TL;DR: Top speed I’ve managed (so far) on my gen1 with 180cc kit, 160cc injector and longer duration cam, with PCV + Autotune/wideband: 89mph at which point we’re still on power but bouncing off the rev limiter.
UPDATE (4/8/2024): I’ve just increased the RPM limiter from 10.1k (standard, according to the ECU itself, not hearsay) to 11k, which should give us a new top speed of 96mph – I’m awaiting an opportunity to test this as with the rebuilt 180cc engine with the rebalanced internals (forged con-rod and lightened crank pin) she’s still very much in the power band at the previous RPM limit of 10.1K. At the time of writing this i’ve only ridden it once with the new limit and it was only locally but she pulled all the way to 11k so I’m hoping we can get close to the theoretical 96mph. I’m gradually getting a bit more set up in my new location and this is something I’m going to be working on in coming weeks/months, so watch this space, and my youtube channel for updates. I think with a little more work 100mph will be possible.
UPDATE (25/05/2026): I raised the RPM limit again, this time to 11500, and have been running that setup for a year or more now and it’s been great. I haven’t recorded a higher top speed yet because there aren’t actually any straight roads where I’m based and I’ve not yet been out onto the main roads on this bike yet, but given she was bouncing off the limiter at 89 previously, I’d be expecting to see mid 90s. With the 11.5K rpm limit she’s geared for 101mph and whilst I doubt she’ll ever push that on the flat, with favourable conditions and a tail wind we might just see triple digits. I’ve been kinda surprised how chill the engine has been about ramping the rev limit up so much and I do run it to the limiter reasonably regularly. I don’t think i’d push much higher (rpm) though without some valvetrain mods.
Please note: just raising your RPM limit may not be wise and may destroy your engine, and a 180cc kit introduces balance changes which may also complicate things. I’m doing it here because I’ve built this engine a certain way which means the power band actually comes in a little higher than stock, so when we were hitting the limiter before we were still very much in the power band, and the engine wasn’t shaking or sounding unhappy, in fact it was smooth and in “mechanical harmony” (best way I can describe it).
You can probably go a little higher on a stock engine (e.g. 500rpm) but mine is most definitely not a stock engine, so don’t try this at home kids, unless you know what you’re doing.
They say time will tell and…well…time has told. Not just on my bikes but the race teams I’ve been working with over the last couple of years have been having good results with the 11.5K limit and *so far* I’ve not had reports of catastrophic failures like I was expecting. I’m having to conclude that these engines seem to be ok up to about 11500rpm (assuming they’re well maintained, YMMV!). Good job Yamaha 🙂
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 and speedo variations could account for the difference.
Continue reading YZF R125 180cc kit top speed
