how fast will a yz 125 go?

[yamahakid88]

looking at 125 2 strokes and was wondering how fast they can go?

[sykal]

about 65 mph

[djmamayek]

looking at 125 2 strokes and was wondering how fast they can go?

I can't possibly see how the top speed of a motocross bike matters. I can't think of a track I've ever ridden on a 125 that required more than a couple seconds of WOT in 6th gear, even hare scrambles guys don't run WFO in 6th very much. Furthermore, wringing a 125 out at WOT in 6th gear for more than a short time is BEGGING for mass destruction, anyway.

So, with that out of the way, a stock 125cc two stroke shouldn't have a problem getting up to 60-65 mph, maybe a little more or less. Fast enough to scare you in the rough stuff, that's for sure.

[Yamaguy55]

I can't possibly see how the top speed of a motocross bike matters. I can't think of a track I've ever ridden on a 125 that required more than a couple seconds of WOT in 6th gear, even hare scrambles guys don't run WFO in 6th very much. Furthermore, wringing a 125 out at WOT in 6th gear for more than a short time is BEGGING for mass destruction, anyway.

So, with that out of the way, a stock 125cc two stroke shouldn't have a problem getting up to 60-65 mph, maybe a little more or less. Fast enough to scare you in the rough stuff, that's for sure.

+1

But being 15, sustained WOT in 6th will happen, with the predicted results.

Snide comments aside: understand that MX bikes are basically dragsters for very rough terrain: as fast as you can handle from corner to corner. They are not designed, nor were ever intended, to spend any appreciable time at all at full throttle in top gear, or even the top few gears. So when it is stated that you will have a serious engine non-grenade problem if you insist on WOT in top gear, no one is joking.

Many insist on using MX bikes for play bikes. It works after a fashion, but even in the old days, where the difference between MX bikes and street legal enduros wasn't so vast, they still left a lot to be desired except for the truly hard core. I did the MX bike as a play bike thing in the 70s and 80s, and probably wouldn't do it again. The one advantage I did have was it was an open class bike, so if it was running, it was in the powerband. It would climb anything I aimed it at.

To each their own.

[crtwostroke]

The power are great on the yz 125's. When my brothers was stock it went 70 now it goes 75 with mods. Thats for shure fast enough for you, as you will never go that fast on the track.

[adam728]

125's gernade if held wide open?

Guess no one told me that when I took my old 99 KX125 to the dunes. Easily holding WFO in 4th and 5th for 45+ seconds at a time.

Jet for it, use the right fuel for it, provide the engine enough oil, and you should have no issues holding a 125 WOT for quite some time. Yes, at a point piston crown cooling becomes more of an issue, and you need to start dumping in too much fuel than ideal to keep it together. But if you want to setup a 125 to go 30 seconds of wide open in 6th, it's no problem.

[scalejockey]

125's gernade if held wide open?

Guess no one told me that when I took my old 99 KX125 to the dunes. Easily holding WFO in 4th and 5th for 45+ seconds at a time.

Jet for it, use the right fuel for it, provide the engine enough oil, and you should have no issues holding a 125 WOT for quite some time. Yes, at a point piston crown cooling becomes more of an issue, and you need to start dumping in too much fuel than ideal to keep it together. But if you want to setup a 125 to go 30 seconds of wide open in 6th, it's no problem.

?^^

[djmamayek]

125's gernade if held wide open?

Guess no one told me that when I took my old 99 KX125 to the dunes. Easily holding WFO in 4th and 5th for 45+ seconds at a time.

Jet for it, use the right fuel for it, provide the engine enough oil, and you should have no issues holding a 125 WOT for quite some time. Yes, at a point piston crown cooling becomes more of an issue, and you need to start dumping in too much fuel than ideal to keep it together. But if you want to setup a 125 to go 30 seconds of wide open in 6th, it's no problem.

Didn't say that... But, since you seem to be calling me out. Take your kx out on the road and fly down the Tarmac WFO in sixth for a few minutes on an eighty degree day. The bikes aren't made for that, they will tolerate it for a limited time, but you'd better be on top of the maintenance schedule, and you'd better be jetted for that kind of riding (e.g. Rich on the main).

Let's be honest, 125 motocross bikes require a ton of maintenance, and if it's not kept up with they quickly turn into ticking time bombs no matter the conditions.

[Greg Y]

You can gear them to go stupid fast, keep gearing until head shake throws you from the bike on every speed run. When you heal from the broken bones and head trauma, reduce one tooth on the counter and you are there.

Better advice is, do not ride over your head. Instead of trying to out run your buddies, learn to out ride them. It is far more embarrassing to have someone school you in a tech section or flat corner then to chase you down in a drag race.

[Yamaguy55]

125's gernade if held wide open?

Guess no one told me that when I took my old 99 KX125 to the dunes. Easily holding WFO in 4th and 5th for 45+ seconds at a time.

Jet for it, use the right fuel for it, provide the engine enough oil, and you should have no issues holding a 125 WOT for quite some time. Yes, at a point piston crown cooling becomes more of an issue, and you need to start dumping in too much fuel than ideal to keep it together. But if you want to setup a 125 to go 30 seconds of wide open in 6th, it's no problem.

Agree, with a caveat: jetting, squish, timing, etc better be right. Most non tinkerers' engines aren't that correct. Bikes have come a long way in the past 30 years, but when I lived in Western Australia, in that heat, with the long straight roads made of sand, lots of people said goodbye to their pistons. And they weren't MX liquid cooled bikes, either.

But short bursts are ok, to a point. They still weren't designed for that use. Used like that, they die as fast as TZ road race engines. Some of them last, many don't.

[dmm698]

I'd be willing to bet no 15 year old kid asking how fast his mx bike will go down the street on a top speed run will have his squish, jetting, air/fuel, and all other variables spot on to make it last for that. Enough said.

[Yamaguy55]

I'd be willing to bet no 15 year old kid asking how fast his mx bike will go down the street on a top speed run will have his squish, jetting, air/fuel, and all other variables spot on to make it last for that. Enough said.

Maybe. I was (still am) oddball enough to have checked that sort of thing. But I've always been a tinkerer, but most people are really more plug and play than they ought to be. My friend had a HD Rapido 125, and if you weren't real careful, it would kill itself any chance it had. The other chum had a CZ, which like all of them, had the nickname "seize-easy" which it did with alarming regularity. Several of us even home formed and welded our own expansion chambers after buying Gordon Jennings two stroke performance book. They actually worked but looked like deranged elves built them. Stupid level loud as well. Probably why I'm deaf today.

Maybe the kid will impress us in a positive way and not only go fast, but even be savvy enough to make sure he doesn't kill off his bike. You have to learn some way.

[rpt50]

On 5/26/2011 at 11:05 AM, ajd187 said:

About 9.8 meters per second.

ha ha yes but 9.8 meters per second per second (meters per second squared).