4th gear


New Member
Hey all,

I have a 125cc and i just changed my sprockets to a 17 / 34.

I have an issue when i get into 4th i am always under revs. So i lose speed rather than gaining any and the engine does not sound like he is enjoying it...

1st : 35km/h
2nd : 70km/h
3rd : 100km/h
4th: 87km/h

Does anyone have an idea how i can change this ?

For info: stock CDI, OORacing orange performance coil, NGK spark plug, Mikuni carb.

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You need serious head work and a much better flowing exhaust and probably quite a few other mods before you can go that tall

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I run 17, 34 on the salt but is too tall for my bike on the road and it has many more mods than yours e.g. 150 cc. 15 , 34 would be a good start even. A good exhaust will get the bike going better. Then head work with bigger valves .

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stroker crazy

crazy as a fox
My 'Ace' runs 17/35 which is perhaps a tad too tall.

And that is with 150cc, better exhaust, carby mods, etc..



Agree with all the above. Too tall! You're a basketballer!

I currently run 17/38. It would be impossible with standard kit. I have larger carb, better header and exhaust, CDI.

I support increasing front sprocket as this relieves the chain.

I will be swapping out the 38R for a 37R tomorrow lifting the ratio from 2.24 to 2.18.

I'll report back on how it goes. It may be a tad too tall. In which case I will have to loose weight. (Which probably is a good thing)

Anything taller than 2.18 should only be for highly moded or 150cc

Don't forget to check the database sprocket sheet:
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