honda cg 125 big bore kits

[Smiley]

hi as previous topics! i have blown my piston twice! im giving it one last chance n replacing the piston over size! bore the barrel and replace con rod and bottom end bearing!

the question im having is would it be worth getting a big bore kit and which is cylinder and piston! n replacing it as a whole!! but the problem is that the piston sleave is bigger than the crank mouth! so it wont fit! but says that it is for a honda cg 125 bike of the age of mine and older and younger!! so can anyone help with this?

cheers
Smiley

[N cee thirty]

yeh you've got to get it milled out abit for the barrel to fit,thread in show and tell about a guy doing it
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'00 Aprilia RS50 > '92 Honda CG > '99 Yamaha Fazer > '91 Yamaha RXS > '79 Suzuki X5 > 01' Honda Cg > 07' Honda Cg > 82' Kawasaki Z200 > suzuki gsxr 400 gk73a > honda vfr 400 NC30 Mod 2 Passed 09/06/2011

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[The Shaggy D.A.]

I'd love a CG150!! If you go ahead with it let us know how you get on!! Post as many pictures of the process as you can!! Or just buy a bigger bike!!!!!!!!!!!!11!!1!oneoneone
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[unitynotsocri...]

the honda's can get bored out more but be wary of trying to match any chinese barrels on though.i think you can go up to about 140 ccc but as you've found the crankcqase mouth needs machining.


don't mix china/brazil/japan barrels unless an engineer has measured them up

Also you have to take note of the oilways near the crankcase mouth,thats why they cant be fitted with bigger pistons,

have a look at some 200cc japclone singles and check the harness compatibilty and engine mounts,it may be cheaper than upboring it.

any upbore of a cg would make em near perfect

A good place to look into it would be a trial bike engine builder who does smaller honda singles like tl's and Xl's

https://hondacg125.awardspace.com/how_to_increase_engine_size.htm
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[tahrey]

Would there be any mileage in getting hold of ACTUAL 150cc engines from Brazil or India (the latter being Hero's, I suppose)? The CG150 is an actual model in certain markets, along with the CG100. It's all down to local regs.

As far as I've been told, the foreign 150 is barely any more powerful than the 125 here, but I presume that's because it's detuned for crappy petrol. If we kept the UK spec CDI unit and adjusted the carb to suit, there might hopefully be a nice 20% power/torque boost in the offing for better speed and easier cruising without too bad a fuel economy hit or needing pricier upgraded tyres, brakes, etc.

(This is something I've wanted to try for a while since passing my test - I'm quite in love with the economy and light weight / nimbleness, as well as the £16 sub-150cc tax class. A 149cc would have the extra dose of power I sometimes crave when squeezing in front of an HGV at +0.5mph at the end of a sliproad, or slogging up a hill with a headwind... allow a trailer or sidecar if I went completely mental... be allowed on various european motorways where sub-125cc is verboten... etc. Really my only other choice at the moment is going up to a 250, which is tempting but a bit OTT for my needs)

But, how much would it cost to import even a handful of second-hand lumps for ourselves and any other interested parties?

[Rogerborg]

The other option would be to try and source some 161FMJ Chinese engines. That's essentially a clone of the 150cc Honda pushrod. You're unlikely to find any in the country already, so you'd be looking at importing a bunch of them (30+ ) from China then selling them on (good luck with that).

Failing that, a 156/157FMI 125cc from a junk bike plus a Chinese-made 150cc barrel and piston (see eBay) should fit together OK, and drop in to a CG125.

I say should, you'd have to do a fair bit of measuring to be sure.
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[unitynotsocri...]

some chinese bikes I've seen appear to have 200cc ohv engines,and are plainly a bored up cg.That extra 25 cc to 50 cc will help,you might even find that they can be bored out an extra 20cc but for all the hassle and cost i'd just get a 150cc or 200cc engine.Try 'Thumpertalk' or 'Chinaparts'.its seems your looking at £300 plus so at that price it may be worth looking at a second hand bike as a donor.

For tuning a cg type engine ,its important to remember that they have symmetric valve timing,using one cam for both inlet and exhaust ,so the only way to tune them is by exhaust,air filter/carb mods.If you wanted to up the compression ratio by having the head shaved then you'd run into problems with pushrod length.Fitting larger valves may help but any 'hotting up' the engine would detract from the good tourque curve that the cg types have,an OHC 150cc engine may seem a better choice. try

www.bikechatforums.com/viewtopic.php?t=25838

Note that early cg's use a single thickness exhaust pipe,this adds a bit more power than the double walled later types.Earlier Brazil type cg's use a carb with an accelerator pump,this can add more oomph/economy but the accelerator pump linkage needs fettling properly for performance.

a good approach if your serious about it is changing the cam,carb/k n N mods and exhaust.The exhaust and K n N mod will add a bit more whack although it adds to the noise,especially inlet noise.
a minor but nifty mod is to fit an exhaust cooler fin ring,as used on the seventies cb125s and if you know what your doing you can also plumb in an oil cooler,
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[pepperami]

[0ddball]

I was just thinking the same. If you have managed to screw a standard engine twice it suggests you are doing something wrong. I'd find out where you're going wrong with the 125 barrel before messing with big bore. If you have that much trouble with a standard engine, the big bore will be a world of hurt.
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ZXR750L

[unitynotsocri...]

you've got to be wary of replacement parts.a lot of later cg china type parts are not compatable.as pointed out in the earlier link,the biggest problem is the crankcases need machining to take the larger barrel sleeve which ,in effect limits big bore ups to a max of 140 cc .If you bore up any 125 then you should fit an oil cooler or you could easily seize the new barrel/piston.
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[Teflon-Mike]

OK, a bit of power theory, for you.
Science Says, as far an making it in an internal combustion engine;

Power = Cylinder Pressure x Cylinder displacement x Engine revs.

As its a direct relationship, suggests that increasing cylinder displacement 25cc, or 20% from 125cc to 150cc ought to increase power, accross the rev range by 20% so your 10bhp CG engine would get up to 12bhp.

BUT, this presumes all other factors equal. and I can tell you straight away, they wont be.

If you have a cylinder head chambered for a 10:1 compression ration on a 125cc engine, it will have a 12.5cc combustion chamber volume.

Squash 150cc onto 12.5cc and your compression ratrio goes up, like the power, 20% to 12:1

This is a BIG difference, and while it would be good for power, and pottentially increase combustion pressure as well as capacaity, giving MORE than theoretical 20% power gain..... taking Compression THAT high, likely to start risking pinking, the fuiel octane rating just not high enough to stand that sort of C/R meaning you have to back of ignition timing and ritchen mixture to compensate, loosing more than you would gain, and reducing mpg...

Worth mentioning also, MPG is 'Inverse Power'... power is rate of energy transfer, fuel is energy, mpg the rate you buirn it or transform it from potential energy to kinetic. If you up the power engine makes by 20%, reasonable you will reduce the MPG same amount... you buy CG25's becouse they are ecconomical, remember?

Anyway.....Convenient most 'big-bore' kits save you a bit of effort trying to get cylinder heads machined to increase combustion chamber volume and reduce compression ratio, and pottential problems in this taking metal away around valves and stuff; they often make the barel longer, or the piston shorter to put more voilume above the piston top at top dead center.

So, you may be lucky and you might get a BIT of extra compression, you may not, but bit of luck, you wont start running foul of octane rating and having to 'loose' pottential gains in power from less optimal ignition and fuel mixture.

OK, back to the bang, and the cylinder pressure comes from getting charge into the cylinder and making it burn... more charge you get in (with more fuel in it... remember that MPG is power too!) more cylinder pressure you get.

Cylinder is 20% bigger so you ought to get 20% more charge, and as much pressure as before.... BUT, sucked through the same holes, which almost certainly will be stopping the 125cc cylinder getting a 'full fill' very often, and certainly not at higher revs..... it doesn't stand MUCH more chance of putting much, if ANY more charge into a pot 20% bigger....

Manifold pressure, the actual 'suck' that fills the pot, more dependent on piston 'speed' than piston size, with the same 'stroke' you get the same piston speed for the same engine speed, and through ports sized for a 125c cylinder you often get LESS complete cylinder fill on big bore engine than you do on smaller one.

Capacity goes up 20% but combustion pressure often falls, due to incomplete fill by almost as much, and you seldom see theoretical power gain, often only a fraction of it, and often only over a portion of the rev range.

Increasing capacity from 'stroking' rather than 'boring', using crankshaft with longer stroke rather than bigger piston, you increase capacity, but importantly increase piston speeds for any given crank speed, giving more 'suck' in the inlet manifold, and consequently the short fill phenomona often less dramatic, and certainly at lower engine speeds often gives a much more significant power gain.

Anyway, third variable in the equation, engine speed.

If you want more power, so you can get a bit more speed.... well.... power is how fast you make or over come force.

Yes, lets you go faster, becouse you are using power to oiver come drag, and at the business end Power = Drag x Speed.
So more power = more speed.... only as drag is also proportional to speed to go a little bit faster, you need a lot more power, and doubling your power will tyically only increase speed maybe 50%....

CG125 makes 10bhp, and does 65mph.

Double the power to 20bhp (about that of an unrestricted NS125, or CB 'two-fifty') you will just about struggle up to 'about' 90mph, so actually already beyond the 50% more speed for 100% more power, and aproaching steeper portion of the exponential, where it starts taking 3x the power to get 1/3 more speed... but anyway.

20% more power, then will not mean 20% more mph, and that extra 2bhp you are hoping for but are unlikely to actually get, will not boost top speed from 65 to almost 80.... realistically, might just get you nudging 70 from time to time.

BUT, road speed is directly proportional to crank speed, curtecy of the gearbox and final drive.

If peak power is at 9,ooo rpm, and that gets you to 65mph, then no matter how much power you have..... 9,ooo rpm will always be 65mph, unless you raise the gearing..... extra power will simply accelerate you to the SAME top speed, more rapidly.....

Though depends on whether the bike is significantly 'over geared' to start with, and what revs you are actually pulling 'flat out', on a little bike, they do tend to be geared pretty close, and will be pulling max speed at peak power revs.

Without up gearing then, likely to not go any faster, unless you can make engine rev to higher speed....

Here boring the engine out, that idea of sucking charge through same ports; at 125cc, likely that the poets will flow enough air to supply the pot at peak power revs, but on bigger pot, likely that it will be reaching its maximum flow 20% further down the rev range...

Consequently instead of making peak power at 9.ooo rpm, its peaking at just over 7,ooo rpm....

Balence of that Power = pressure x capacity x revs equation, you get no more power from extra capacity, you just get what you had, at lower revs....

Now, your bike, has same power so same speed, but geared for 65mph at 9,oooo revs, you still have to rev to 9K to go that fast, only you now has LESS power at that engine speed and not enough power there to over come the drag.

With peak lower down the revs, you accelerate harder and faster to 'peak' but reaching peak at just 50mph, power starts dropping off, and no higher gear to change up to.... you might struggle to get much more than 55 from the thing......

Which is ALL a long way round saying that its a bit more complicated than you might presume..... and its NOT as simple as 20%cc meaning 20% more power and 20% more power meaning 20% more mph, AND it all happening with no change in MPG!

You will rarely get 20% more power JUST from 20% increase in capacity, without optimising the whole engine to get most from the extra displacement, AND optimising the gearing to exploit the extra power.

Usually power gains are a lot less than simple science predicts with the competing factors taken into account, the spoeed gains from them, even less impressive, and the increase in mpg, usually higher...

You rarely get owt fer nowt in this world, and messing with engines, theres ALWAYS a price..... and THAT is before we look at the effect on component load ratings and how a presumed 20% more power can put an extra 50% ed=xtra peak 'load' on them, and deminsih service life 100%....

Ie: carrying loads of 10bhp, engine may have 30,ooo mile 'life'. Up that power to 12bhp, and loadings increase such, and compund such that engine life deminsihed to 15,ooo miles....

No great shakes you may think, thats still a lot of miles..... but doubles the cost of the engine, for a fractional improbvement in performance.... and may not be abig deal when you have a brand new engine with pottentially 30K miles life in it....

If you are tuning an old engine, thats already blown up a couple of times thats probably already exceeded its 'normal' 30K mile life.... shoving a new piston in it to get maybe 3ooo miles out of whats left in the bottom end...

NOT a great idea to stick piston 20% bigger over it, for miniscule performance benefit, to eat that 3ooo mile 'residual wear' in double time, and see the big end bearings disintegrate in maybe just 1ooo miles time.....

If you are trying to eek out life in an old bike, on its last legs, and keep it going that bit longer, then you REALLY dont want to be trying to sacrifcde its already compromised reliability trying to incorporate 'performance' mods.

Want performance; start by putting as MUCH life ionto the STANDARD engine as you can, THEN think about how much of that you CAN sacrifice for performance gains, and THEN ponder whether its really worth it.....

Dont blithy bolt on go faster goodies, to a thrashed to death bike, thrash it some more, and then grumble that it keeps breaking!
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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!
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[unitynotsocri...]

So,fit a bigger bore exhaust pipe, a K n N and be happy it will pull a little better lower down the revs,i.e practical useable power ,or fit an OHC engine?
Most bikes use square /over square bore/stroke dimensions for higher revs ,is it possible to make a longer stroke cg so it has more pulling power?i used to love my old triumph car engines,they were long strokers and lasted longer because they revved LESS.
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[Teflon-Mike]

[unitynotsocri...]

Thanks.....i will forever shut my mouth when anyone asks about cg tuning.That will keep me busy for the next half hour ,

thanks Teflon Mike ,two of my older triumphs were a 1500 dolomite rear wheel drive leyland,and a 'K' registerd 1500 front wheel drive,always thought the front wheel drive was a better car,same engines as spitfire/mg clones.

Have you thought of being an engineering lecturer Teflon Mike?in these times of deteriorating education your country may need you!
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[0ddball]

[pepperami]

[chris-red]

It took me longer to scroll past Tef's posts than I usually spend reading an average post.
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Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.

[unitynotsocri...]

SSSHHHHSSH, dont tempt him!
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS

[stonesie]

I tried it, got it running and it turns out I have a serious crank problem and no idea how to set up the carb, lost interest and bought a Daytona

Might have another crack at it in the new year or just return it to a 125 and sell it to a learner.