Need a bit more power from my 125.

marc1024

marc1024

marc1024

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20 Oct 2010
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Scotland, My home.
As some of you may know i have a 1984 honda xl125 :) its blown up a few times but now iv spent the money on it and its had a complete rebuild!(bottom and top end) It actually feels now its putting out its full 12hp :)!!
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Iv recently changed the rear sprocket to give me more top end, i can sit at 60mph now at 7000rpm, instead of 8500rpm to save me a bit of fuel. But going 2 teeth down at the back has given me a dramatic reduction in acceleration.
Iv given the carb a full clean/rebuild and all the float levels and jets have been set to standard. I tried leaning it off but as soon as you thrash it, it will just overheat and it will become impossible to start when it shuts off when you slow down or get to traffic lights. Right now i can leave it for a week, go out one morning and it will start first kick with no choke!

Anyway, do any of you lot have any old school tuning tweaks to give me that extra acceleration? i dont want to spend £2000 on a 135cc bore kit with high compression pistons and a stronger crank to support that and high lift cams etc. I was considering getting a new style exhaust system for it to make it go a bit better but i find it hard to find a good looking one. My dad made a very simple straight through pipe including the manifold. Honestly iv never heard anything so **** loud! the manifold has a small step down, the exhaust port is about 1 1/2inch but the manifold steps down to 1/2 inch so that will be restricting it. Rightly so, the straight through pipe gave me tonnes of acceleration at the expense of killing my ears! so thats a no no.
 
There are no real 'quick' tweaks that you can do without spending money on parts and shortening the life of your engine, not to mention the increase in fuel consumption.

It really isn't worth it mate, if you want a quick 125 then buy a new bike like an NSR or RS125. The thing is though, these 2 strokes are a pain in the arse to upkeep.

You're best off getting a bigger 4 stroke bike.
 
You'll struggle to get cheap improvements from a 4 stroke 125, you could do what we used to do as youngsters and stick in a bigger engine from the same manufacturer say a 250 or something but you'll just invalidate your insurance and assuming your on a CBT won't have the licence to ride it.

At the end of the day the xl125 wasn't really designed for speed just reliable commutering on a m/x look.

Are you on a CBT? you could sell it and buy a bigger bike as 125's hold their money well
 
Dogbreath said:
Fit a larger rear sprocket, an extra two teeth should help greatly ;)

Economy and performance are not really compatible on a 125.
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well it works out i got 50-55mpg when sitting at 60mph at 8500rpm, when it put a 2 tooth less sprocket on the back it went down to 7000rpm at 60mph with a minimum of 70mpg, not worked it out exactly as iv not done a long run with the new sprocket. I just want to get up to 60 slightly quicker :(

Also i dont really want to get a bigger bike until iv done my driving test, i keep failing on the theory so ill be on my CBT for a very long time loll! im not to keen on bigger bikes, but i have been eyeing up KTM super moto what is road legal. Dont know what size though, probably 450cc.
 
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marc1024 said:
i keep failing on the theory
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Get your hands on the official DSA program, it will take you through all of the questions.
I've also got the AA Theory Test iPhone App, which is great for doing a few questions whenever you find a free minute.

Using the two of these I only got 2 questions wrong when I did mine.

And, since I'm a lazy goit who's let it expire :o, I'm using them again :)
 
Doug said:
Get your hands on the official DSA program, it will take you through all of the questions.
I've also got the AA Theory Test iPhone App, which is great for doing a few questions whenever you find a free minute.

Using the two of these I only got 2 questions wrong when I did mine.

And, since I'm a lazy goit who's let it expire :o, I'm using them again :)
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got both but my brain cant cope any more with questions and revision! no matter how hard i study i just cant get it into my head. i took week of work once just to study. did about 5 hours a day (not all at once) and 5 hours before the test. Went in and only got 25 in the multiply choice and 65/70 for the hazard perception. Going for my 4th time this sunday!

Another thing i would like to ask about my bike, what about changing the carb? as in getting a completely different carb, would that give it more power?
 
marc1024 said:
Another thing i would like to ask about my bike, what about changing the carb? as in getting a completely different carb, would that give it more power?
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It's unlikely to make any noticeable difference unless the one that's fitted is particularly restrictive, and you'll have all the hassle and expensive of getting the different carb correctly jetted for your engine.
 
problem with a 4t 125 is there are pretty much no magic solutions to extract more power...that motor is built for 12bhp nothing more. most the 2t 125cc machines are restricted as they naturally make ~25bhp or more, so liberating more power is usually a case of removing the restriction

you don't have reed valves / expansion chambers in pipe / ypvs systems etc etc...

for a guranteed boost in both acceleration and terminal velocity I suggest riding off beechy head, bike will only be good for this once though :)
 
You can tune these engines; larger valves, ported head, wilder cams etc. are all possible, but it will be expensive and anything you do to significantly increase the power is going to have a fairly big impact on fuel economy, which appears to be important to you.
 
Did anyone see the feature PB did on an R125 a few months ago?. must have spent the thick end of about £2k iirc (inc a Mallossi 180cc kit) and the thing still failed to crack a proper 90mph...
:o
 
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