Speeding Up The CG125?

Increase top speed

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23/02/2008 at 22:39

I've got a 02 plate CG125 with L-plates.

I'm getting towards 1000 miles now, and am thoroughly fed up with it's quite low top speed.
The best i can hope for is 65mph, and up hills it can often sit at 50, making me feel like i'm crawling.

I've read websites, googled it, asked a couple of mechanics.. but it seems like no one can really suggest much, and i wondered if anyone here could help instead.

Ideally, i'd like it to be able to hold 70mph, so i can take it on the motorway without causing high blood pressure to other (car) drivers.
Obviouslly this is probably not going to happen, but i want to know if there's anything i could do to give it a further little boost.

I'm already aware of wind resistance, and make sure to wear a reflective jacket to keep it down. I'm also aware i can lean right down (looking like abit of a plank no doubt, but it will work).

Apparently i can replace the spark plug with a NGK DPR9EA-9 for "extended high speed riding", which sounds ideal because it will apparently disappate heat better than the NGK DPR8EA-9's used as stock (hence maybe a few more sustained mph top end).

I use Shell V Power petrol (just on the chance it helps me out).

I could also change the sprocket, but i'm told this will murder the acceleration so i'm not willing to touch this option.

Short of boring out the engine (which would be illegal for learner use), what can i do?
I'm not looking to spend alot on this, i just believe there's got to be something more i can do.

And before you suggest it, yes i'm aiming to pass my test, but i don't think i will be able to afford to buy another reliable bike. (at least not for the small amount the CG's sale will cough up, and even then, i must be bikeless for an unknown length of time, which is not attractive to me at all!)

23/02/2008 at 22:58

When you want to go faster lie on the tank and put your feet up over the indicators, used to give me a couple of mph on my old 125 apart from that go for the RS125.

You won,t get a lot more out of the motor as it is.


Bringing bling to life


Go to track 2 Come the Night written by our very own Katseye

Please visit the Charity Trackday site

A mind of useless info. If i can ever remember anything.

Demolish in a day. Rebuild in a decade

23/02/2008 at 23:57

I got an indicated 73mph on my CG, once. Down a hill, with a tailwind. And a good long run-up. And I weighed well under 10 stone back then. And the baffles in the exhaust were knackered. Did I mention, that happened once in the ten months I owned that bike?

Sorry mate, it's not going to happen for you. Go get a faster bike. 

24/02/2008 at 00:24

Wearing a reflective jacket will do sod all. Lying down on the tank will help. Removing the mirrors will also help, but it's a pretty stupid thing to do.

On an ancient (design) pushrod engine with low compression, changing the spark plug and running on higher octane fuel will also do sod all. 

Changing the rear sprocket down one tooth will make the engine rev slightly lower, but I'd be very surprised if this helps. The CG's power(10.5?) delivery isn't exactly peaky, so it's unlikely that with standard gearing you're leaving the power band at those speeds - it's simply that there isn't enough of it.

To get more power you'll need to increase torque or revs. Increasing revs on a pushrod engine is expensive, so you're left with increasing torque by way of:

free breathing exhaust

free breathing carbs


The first two won't be illegal for a learner as long as they don't take the power above 14hp. But they will invalidate your insurance, possibly fail an MOT, and will make it a pig to ride around town.

The over-bore will make it illegal for learner use.

All of them may risk lunching the crank or con-rod. 

But as size12 says, it's illegal to go on a motorway as a learner and you seem intent on doing that anyway.

I'm not sure I'd want to make a CG any faster given the way they handle either - it's generally wise to improve a bike's suspension and brakes before increasing power. 

If you wanted something fast, why did you buy a CG? How much did you pay for it?

Pass your test, sell the CG, and buy a stink wheel. You've got to have one while you're young (I assume you are)! If you want to use the motorways for any distance, you'll ideally need a 500 - there are millions of GS500Es going for sod all money. They can be made to handle well, cost naf all to run, and will sit on a motorway all day.

24/02/2008 at 09:44

My burd has a cg, I got just over 70 on the clocks on it down a hill with the wind behind me and I was impressed!!! It will slow to 50 up a hill.

At 60 it starts to wobble, it's not safe. It's funny, but I realise how dangerous it is.

What you gotta do is wait pass your test and buy an old cb500 or knackered old fazer, then you can get past 70 on the motorway.

24/02/2008 at 13:25
you could try fitting NOS?
24/02/2008 at 18:34
Rolling on the floor laughing. I've got some rope.

24/02/2008 at 18:35
Good point from coronajones tho' - Nitrous Oxide and supercharging (including turbo) would make it faster - albeit very briefly.

24/02/2008 at 20:55

Just to clear up a few things:

1) I have no intention of going on the motorway with the L-plates (my apologies for phrasing my post in a way that caused that confusion). I'm thinking of the period after i've passed my test and before i've managed to obtain a more powerful bike (which could be actually a few years, which is a reason for the original question).

2) I would highly recommend a read of this site to anyone that disagrees with the wind resistance concept. The fuel and spark plug are also mentioned there. Now i'm not a mechanic, and i know next to nothing about bikes, but i did do one test to check this guy's info: i did the same route with reflective jacket, and then with textile coat (the two opposites of wind resistance according to the webby), and i noticed i was pulling an extra 5mph top speed with the reflective jacket (something like 67 vs 62). So i'm going to take it that the guy was right .

3) I'm not actually crazy, the rope idea just isn't happening, amusing though the thought is ^^ .

 My (main) reasons for the CG were it's ultra-reliability, and fuel consumption. Very useful for a broke student with no mechanical ability!

24/02/2008 at 21:31

Reflective jacket bends light to increase speed by creating a sencondary magnetic impulse field.

Not effective below 50mph as a plasma trail will not ignite below this speed.

Theory of phisiobullsyntheticsyntheshite by Dr Whom


Bringing bling to life


Go to track 2 Come the Night written by our very own Katseye

Please visit the Charity Trackday site

A mind of useless info. If i can ever remember anything.

Demolish in a day. Rebuild in a decade

24/02/2008 at 21:39

why not just factor the average speed into your journey plans.

leave 20 mins earlier. 

or, even better, fit an electrogravity drive / polarized hull, just like on the b-2 stealth bomber. 

24/02/2008 at 21:59

Air resistance is caused by two things.

The first is form drag. This causes turbulence, vortex shedding, stagnation points - all of which generate areas of low pressure behind the object, causing a rearwards, or decelerating force on the object.

The second is skin friction. This is a thickening of boundary layer due to general pressure gradient. As the the Reynold's number increases, the boundary leyar thickens - it gradually moves from thin (laminar flow), to transitional (damped eddies) through to unstable (vortex shedding). 

Now, think about it. Your helmet is already smooth. Yet it still suffers buffet. If you covered your helmet with textile it might actually help - the reason being that you can trip the boundary layer from laminar, to turbulent, before it goes through the lossy transitional phase. This is the same principle as the "golf ball effect", and also they way that swing bowlers make a cricket ball move by polishing one side and picking the seam on the other. The polished side doesn't trip the boundary layer and actually undergoes MORE drag!

In terms of aerodynamics motorcycles are a short, and therefore relatively blunt, or "bluff" bodies. Most of the drag is form drag - you can feel this from the buffeting on your helmet. Bikes with panniers feel this as shimmy, etc, etc.

SO. It is more likely that leathers will suffer less drag than textiles because they're tight and don't billow, so they don't create extra "form" drag. The effect of them being smooth could - depending on reynolds number - actually be more damaging than them being a bit rough, but this will pale into nothing compared to the form drag effects. It seems like sticking a Hi-viz over the top of a bulky textile jacket will do little unless the jacket is actually letting air through..

The guy you quote is right to say that fitting lower bars won't help much, but that's simply because they only drop you by a couple of inches. If you laid with your arse on the pillion seat and your chin on the top yoke, it would help a lot more - look at the way GP racers tuck in. The bar position is more about allowing them the space to tuck in OR sit upright for braking, than actually dropping  their body down.

As for the spark plug issue, it seems that the main reason for the change in plug rating will be the changes in carburation. Newer models will run leaner for lower emissions. Ths will make them run hotter, so need a spark plug that dissipates more heat. This leaner running and cooler running plug will also make it harder to start - probably why there are so many more people struggling to start CG125s these days.

In all honesty, if you're a skint student and want something faster, your best bet is to get friendly with someone very mechanically minded. That way you can pick up half decent bikes for not a lot of money, sort the problems out, and have a good bike at the end of it. Spending any money on making a CG125 faster is not going to be well spent - "there ain't no substitute for cubes" is an old truism. You could do SO many things from freer breathing pipes, filters, carbs etc all the way through to serious engine work. All that would happen is a decrease in reliability, increase in fuel consumption, and at a cost that would enable you to buy something faster and still have petrol money left.

25/02/2008 at 13:31
250 superdreams go for peanuts on ebay and they will hit 90 just
25/02/2008 at 16:31
26/02/2008 at 00:06
Isn't that just a police biker without his top leathers on???

05/11/2010 at 09:47

I know this thread's been been put to sleep, so this is a bit of a long shot. Just wondering, when tappy says that a freer breathing exhaust would make it a pig around town what he means. also, how easy is it to take the baffles out of the exhaust, is it just a simple question of a long bit of metal and bashing them through?

Also, tried taking the airbox cover off my '51 plate CG and found i got a HELL of a lot less power than with it on, any ideas/fixes?



19/11/2010 at 01:07

What does "Sorry, this thread has been deleted" mean exactly, and WHY?

 Since I can type here, I'll assume its still "open", in some way.

If you REALLY want to put some effort into this, I'd say you are more likely to get payback (and less likely to break your bike) if you leave the engine alone and try and  improve the aerodynamics.

Since standard naked bikes are so bad, significant improvements are possible with fairing, though the biggest have been achieved with a "feet-forward" configuration that won't be achievable with a CG125.

DIY fairing could be low-tech and quite cheap. For temporary, experimental attempts you could use skin-on-frame techniques, or papier mache. If you get something useful you could do a more permanent job in glassfibre. As a bonus you might get some weather protection, and maybe some crash protection too.

 There are potential dangers, of course, like reduced steering lock, sidewinds, (see the controversy on "dustbin fairings") sharp edges, so you'd have to be careful, and regard it as an experiment, but if you survive your bike should be easily restorable to stock, and anything learned could be transferable to another bike,

Checkout Craig Vetters site


Or this streamlined Honda Innova. A crude prototype increased top speed by over 10mph, with more to follow (though this is a feet-forward design, and the focus is on mpg)




23/11/2010 at 21:54

empty your pockets befor eyou get on the bike! loose some weight, go for a jog 3 times a week!!!

leave it alone man, just ride it and sell it on when done and buy a faster bike later. its really not worth the effort

How fast do you want a 125 to go? how much money have you got?!!! you could spend a few hundred quid having the cam reprofiled and maybe a re-jet, bigger carbs, high flow air filters blah blah blah, you could have the head skimmed blah blah blah.........but theres really no replacement for displacement on a road bike. bigger engine; more power to you.

24/11/2010 at 00:26
it might touch 120mph if you threw it off a cliff
13/03/2012 at 07:51
just thought i would tell you what ive done to my giantco 125 hunter (cg clone), changed the front sprocket from 15t to 17t,rear from 49t to 36t,fitted a 28mm delorto carb and fabricated a new manifold as well as ported the head,machined out the barrel and fitted a new liner,bored to 69mm using a honda g200 piston,also had to machine out the crankcase bore to take the new liner as well as fitting packing under the clutch springs to stop it slipping,the next step is to make a new single walled down pipe for it out of stainless steel,all these mods have taken the top speed from about 60mph to over 90mph,time will tell how long the bottom end will last!,fuel consumption has gone from 100mpg to about 60mpg!
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