Tuning A 125cc 4-Stroke

[bud]

6 hours ago, Grumpy Old Git said:

I once got over 60 on a Honda 50 (down-hill wind behind). The engine seized just after that!

I wonder what it was reving to?

[Grumpy Old Git]

13 minutes ago, bud said:

I wonder what it was reving to?

No Idea,

 

I had the cylinder rebored afterwards and continued to ride it for over a year. Reg was PTO 69G (Honda C50).

[JRH]

22 hours ago, Grumpy Old Git said:

I once got over 60 on a Honda 50 (down-hill wind behind). The engine seized just after that!

I did that on a lambretta LI150. Speedo needle was so far off the clock it couldn’t be seen. When it locked up don’t know who was more worried, me or the car behind me.

Pulled clutch in coasted to bottom of hill and when the Speedo was reading about 30 dropped the clutch, engine fired up so I rode home.
Took the head off didn’t look too bad, put it back together and continued to ride it. 

[Grumpy Old Git]

54 minutes ago, JRH said:

I did that on a lambretta LI150. Speedo needle was so far off the clock it couldn’t be seen. When it locked up don’t know who was more worried, me or the car behind me.

Pulled clutch in coasted to bottom of hill and when the Speedo was reading about 30 dropped the clutch, engine fired up so I rode home.
Took the head off didn’t look too bad, put it back together and continued to ride it. 

No 'clutch' on the C50 - The 'clutch' only engaged when you moved the rocker gear change pedal to change gear.

[Fozzie]

On 02/02/2025 at 02:17, m0tion said:

Hi, was just wondering how the hell you got your rs50 to go 80mph

That's a long journey down memory lane as it's been about 16 years since I had that bike  

 

It was an 04 model, bought for £1400 in 2006. My dad found out it wasn't limited to 30 and would pull to an indicated 50. 

Once I got my CBT, I spent all my money from extra shifts in Somerfield's to pay for a used arrow exhaust, a bigger carb, fuel reeds, spark plug, air filter, and promptly blew the bike up. 

I rebuilt it, and tried again, but blew the crank case seals. So it was rebuilt again, with me getting at least a little wiser to my stupidity each time. It went off the speedo easily, though the speed stopped at 50, but the euro models went to 70, and I found it was doing an indicated 65-70mph when going downhill. I think the mods I did took it from 8-9bhp to about 11-12bhp based on what other friends claimed to be getting (a dad of one owned a workshop with a dnyo). 

 

I rebuilt it again, installing an unmarked malossi top end (I reckon it was the 82cc kit, which was popular at the time) and then had to mess with the fuelling for weeks, as well as the oil feed to stop it seizing. When I got it right, I stripped it of all extra weight, and tried to top it out but I went off the end of the rev counter at 14,000rpm and felt I was going to blow it up if I pushed it any further. So I started putting big front sprockets on it, and trialling different ones until I found one that didn't cause me to lose speed in 6th while in the power band. 

With over inflated tyres, and a 2 mile stretch of road near High Legh garden centre, and a cheap GPS speedo, I got over 70 on a flat. So I took it down Agden Brow hill in Lymm catching the left hand bend just right with the throttle wide open as it goes into a national speed limit, and there I got it to 82mph for all of 3 seconds. Once out of the power band I couldn't be in 6th gear as it just slowed to a stall. The biggest bragging right I got was when I went for a ride with a friend on a CBR125, and he couldn't catch me on a straight, although I was doing 14,000rpm+ in 5th 

But as with many teenage stories... Some details were conveniently left out. The friend on a CBR125 was in full gear, a lot heavier than me, on a heavier bike and not tucked in. I had my feet where the back pegs were, head under the screen (could barely see), was wearing steel toe capped boots, jeans, a river island leather jacket, a cheap £60 helmet and no gloves. The GPS speedo I used also had a habit of drifting between 30 and 50 when in a car sat doing 30 the whole time, so it wasn't trustworthy. And to top it off the top speed while it could hold it on a flat, it would never reach it without a hill or strong tail wind assist. 

 

At 16-19, I felt like a king who got an RS50 to do 82mph. At 34 looking back, I laugh and think "what a twat, he wrecked a reliable 50mph bike, and definitely didn't do much over 70 for all that trouble"  

[billysugger]

I tuned up my 1977ish Honda 125s after deciding to strip the engine to learn about mechanicing I had the inlet and outlet bores gas flowed and polished to a mirror finish, the head was skimmed to lose about 10thou less than standard,the pistons were changed to a different profile with cutouts  to accept the valve heads.  All in all, it went from 16.5 bhp stock to closer to 28bhp. I never got to find out it's full potential because on my first ever speed run on it, out near Yelverton, due to crazy levels of steering wobble. I was showing an indicated speed of 110mph at 2/3rds throttle.

The fitted tyres were Avon Roadrunners, but the speed rating was about 88mph and at the time I don't think there were any out there of the size needed but a higher speed rating so I never tried it again .Incidentally ended crashing on black ice and doing considerable damage to my right knee and by the time I was sorted out and released from hospital the bike had disappeared from  where it had been parked up while I was fixed up

Edited February 8 by billysugger
Punctuation