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Tuning A 125cc 4-Stroke


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Hi, i'm looking at buying a nice little 4-stroke for my first bike just to act as a commuter and to get my experience up. The problem is that if I were to get a 125cc 4-stroke it would be barely kicking out more power than my old Aprilia RS50, which was tuned to the point it did 80mph.

The bike i'm looking at has a Yamaha TTR engine. Does anyone know how to tune these engines so that its giving hp thats in the mid- 20's. Ive got plenty of money to spend and I DONT WANT A BIGGER BIKE.

Is there anyway to change the timing of the valves? Sorry i'm a noob with 4-strokes because i've lived with 2-strokes all my life.

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Ok personally I would save your money, pass your test and buy a bigger bike. Without big boring a TTR125 you could get a racing can and a air filter ( such as a K&N ) and upjet it obv. You could also change the sprocket size but you will loose acceleration power but gain top end.

You can buy big bore kits for the TTR 125 from here:


I must add unless you know what your doing from back to front don’t even try to attempt to bore it yourself. You can bore it to a 150cc for example which includes: sleeve, forged piston, pin, rings, circlips, headgasket, basegasket but as that website said it will also require resleeving and boring your stock cylinder and rejetting which ive already mentioned doing anyway.


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I was talking to my uncle, who has a cbr600 and he said if I gave him a 125cc 4-stroke engine he could change the power output from 15 to 25 hp if i gave him 3 hours and £50 using nothing but tools.

I asked how and he just said two words "valve timing". Can anyone validate that this may be possible? lol

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head skimmed and gas flowed, then reprofile the cam gave my XJR

another 18bhp then carbs and exhaust took it to extra 35bhp although it

is a big engine it is still 1/3 extra power, (105 standard) :wink:

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Its so damn complicated to give a 125cc 4-stroke more power. A 2-stroke, you just put a new exhaust, carb, air filter and race reeds and the power doubles. I'm probably going to sack the idea of a 4-stroke.

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