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Discussion Starter · #1 ·
Ive got myself an 2010 r125 and for some reason the performance is terrible. The max i can get is 104km/h(64mph)-115km/h(71mph) while my friend with 2008 r125 that got its engine group(correct me if i miss use some words) changed and can easy do 134km/h(83mph)
The sprockets are 14, 48 and the engine was honed before i bought it ant the piston with rings was changed. The seller told be that it is straight piped and it has the older akrapovic revolution i think clamped on and it lets out a little bit of air trough the joining(its weaker then breath). I weight 90kg(198 pounds) and im 185cm(6”ft). What could be the problem?
 

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2008 YZF-R125 (180cc)
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congrats on the bike :), though that does seem a bit disappointing performance. 83mph should be about the most you'd get out of a stock(14:48) geared 125cc cylinder and that seems pretty consistent even across the gen1/gen2.

tbh several things - this is the first time I've heard of anyone honing the cylinders on these bikes. Not saying it can't be done properly but it seems like an unusual step when a new nicasil coated cylinder and piston is only like £85. If this was not done properly that alone could explain your performance. I guess you would find out by doing a compression test. Lousy compression, probably that's the issue. Good compression, it's something else. As far as I understood it the aluminium cylinder would need re-coating with a ceramic coating like nicasil after honing to make it last.. but i might be wrong so if someone else knows better then pls say so. Honing steel blocks/sleeves is perfectly normal practice on cars but they're steel not ali.

second thing - unless you're adjusting your fuelling somehow (e.g. power commander, rapidbike evo etc) the chances are your straight pipe is making it slower. Yes the better exhaust makes it breathe/scavenge exhaust gases better, but as a result of that better scavenging it sucks some of the air-fuel mix out of the cylinder while the exhaust valve is open, which means it potentially runs a little bit lean. If you don't have something putting extra fuel in to compensate, there goes a bit of your power (but it sounds better soo..). When I got mine and put the k+n and exhaust on (in preparation for the PCV) it got slower. Only by a bit - maybe 3-4mph but it was noticably slower. Even now wiht a 180 kit, PCV, autotune and all the turismo bits, I still run 2 baffles in the exhaust - one just before the silencer and one in the end of the silencer. I have repeatedly tested without either/both baffles and every time it's slower without them, hence they stay.

For a 125 something like this would be what I'd look to fit in the end of the can (check the diameter matches your exhaust) Pair of Demon Slash Stainless Exhaust Baffles - 50.9mm/2" Straight DB Killers | eBay and something like this (also check diameters) goes just before the silencer (with the thin end pointing into the silencer normally) 1X Motorbike Motorcycle Exhaust Muffler Baffle DB Killer Silencer 48mm Universal | eBay. . Your exhaust manufacturer may have a specific baffle they offer which you'd have to buy separately, but generic baffle are probably fine too. This stuff is pretty low tech. Should get better midrange response and top speed, though only way to be sure for your bike is to test for yourself.

third thing - perhaps if you get the chance take the tank off and have a look at the hoses and whatnot going to and from the airbox (it's much easier to see with the tank off), around the throttle body etc. There's the little hose which goes from the airbox to the exhaust pipe, and one which goes to the coolant pump, as well as the main pipe whcih goes to the throttle body. Have a look round there and see if any of these hoses have come off, or have cracks in etc or are not connected properly. It happens quite easily as they age and leaks there will also hurt your performance.

any one or two of these things could explain your top speed situation and these would be the easiest things to fix too :) .

It might be an idea to have a look at your spark plug too and see what that tells you - perhaps take a pic and post it here if you want help reading the plug. I'm no expert but afaik if all is well it should be a sort of chocolate brown like it's been dusted with cocoa powder. White/lighter is lean, black/darker is too rich

hope that helps..
 

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Discussion Starter · #3 ·
congrats on the bike :), though that does seem a bit disappointing performance. 83mph should be about the most you'd get out of a stock(14:48) geared 125cc cylinder and that seems pretty consistent even across the gen1/gen2.

tbh several things - this is the first time I've heard of anyone honing the cylinders on these bikes. Not saying it can't be done properly but it seems like an unusual step when a new nicasil coated cylinder and piston is only like £85. If this was not done properly that alone could explain your performance. I guess you would find out by doing a compression test. Lousy compression, probably that's the issue. Good compression, it's something else. As far as I understood it the aluminium cylinder would need re-coating with a ceramic coating like nicasil after honing to make it last.. but i might be wrong so if someone else knows better then pls say so. Honing steel blocks/sleeves is perfectly normal practice on cars but they're steel not ali.

second thing - unless you're adjusting your fuelling somehow (e.g. power commander, rapidbike evo etc) the chances are your straight pipe is making it slower. Yes the better exhaust makes it breathe/scavenge exhaust gases better, but as a result of that better scavenging it sucks some of the air-fuel mix out of the cylinder while the exhaust valve is open, which means it potentially runs a little bit lean. If you don't have something putting extra fuel in to compensate, there goes a bit of your power (but it sounds better soo..). When I got mine and put the k+n and exhaust on (in preparation for the PCV) it got slower. Only by a bit - maybe 3-4mph but it was noticably slower. Even now wiht a 180 kit, PCV, autotune and all the turismo bits, I still run 2 baffles in the exhaust - one just before the silencer and one in the end of the silencer. I have repeatedly tested without either/both baffles and every time it's slower without them, hence they stay.

For a 125 something like this would be what I'd look to fit in the end of the can (check the diameter matches your exhaust) Pair of Demon Slash Stainless Exhaust Baffles - 50.9mm/2" Straight DB Killers | eBay and something like this (also check diameters) goes just before the silencer (with the thin end pointing into the silencer normally) 1X Motorbike Motorcycle Exhaust Muffler Baffle DB Killer Silencer 48mm Universal | eBay. . Your exhaust manufacturer may have a specific baffle they offer which you'd have to buy separately, but generic baffle are probably fine too. This stuff is pretty low tech. Should get better midrange response and top speed, though only way to be sure for your bike is to test for yourself.

third thing - perhaps if you get the chance take the tank off and have a look at the hoses and whatnot going to and from the airbox (it's much easier to see with the tank off), around the throttle body etc. There's the little hose which goes from the airbox to the exhaust pipe, and one which goes to the coolant pump, as well as the main pipe whcih goes to the throttle body. Have a look round there and see if any of these hoses have come off, or have cracks in etc or are not connected properly. It happens quite easily as they age and leaks there will also hurt your performance.

any one or two of these things could explain your top speed situation and these would be the easiest things to fix too :) .

It might be an idea to have a look at your spark plug too and see what that tells you - perhaps take a pic and post it here if you want help reading the plug. I'm no expert but afaik if all is well it should be a sort of chocolate brown like it's been dusted with cocoa powder. White/lighter is lean, black/darker is too rich

hope that helps..
Db killer kind of fixed the problem as i could reach 120km/h(74mph) and i still think theres some kind of a problem apart from the exhaust(isnt it a problem that my db killer is home made as ir could block the particles from leaving the exhaust and could enter to my engine?)
 

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2008 YZF-R125 (180cc)
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oh wow that's quite an impressive gain form just sorting the baffle. Good work :)

so is that a baffle/dbkiller in the end of the pipe or at the forward position before the silencer? On mine i found it was slower unless it had both, though might be different on diff bikes.

In terms of what we want an exhaust system to do...it's sometimes easier to think of the extreme cases to illustrate it. Bear in mind that the same volume of gas will need to move quicker through a smaller opening than a massive one.

So the one extreme would be imagine trying to push all the exhaust gas thru an exhaust system which had a diameter the size of a drinks straw..the air would have to be moving so fast through it, you'd defintiely see some restriction (aka back pressure) there. You'd have good gas speed but too small diameter so too much restriction. Would give you great torque initially (low rpm) but quickly become a limitation as the rpm increases.

The other extreme would be if you had an exhaust system with pipes say a foot in diameter. The gases would exit the exhaust port (~30mm dia) and basically just stop. With that size of pipe it's basically like open air so you would have 0 gas speed. That means the exhaust gas doesn't have any momentum and then doesn't actually help scavenge the cylinder at all, which is useless, also insanely loud.


Whilst obviously you'd never build an exhaust like that it helps illustrate the principle about what the exhaust system is meant to do, and why putting a massive pipe on a tiny motor will make you slower.

Really what you want is a balance between having good gas speed,small enough diameter that you don't lose all low-down torque, but big enough that you don't start restricting the airflow at high rpm (drinks straw). Also in there somewhere you're trying to reduce the noise to an acceptable level.

Also a factor is the actual sound waves caused by the exhaust pulses. This is more into how 2-stroke exhaust tuning works but there's some crossover with 4-stroke in this case. So again the shape and format of your baffles will affect sound waves (and hence the scavenging), as well as gas speed.

Imho the theory on this is effectively so complicated that it's much easier to just pick some values which seem reasonable (e.g. baffle hole size) or copy them approximately from existing designs and then build a second variant and A/B test them on your bike. One will behave better than the other so then if bigger holes in the baffle give you more top speed and that's what you want, maybe make another one with bigger holes still. Eventually the holes will be too big and you'll see the top speed drop off again...so then you go back to the one which worked best. This process is known as "exhaust tuning".

In general you should find more restrictive baffle will give you more low-end torque but might be a little restricted at higher rpm, and less restrictive will be better at high rpm but you'll lose some low/mid.

Home made db-killer is fine. If you feel the urge you might make a second one with bigger/smaller/more/less holes and test again to compare. You'll probably be quicker with one over the other.



I just today made a new baffle for the end of my silencer too, also home made, but if you get stuck finding a baffle to go in the end (e.g. bc custom diameter) then let me know, I might be able to make one to fit.


Im literally doing what I've just described/suggested with A/B testing. On the left is my old baffle which i made a while back but since the bike has got quicker I'm finding it a bit restrictive at high rpm. Also it's quite "clacky", probably because it's quite thin. So this new one is made of much thicker aluminium (so should be less subject to vibrations) and also has the holes around the edge now to allow a little more air through. Same main hole diameter.

The new one will either be better or worse, in which case I'll make another variant and test again till I find the right combo. These baffles can also be tested different ways round which (due to careful position/offset of the lock-bolt) means you can also vary the depth the baffle is inserted, which can again make a difference because wavelengths. The old one for some reason was better the one way round than the other.


hope that helps :)

33922
 
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