YZF R125 180cc kit top speed

TL;DR: Top speed I’ve managed (so far) on mine with 180cc kit, 160cc injector and longer duration cam, with PCV + Autotune/wideband: 89mph.

So I’ve noticed there’s people searching on this topic so I’ll share what I’ve discovered so far, hopefully it’s of some use.

Top speed on mine when I got it, as 125cc on stock 14t sprocket was 83mph (indicated). That was absolutely throwing-it-down-a-hill-with-the-wind-behind-you, the most I could get out of it. 83 seems to be a fairly common top speed for stock bikes. Some other people have claimed 86 out of a stock bike but I can’t verify that myself and that wasn’t my experience.

Once the PCV, K+N filter and Danmoto Carbon GP exhaust were fitted and it had been dyno tuned by RGS we managed 85mph (indicated), once, downhill on a 15t sprocket (with a full backpack and tail bag). It didn’t really have the power to push a 15t in general so I went back to 14t for general riding and maxed out at 83 (indicated) was something could reach fairly regularly in moderately favourable conditions.

That 85 record stood (for me anyway) for a long time till I’d added the 180 kit, autotune and till i’d figured out the injector and the cam to suit and the top speed started to improve. Then after switching from my cut-down stock screen to the double-bubble we managed to break 86 whilst drafting a bmw and the map started to come in properly from there. The autotune was busy having to build a map for this on the fly so progress was incremental but steady. With repeated runs we saw 86 (without a leading bmw), then 87, 88…

In the end the most we managed (so far) was 89mph (indicated) and that seemed to be where we hit the limiter, though according to gearingcommander on the 15t we should be geared for 92mph. The current top speed was actually set with the wider bars on too which is interesting cos they usually cost a couple of mph in top end due to drag.

Around the time of that top speed she’d pull 85 on the flat reliably in neutral conditions, with 88/89 attainable in moderately favourable conditions.

I’ve calibrated the speedo in the past and it usually reads 4mph over the gps at 85 so I’m guessing actual top speed was more like 85mph by gps. I’ll use indicated speed throughout her just because those are the readings i have, and we know how much over it reads so work you can it out if you care, or don’t.

The bike was able to sustain 88 (which honestly feels a lot faster than it sounds on that little frame) and we managed to bounce off the limiter shortly after at 89mph which was interesting and somewhat terrifying when it comes as a surprise and you feel like you’ve lost power or maybe melted something xD.

Whilst it could maintain those speeds, it didn’t really seem as happy screaming its head off like it used to as a 125, which is how I got digging into the whole balance factor thing. It also kinda explains why that engine has gradually shaken it’s bearings to bits. It didn’t really seem to appreciate the high rpm stuff, though there was no problem with overheating (since the fuelling was correct), and I was always very careful about warming it up before we’d play, which is probably a fairly major reason it lasted as long as it did .

The 180 seemed happiest on a 15 tooth front sprocket. I tried 14t which was super fun and happily lifted the nose in 3rd but it was forever maxed out and running out of top end which got old surprisingly quickly. The rear sprocket is standard, with a PCV+autotune (wideband), 160cc/min injector and a longer duration cam profile more suited to the cylinder (stage 2), and a pretty nicely refined map thanks to the excellent auto-tune module on the PCV. This is also with my R15 upgraded clutch to make sure there’s no slipping.

“Never let me slip, ’cause if I slip, then I’m slipping”

Dr Dre
all testing carried out on private tracks ofc. “71” is the temp and you should be able to just see it reading 88 on the speedo, confirmed by the tacho needle firmly in the red (sorry baby!). For context it’ll hit 90+degrees C waiting at a slow traffic light so 71 deg C when it’s at max warp is a happy engine from a cooling perpsective. Also if the fuelling was wrong that would have been a big melty mess of piston parts by now.

I didn’t really find the 180cc kit improved the top speed that much (or even at all) initially, and every mph over the stock 82 was hard & carefully won (and usually expensive), but it was immediately much more responsive in the low and mid, which was something the bike simply didn’t have before and massively improved my enjoyment of riding it, especially on twisty roads.

Whilst not being that much faster, it was enough to mean I no longer got bullied by literally everything else on the road but it’s still as light and responsive a bike as before. Cars became more like “slow things that you ride around”, but without there being too much power on tap for lack of self-control to allow sliding into the realm of getting into *too* much trouble.

figuratively me once my hand touches the/any throttle

I think part of the less-than impressive top speed gains was that the bigger 180cc piston wasn’t as happy at high rpm. This was partially because it also needed an injector upgrade (which helped a bit) and a cam with a longer duration (which also helped a bit), and then it ran pretty well but it didn’t help that my engine had seen better days *before* we ended up putting twice the power through it. Cam-wise if you’d got the malossi kit that comes with a cam to suit, but you can get a cylinder for ~£100 and a cam for £60 or so delivered which, whilst needing 2 separate orders is still a lot less expensive than the malossi kit.

But even with the fuelling and breathing sorted out I still felt like the bigger piston changed something with the engine’s internal balance. I’ve gone into this on my balance factor video – so a bit like on a merry go round/roundabout you can change how it spins and change the speed by varying how far in or out you lean, same sorta deal but instead of leaning in or out we’re varying our weight instead.

Pretty accurate simulation of how a big bore kit affects engine balance

My engine’s reluctance at high rpm could also be in part down to it just being pretty well used, with worn bearings and a stretched timing chain which looked to be about half a tooth (at least) stretched when I fitted the cylinder. Even so she went pretty well, so fair play to Yamaha.

I put the 180 cylinder on at 23333 miles and the bearings pretty much shook themselves apart at about 28500. No dramatic failure, just a decade of being carefully warmed up and then beaten on relentlessly by my friend and then later by me, and being raised from the dead a couple of times.

The A-motor is still running but has obviously bad rattles and will be rebuilt, though it’s a bit of a state by now.

I’ve just finished rebuilding the new engine (B-motor) which is almost exactly the same as the current A-motor, so also 180cc with the same exact make of cylinder bought shortly after the previous one, but with new internal bearings throughout, a heavier forged con-rod and a lightened wrist pin, whose mass changes *should* (if my maths is correct) fix the balance factor of the engine internals back to almost stock (stock is ~0.48, stock rod on 180 cylinder is ~0.4, and forged rod and 180cc cyl should be ~0.49), so hopefully this version of the 180cc engine should scream like the 125cc version because the internals are (hopefully) balanced to match the bigger cylinder’s weight whilst operating in the optimum rev range of 8-10k.

The new engine (aka the “B-motor”) is just going into the bike at the moment, so hopefully it runs and then I guess we’ll l know whether i was right or not soon enough 😉

[edit: – Since writing this the B-motor is in and working!! wohoo!]


Impractical as it is, and difficult as it’s been to get to this point, I absolutely love the 180 motor the way it’s set up now (minus the knackeed bearings). That’s why the B-motor I’ve just built is exactly the same setup, but hopefully with the balance factor thing fixed, and fresh bearings throughout. If I’m right about the balance factor thing then this should be an extremely fun motor 😀

One thing to bear in mind is what you’re able to ride on your license. In the UK you cannot legally ride a 180cc kit on a CBT/provisional license. As far as I undertand it you can ride up to 125cc which is not more than 15bhp. The 180cc fails on both counts…in which case, if you need a full license to ride it why not get a bigger bike?

Well, because I kinda like this one, and if the balance factor thing works then the engine should be about perfect for me. I’m not looking to set land speed records and I’m not particularly strong so I don’t really want a bigger bike. I already find the R125 quite heavy. My other bike weights half that and has more than twice the power. The R125 fits me nicely though, I just wanted it to be a little bit faster. Plus what’s more fun that a racy little sportbike that just want’s to GO and which you can rinse literally everywhere without really annoying anyone or getting into too much trouble, and which costs very little to put fuel in? *especially at 2 quid a litre as it is now.

I’ll wait.

The B-motor,. The timing marks line up *perfectly* on this one (unlike the last). That might hopefully make a positive difference re valve timing.

Ah but…is there a better way?

So, obviously the top speed is something we’ve talked about in our discord server (link for that is on contact menu), and Legend of Legend’s Garage reckons he was able to get 95mph out of his 2nd gen R125 by adding only a Tuneboss ecu which extended the rev range by 1500rpm. Stock 14t front sprocket, just the Tuneboss which is a direct replacement ECU, so not a piggyback (as far as I know). I’m not sure if the gearing works but it seems plausible given it’s adding mph range at the most advantageous point to do so.

95 tho?? holy smokes.

Whilst I’ve not seen photo proof, I believe him, and gearingcommander suggests it ought to be possible too. Part of me wonders whether adding RPMs is a smarter approach than adding CCs, given all the balance factor nonsense you then have deal with as a result of big-bore.

bottom row suggests that addign 1500rpm to the top end would indeed allow 95mph by way of gearing, plus you have all that 14t poke to help you get there (rather than a 15t whcih has to work harder).

Another possibly significant difference is that by adding RPM you can still arguably ride it as a 125cc on a CBT (assuming your map isn’t putting out over 15bhp (which is very difficult to prove or disprove without a dyno run) whereas a 180cc cylinder is not legal to ride in any way on a CBT. So if you’re on a CBT, maybe stick with adding rpm as a way of adding top end.

I was wondering whether it would be possible to recreate Legend’s situation but the tuneboss ecu was never really intended for the euro R125 market so it seems like what he got was a rare/one-off, possibly a R15 unit with a map tweaked a little for an R125…so actually not a setup we can duplicate easily. I spoke to Tuneboss but they only did a limited production run and those units are all sold, so unless you find one used, they’re not to be had.

There is one unit which might be a possibility but I’m not sure whether it’ll work on any of the bikes/engines I have (Gen1).

Namely: https://powertronicecu.com/ and this is the specific unit for the R125 I’m interested in testing, though there is also an early R15 version which might be an option too/instead.


I’m merely pointing out what might be possible solutions based on the info I’ve managed to find..

I haven’t tested this unit yet but if they wanted to send me one to try out (or someone wants to sponsor the build) I would be interested to test it and I have an engine to put it on. I’m sure there’s scope for some youtube videos too.

Interestingly the model for the R125 says 2013-2015.. so it looks like it ought to be gen2, but 2013 is still gen1.. and if it works on a 2013, then it ought to work on a 2008 too..right? Livechat support insisted it will work on a 2013 bike even though 2013 is gen1, which contradicts how I understand the generations/models run…but hey, perhaps there’s a difference I’m not aware of which makes it possible or maybe support are wrong.

Or maybe (as I suspect) they ignore the O2 sensor from the gen2 and that’s either a signal we can ignore on gen1 or I’ll make it work somehow, so maybe actually it’s fine for 2008-2018. Who knows?

As far as I can tell the powertronic ecu allows adjusting the ignition timing as well as the fuelling, whereas PCV, PCFC etc will only allow fuel adjustment, no timing.

The PCV/PCFC have ignition tables listed in the software, but it’s not implemented in the R125 version of the hardware. So given the unavailability of the tuneboss system, the powertronic seems like the closest thing available.

The tuning software looks pretty decent and reasonably similar to the Power commander software but as far as I’ve seen there doesn’t seem to be any sort of auto-tune based off the narrobwband O2 sensor (like Tuneboss has), or any option to add an autotune (wideband O2) unit like the PCV allows you to do, and like I have on my bike.

So that means you would likely need a dyno tune (£200~) to actually get it set up properly and/or do some A/B testing to refine the map, which could be expensive and/or slow. Till I get my hands on one I won’t know for sure what it can and can’t do.

I have a frame I’d like to put a high-revving 125cc in and compare how that goes vs the 180, but for what a powertronic costs (~£300), I think i’d rather go speeduino, which has control over *everything* including ignition timing, has autotune (wideband) built in and costs about the same, and is open source and has great forum support. Speeduino would also handle stuff like boost control if you were so inclined (I am very much so).

It is however quite a large device and needs a custom loom adapter making which is likely to be quite challenging, or the loom itself needs hacking (also challenging), which I’m not super keen to do, but will likely end up doing nonetheless, as I’ve not found a socket to go into the ECU plug on the loom (yet).

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