vs. tuned configuration. Again, way too fuel and rich AFR from factory, but not as bad as carby U.S. version:



There were NO MODS on this bike at all! Everything bone-stock with just dyno-tuning, reprogramming ECU to adjust WOT mixtures to most powerful 13.5:1 AFR. Due to precision of EFI, perfectly flat AFR curve is possible to keep engine at most-powerful mixture. It needed a -10% fuel adjustment then +2% within just 500rpms!!! Overall, it gained +4hp (+15%) from just leaning out fuel-mixtures alone! Remember that ‘60s hot-rod saying….



Here’s an article from time I started racing & modding Ninja 250: Sport Rider - Ninja 250 On The Track - Racing On The Cheap Scroll done to The Nuts and Bolts section on how they prep bike for more power and racing…

 

In my 3rd season, I build cheater 250 Superbike engine using Ninja 300 crank and 4mm oversized pistons and con-rods from ZX-6R for 335cc…

< more build info & dyno-charts to come here >

I am discovered and they bump me up to 300 WSS/Superbike where I go up against… Rocco!!! Such a nice young man.

 

Guys, thank you for such amazing answers. The topic is extremely interesting, I see that it is not something that can be done quickly, the topic is quite complicated but worth your interest in it. Well so buying just a jet isn't a good idea. I will try to find larger airbox that won't be easy - getting new exhaust is not so challenging here, yet quite expensive. I think that i will need some mechanical help here, it would be usefull too.

 

After analysing your posts i think another great idea would be change of the cylinder to 170cc (athena), there are some available parts for this bike, then changing the carburetor for cvk 30 carburator and change of exhaust, then i guess is should also think about getting other gears with that stuff to achive better v max, but that should give a little bit more, what do you guys think?

 

Yes, bigger displacement is always best start. Most power gains for amount of money spent. Be sure to dyno-tune to dial in AFR. Will leave potential power if too rich. Or melt pistons and destroy engine if too lean.

 

Hm i think i will need some expert to do that, also wondering is it better to change carburator or stay with mine but change main jet for bigger one, this all will make a small beast from this bike, with the right gears i think i will be able to achive something like 140km/h of v max