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CBF 125 bore kit???

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giggl3fac3
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PostPosted: 21:14 - 06 Sep 2018    Post subject: CBF 125 bore kit??? Reply with quote

I'm using a CBF 125 as a commuter to work. It's great on fuel but I want to be able to sit at 70 comfortably and not get stuck behind lorries Rolling Eyes .

Has anyone fitted a bore kit?
Is it worth it?

I have a full licence so I could get a bigger bike with high mpg but I cant find any that come close to my CBF.
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MarJay
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PostPosted: 21:22 - 06 Sep 2018    Post subject: Reply with quote

Doubt it'll improve top speed any. You might get an extra couple of lbft of torque, but that's pretty much it.

Buy an NC700S.
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FretGrinder
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PostPosted: 21:34 - 06 Sep 2018    Post subject: Reply with quote

Leave it alone. It's not worth it.

Get a full licence.

Get a bigger bike.

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giggl3fac3
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PostPosted: 21:54 - 06 Sep 2018    Post subject: Reply with quote

FretGrinder wrote:
Leave it alone. It's not worth it.

Get a full licence.

Get a bigger bike.

Thumbs Up


I have a full licence... Read the original post... Rolling Eyes
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giggl3fac3
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PostPosted: 21:56 - 06 Sep 2018    Post subject: Reply with quote

MarJay wrote:
Doubt it'll improve top speed any. You might get an extra couple of lbft of torque, but that's pretty much it.

Buy an NC700S.


Fair enough, saw some cheap kits and thought it might be worth it.

NC700S not a massive fan but better than a CBF Razz
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Teflon-Mike
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PostPosted: 23:24 - 06 Sep 2018    Post subject: Reply with quote

In long hand, engine power is Cylinder pressure, times Cylinder displacement times crank revs.

More cylinder capacity should then equate to more power, but only if you can maintain the same cylinder pressure. Here-in lies the niggle.

Power tends to increase with engine revs; up to peak torque engine revs, so does cylinder pressure. There the valve timing and port sizes etc reach an optimum to cram as much charge in the pot as possible, and make as much cylinder pressure as you can. Below peak torque revs, the optimum hasn't been reached and cylinder pressure will be below maximum, and so will power. Above peak torque revs, the cylinder pressure is starting to fall, as the ports cant keep up the rate of flow to fill the cylinder to capacity each cycle, but, more cycles at less pressure, make up the difference, and power tends to continue increasing up to peak power rpm, where the flow capacity starts to choke the cylinder filling, and cylinder pressure drope more than revs can make up.

With the same cam, valves, same valve lift... a bigger pot, will tend to drag peak torque down the rpm range a bit, as the ports flow the amount of charge they might up to thier capacity, so fill the larger cylinder earlier; but then, the ports start to choke the flow, and peak power will also be reached earlier, as the cylinder is short filled each cycle as revs increase; and you 'probably' wont get any more power from the bigger engine.. you might.... but not a lot; because the cylinder pressure has been falling for however many rpm up the range.

70mph?

To achieve that, well, you might get it with about 12bhp, but only in more ideal conditions. To achieve it more often and more reliably, you really need to have a surfiet, around maybe 20bhp... that would mean doubling the power of most humble soft-tuned 125's, and that is not an easy job, and would take an awful lot of tuning above and beyond merely boring the thing out; where even boring the thing to 250cc wouldn't do the job, as the competing factors of cylinder pressure and revs work against the one of cylinder displacement.

Could be done...... but..... I VERY much doubt you'd get that sort of power from a humble air-cooled four-stroke single.

Honda CB125Twin, with two pots and a pretty wild cam, and hemi heads, managed 17bhp, and not all that reliably. The Yamaha / Minarelli YZF-R125 motor, as stock makes A1 complient 15bhp.... tuning can get them up to around 17/18bhp or so, with varying degrees of UNreliability, and ading a big-bore kit 'may' as long as they dont sieze, get them scaring the 20bhp mark....

BUT, point is, you are still looking at achieving something that has 70, probably even 80+mph potential, you are not realistically looking at creating something capable of sustained 70mph 'cruising' with a bit of spare to get past artics and dodge audi-enemas.

MY answer is, stuff it, sit it out behind the artic.... at 55mph; the m-way is boring as feck anyway, relax, enjoy the fact that you are still making a batter average speed than trying to batt it out cross country, where roundabouts and traffic lights bring a 60mph road-speed down to more like 30 average, very quickly; enjoy the lack of SMIDSY stress, and if you are a bit brave.... being in the 'bubble' of an artic's slip stream, being sucked along in its wake, in a bubble of effectively still air on a whiff of throttle, pushing MPG up to moped MPG's!

Or, get a bigger bike... and even then.... I tend to avoid the M-Way cos of boring, but when I do use it, 55 in the truck-stack is still easy riding and more progress than trying to point to point cross country; saves fuel and stress, and an Audi-Enema.... and on a 100+ motorcycle, that is something the extra is inclined to beg you engage in... trying to play with the blitzen-bimmers in the middle and outer- lanes... and its a game you really just cant win... dey got a box round'em! They aint putting the same stake on the table when they call your bluff! so why even get in the game? Its just extra stress, extra fuel, and risk to life and licence you really dont need.

But your call... but a big bore kit really isn't much of a solution to very much, I'm afraid.
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temeluchus
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PostPosted: 00:32 - 07 Sep 2018    Post subject: Reply with quote

If you fit a bore kit it will impact economy. More CC more power=more fuel.

Slipstream behind a lorry or get a 250.
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Ste
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PostPosted: 01:32 - 07 Sep 2018    Post subject: Reply with quote

I see you like turtles.
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pits
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PostPosted: 11:06 - 18 Sep 2018    Post subject: Reply with quote

Ste wrote:
I see you like turtles.

https://i.kym-cdn.com/photos/images/original/000/047/813/xzibit_turtles.JPG
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Wonko The Sane
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PostPosted: 17:01 - 18 Sep 2018    Post subject: Reply with quote

Teflon-Mike wrote:
In long hand, engine power is Cylinder pressure, times Cylinder displacement times crank revs.

More cylinder capacity should then equate to more power, but only if you can maintain the same cylinder pressure. Here-in lies the niggle.

Power tends to increase with engine revs; up to peak torque engine revs, so does cylinder pressure. There the valve timing and port sizes etc reach an optimum to cram as much charge in the pot as possible, and make as much cylinder pressure as you can. Below peak torque revs, the optimum hasn't been reached and cylinder pressure will be below maximum, and so will power. Above peak torque revs, the cylinder pressure is starting to fall, as the ports cant keep up the rate of flow to fill the cylinder to capacity each cycle, but, more cycles at less pressure, make up the difference, and power tends to continue increasing up to peak power rpm, where the flow capacity starts to choke the cylinder filling, and cylinder pressure drope more than revs can make up.


I got about that far and thought "turbo"
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Pigeon
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PostPosted: 18:57 - 18 Sep 2018    Post subject: Reply with quote

Possibly could fit a 1 tooth bigger front sprocket. Would lower the rpm by around 750 at 60mph and you might find it does 70.

Simply changing the gearing won't increase the power of the bike, but it might help when slipstreaming lorries, might increase mpg a bit too.

Anything more than 1 tooth on the front and you'd notice the bike struggles to accelerate as well probably.

Don't know about big bore kits with EFI. On a carbed bike you'd need to re-jet it. But with EFI, does it figure it out, or need re-mapping. Sounds like a world of pain.


CB500F does 85mpg apparently.
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stevo as b4
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PostPosted: 19:11 - 18 Sep 2018    Post subject: Reply with quote

I'm not suggesting OP does this, but it'd be semi interesting to know what the CBF head flows in cfm, and what it could flow if messed with, and if there's scope to increase the compression ratio by skimming? Then I'd be wondering what a custom ground cam would cost, and a slotted cam sprocket etc. I bet it would be possible to get enough breathing and extra top end rpm to overheat the oil and grenade the bottom end?

OP don't listen to the above garbage ok!
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Triton Thrasher
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PostPosted: 20:52 - 18 Sep 2018    Post subject: Reply with quote

You’d be turning your reliable commuter into a go-faster special.

Specials builders travel by bus.
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Evil Hans
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PostPosted: 22:20 - 18 Sep 2018    Post subject: Reply with quote

Trade it for a CBF 250? They still top 100mpg and 80mph
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stevo as b4
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PostPosted: 22:47 - 18 Sep 2018    Post subject: Reply with quote

Triton Thrasher wrote:
You’d be turning your reliable commuter into a go-faster special.

Specials builders travel by bus.


^This x100

Must be why I'm very used of using trains and buses. Any savings in fuel and running costs all go on the next useless upgrade or replacement of broken parts.
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Ste
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PostPosted: 22:52 - 18 Sep 2018    Post subject: Reply with quote

pits wrote:

https://www.instagram.com/giggl3fac3/
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pits
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PostPosted: 00:55 - 19 Sep 2018    Post subject: Reply with quote

Ste, need more Instagram pal to see it, but I get the idea.


Op, have you thought about a mk1 Blade?
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Northern Monkey
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PostPosted: 10:44 - 19 Sep 2018    Post subject: Reply with quote

Is a cbf250 not just a cbf125 with a factory fitted big bore kit?
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Evil Hans
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PostPosted: 10:56 - 19 Sep 2018    Post subject: Reply with quote

Northern Monkey wrote:
Is a cbf250 not just a cbf125 with a factory fitted big bore kit?


Nope. 4 valve OHC, six speed box and completely different frame. When I was thinking about getting an uber-frugal commuter I looked at them, but I had a moment of mental clarity and decided I needed maw powah. Obviously.
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