Big bore kits for 125cc - Yes or No?

[eNtak]

Okay so now I have a 125 (ybr-07-carb) I have been doing general maintenance since I have got it and fixed things that were broken and added bits and bobs, but then it comes to the question.

How much (if any) performance could I squeeze out of this little engine?

Before someone says " it's a 125 " or " it's a 125, if you want a faster bike then buy one" or something along those lines, I get it... it's a 125 which is slow (60-65 ish)

But something I have always wondered and done a little research into is BIG BORE KITS!

If you look at it from the following stand point it's not worth it but can it be made worth the effort?

This is my thinking , you have a 50cc bike say 4 stroke to keep power relatively the same. let's call it the YBR 50, if you upgraded to a YBR 125 then you will notice the difference in performance, but..... if you throw a big bore kit on the YBR 125 to take it to a 150cc ...then would you really feel any difference?

Here are some questions :
* Is it worth the added stress on the crank and bearings etc ?
* Are there 200cc kits possible?
* what engine modifications would need to be made
* finally is it all worth it?

Hope someone can shed some light on this

Thanks
Drew

[Commuter_Tim]

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The above post is most likely nonsensical.

I ride a Bandit 600... badly.

[Teflon-Mike]

It's an air-cooled four-stroke single, in a deliberately low state of tune to start with, it makes, what, about 10-11bhp?

IF, and its a very big if, you got an increase in power proportional to the increase in displacement.... you may get what, 0.08bh per extra cc? So IF you could stretch it to 200cc, you might 'just' break the power permitted by A1/LL licence limits....

But you wont.

First up, doesn't matter how big the cylinder is, it can only make as much power as it an cram charge in it.... with valves sized for a detuned 10bhp engine and cam timing set rather conservatively for emissions & economy, the extra cc's mght help chunk up a bit of low and mid-range power, but t will run out of puff at much the same 'peak' power, where valves and cam are already struggling to let in enough charge to fill a 125c cylinder, let alone a bigger one.

If you have a knackered engine, that needs a rebore, anyway... then regs actually permit it to be bored to max service limit, and still be classes as a 125 for licence & tax regs... that would probably allow you to bore to something close to 150cc, and still be learner-legal ad not ausing a tax fraud.... but in all liklihood, almost certain to be cheaper to replace the barel & piston.. where standard parts wouldn't gve you any capacity gan, and a big bore kit would advertse the displacement change being stamped with capacity change on the bottom of the barel.... so rather risking licence and tax fraud, for the bog all gain you'd likely get.

If you seriously want to see how much power you could get from the thing, you'd need do far more than wack on a big-bore kit... you would need to work the head as best as to give it better flow capacity, probably fill and recut valves to get larger ones in the head, and probably reprofile the cam to get some extra breathing potential... match that to revised carb and ignition, and you stand some chance of MAYBE achieving a walloping 13bhp or so.. which is about the best any of the factories ever managed to get from an air-cooled four-stroke single, with two-valves per cylinder, around 125-150cc and NOT with any huge amount of reliability!

YBR 125 is an low maintenance economy commuter..... that's ts strong points, ECCONOMY.... trying to tune one for more power, EVEN if you were successful, would result in lots of money having to be chucked at the job, rather dedeatig a large chnk of ecconomy at the outset, to result in a machine want to do itself damage, and need a lot more maintenance to conserve what performance you achieved, and still face the fact it would need more expensive repairs more frequently, whilst burning more fuel than you got extra power for it...

So if you dot really want a low cost, low maintenance commuter.... buy something that isn't a low cost low maintenance commuter, and you stand some actual chance of actually seeing something that actually has more power, without breaking A1/L-L licene restricts or if you DGAS about them... then ope book on how much performance you can afford....

I mean, you can pick up a 140bhp / 175mph ZZR-1100 or for something slightly more sporty a 100bhp / 160mph ZX6-R for less than the price decent used YBR125... how much power or performance do you want for your money?

Or take it to the max...buy a Busa, and when even that isn't enough, start tuning that, add a turbo, add nitreous, and join the double-ton club.... it's probably easier and cheaper to build a 200+bhp 2000+ mph busa, than try get a four stroke air-cooled single cylinder commuter to break A1 15bhp power limit and potentially reach a genuine 80mph!!!!

So bottom line just NO. it isn't worth the effort.
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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]

I reckon Tef is right, but I'd encourage you to try it anyway because I don't want to live in a world where that is true.
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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

[stevo as b4]

The bigger bore, assuming piston to head clearance is ok, will slightly increase the Compression ratio, and maybe depending on the head design, it'll de-shroud the valves slightly, allowing more air flow at a given lift.

It'll only be very small differences in flow though, and a std unmodified head will as said make it run out of puff early at the top end. Still think with exhaust mods and carb tuning, that such a kit could add a couple of bhp or close to it.

Say a 3bhp gain on a light 100kg? 125 is quite noticeable and significant, as would be a 3kg weight saving etc too.

[ThatDippyTwat]

Lad a few streets over did this (Full license, it's a commuter that does 4 miles).
YBR is 9BHP.

There are 170/180cc kits out there, from Brazil I think the lad said. Will cost you as much as a doggy but running YBR by the time you import it though.
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'98 VFR800 (touring) - '12 VFR800 Crosrunner (Commuting) - '01 KDX220 (Big Green Antisocial Machine)

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Oh God, not another!

It's a low powered 4 stroke engine, not a 2 stroke. Even throwing a few hundred quid at it on a cam and bore kit will only add a few bhp. It wont improve the handling or brakes either.

With many 2 strokes, the bikes were naturally capable of twice the power of the bhp learner limit, so were restricted. Derestricting was usually quite easy and up to an extra 15bhp could be released.

A modern 4T does not have this spare capacity. It was built for 10bhp, trying to add more will be expensive, pretty pointless and make it less reliable.

I recommend you do it and keep us all informed.
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[Polarbear]

Another one agreeing with Tef here
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Triumph Trophy Launch Edition

[Courier265]

Can't believe in 2018 people are thinking of reboring a 125...

[MarJay]

Teflon Mike Translation:

No.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

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[ThatDippyTwat]

[redeem ouzzer]

I’m with Roger. Tune it just to spite teffers.
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Be a REAL MAN!

[stevo as b4]

I don't see what the problem is with wanting to fit off the shelf big bore kits to modern bikes? As long as your not expecting a 10bhp Chinese 125 to become a monster by doing so.

People have been fitting big bore kits to 125's for over 30 years, and I bet a few RD125LC owners put their first Autisa 175 kit on circa 1983?

Once you start seriously playing with engines, the one thing I've learnt in life is to either stop trusting them as dependable daily hacks, or to have a big supply of spare engines at the ready for when you start ruining them quickly.

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[monkeyman]

With regards to the legality of it all, surely I wasn't the only one who derestricted my 125. Back in the day there was reams of info about getting the most out of your NSR/TZR/MITO in this very forum

In fact it was the reason I joined... What has happened to this place
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Im not the best, but im in the top one
RM 250, Husky TE610, K1 GSXR 750

[stevo as b4]

What's happened is that you have to be a complete weirdo with no sense and a pervert to like messing with small bikes now.

If your thinking of starting a new forum for these special people, then add me to the joining list ok.

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[MarJay]

[Fizzoid]

[Bozzy.]

OP,

Forget big bore kits.

What you really need to do is bolt a dirty great big turbocharger to it. The power will be guaranteed to rip your arsehole clean in half.

Don’t believe it’s possible? Check out the link.

https://youtu.be/TD6YAvw_B6o
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SV650S>>>Street Triple R

[Bubbs]

Buy YBR 250 engine and put in it's place. Easy.
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Life begins at the end of your comfort zone.

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[Pigeon]

[Courier265]