Cb125t

[Hilleye]

Working on the principle that "to finish first, first you must finish", and "if not finishing first at least you got more racing in because your bike didn't blow up". Lets say for arguments sake, I was looking at options to try and reduce the risk of blowing up a CB125T engine due to being over revved on a during a 2hr endurance race. Hypothetically, lets say it was running a GSXR250 CDI with no rev limiter built in, which may have allowed this to occur in the past.

Some questions:

What should stock CB125T motors safely rev too? Assuming it's not a day it's just decided it's going to go nuclear.

and

Would a GSX250 CDI, which does have a limiter built in, achieve the effect I want (i.e. prevent the bike being over revved to the point of killing it)? If so, can anyone recall at approximately what revs the limiter kicks in on a GSX250E CDI (or recommend another appropriate CDI)?

[Askor]

lets say it was running a GSXR250 CDI with no rev limiter built in,

The stock GSXR250 CDI does have a rev limiter built in (at about 19,000rpm :woohoo:)

Yow Ling and I dyno'd my mates one a while ago and it sure did hit it's rev limiter at the end of the pull

I've also done a burnout on one, can confirm, they have a limiter :Punk:

[Hilleye]

Ok, but that rev limiter isn't very useful for a CB125T because the valves and pistons have become very 'intimate' well before it's hit 19k. I was hoping the GSX250E limiter was going to be a little more conservative at something like 12k.

If CB125Ts can only do something like 14k reliably then 2000rpm this is probably a reasonable trade off to ensure you're running till the end of the race.

[F5 Dave]

30 years old. They art going to die anyway.


Its better to burn out that fade away.

[Grumph]

Working on the principle that "to finish first, first you must finish", and "if not finishing first at least you got more racing in because your bike didn't blow up". Lets say for arguments sake, I was looking at options to try and reduce the risk of blowing up a CB125T engine due to being over revved on a during a 2hr endurance race. Hypothetically, lets say it was running a GSXR250 CDI with no rev limiter built in, which may have allowed this to occur in the past.

Some questions:

What should stock CB125T motors safely rev too? Assuming it's not a day it's just decided it's going to go nuclear.

and

Would a GSX250 CDI, which does have a limiter built in, achieve the effect I want (i.e. prevent the bike being over revved to the point of killing it)? If so, can anyone recall at approximately what revs the limiter kicks in on a GSX250E CDI (or recommend another appropriate CDI)?

AFAIK the ignitions you're talking about have a hard cut at the point where the limiter cuts in. This is not good if you're using it as a change point....
You do realise that an experienced pilot should be able to tell where it runs off the end of usable power and use this as the change point ?
Fitting a tacho would be cheaper than finding an ignition with the required characteristics.

[mr bucketracer]

14k would be the max i would rev it

[speedpro]

Even the John Connor 125Ts let the rider know when to change gear. The power fades off a bit. Riding to a limiter is never a good way of getting round a track. The engine power hardly ever climbs all the way to the rev limit.

[Drew]

Even the John Connor 125Ts let the rider know when to change gear. The power fades off a bit. Riding to a limiter is never a good way of getting round a track. The engine power hardly ever climbs all the way to the rev limit.
Unless you set the limiter at peak power and your box doesn't drop you out of it again.

My frankenengine has a home made CDI, I don't think it's that hard to do.

[Henk]

Power does go flat before the limiter but sometimes it's still faster to run on the limiter for a bit rather than throw both an upshift and downshift into the mix.

[Hilleye]

Just to clarify. The intent is not to deliberately rev it until I hit the limiter (or use this as a change point). I'm just looking for insurance against possibly over revving it and damaging the engine. The intent is also to have a tacho next time.

Henk perfectly described the situation that resulted in a beautiful come together moment between piston and valve, which subsequently resulted in a much less harmonious operation of both the aforementioned components. Engine got over revved in an effort to avoid an up change because it was so close to my braking point. Short shifting early I guess is another solution.

[Grumph]

Just to clarify. The intent is not to deliberately rev it until I hit the limiter (or use this as a change point). I'm just looking for insurance against possibly over revving it and damaging the engine. The intent is also to have a tacho next time.

Henk perfectly described the situation that resulted in a beautiful come together moment between piston and valve, which subsequently resulted in a much less harmonious operation of both the aforementioned components. Engine got over revved in an effort to avoid an up change because it was so close to my braking point. Short shifting early I guess is another solution.

Or running a gearing more suitable.......

[Hilleye]

Can anyone give me a steer on a reasonably priced replacement clutch for a CB125T? Probably need friction plates an springs (maybe HD?) at a minimum. Also any tips to work out whether the steel pates need replacing would also appreciated. Clutch was slipping pretty badly on the up change coming out of the sweeper in the 40 lapper, it was like running a CV transmission.

[speedpro]

A lot of the small Hondas had similar clutch plates from around then so they shouldn't be hard to find.

[Drew]

Steels will be obvious if they need changed. Any distortion shows clearly as a hot spot.

[Hilleye]

Has anyone replaced the stock valve springs in the Honda CB125T with something that can handle higher RPM? Stock springs seem to be limited to about 10-11k RPM safely. Aiming for approx 14k RPM, which the crank/rods/pistons should cope with. Thought some of the more 'experienced' racers may be able to cast their minds back to help prevent a reinvention of the wheel because this can't be the first time someone has gone down this route for better engine performance.

All help is appreciated.

[Grumph]

First you need to measure the fitted height of the springs. That's the length, top to bottom with the valve closed.
If you're still in ChCh take your springs - with retainers & spring seats - to Kelfords.
If they are feeling kind, they can measure the pressure at fitted height and possibly find you an off the shelf replacement.
I'm guessing but I'd be looking for something around 70-75 pounds pressure at fitted height. More than that and you risk wearing out the cam and followers
quite quickly.