1980 cb125s restore with eco mods

[johnathanf1lm]

Hi i just picked up a 1980 cb 125s from a farm land area. To my amazement the bike only has 4.6 miles on the bike (whole story behind it too) the bike was poorly kept saddly so it will need alot of love. And along with that love i figure i will mod it abit. Now im not going to fairing it or anything just bc of the wonderful old honda look that i love, but i was plaining on shaving as much weight as possible off of it. Smallest rear sprocket as possible to up that top speed and lower the rpms a bit. Led everything and go with a brighter headlamp (its a 6 volt system so may be a pain a bit). And finally this one i may save for a spare engine or if i got a spare bike since this one has 4 miles, but i was looking at deleting the stator and just running the bike off the battery (going to lifepo4 set up and lowering the draw as much as possible). The last one i will prob wont see this bike but maybe the next cb125s i get.

[Ryland]

The electrical system is fussy, stock everything is the only way I could get the turn signals to work, the headlight is also an odd size.
The rims are heavy steel, so if you wanted to shave weight that would be your biggest savings.
Go extreme with the sprockets, at 250lb mine would go 60mph after regearing.
For a while I also ran 10% vegetable oil without any issue and saved that 10% gasoline, 20% reduced performance at high revs.

[johnathanf1lm]

What rims did you go with? And why veg oil? And yeah ik the headlight is a odd size.

[Ryland]

I never found aluminum rims I could afford.
I tried veg oil to see what happened and it burned, same power and it displaced gasoline so 10% less gas burned, I installed an o2 sensor at the same time and it wasn't harmed, no carb issues.
When I had to replace my head light bulb, I used JB Weld putty a plastic bag as a release and formed a holder to the back of the old bucket, the same could be done with LEDs.

[johnathanf1lm]

Nice. Why the o2 sensor?

[Ryland]

O2 sensors are sensitive to contamination, if the oil didn't burn fully it would foul up the sensor.

You can also sometimes find tires sold as high speed moped tires that would be a better choice for street use, instead of the stock dual purpose retro tires.

[Tugger]

Hmmmmm doubts and misgivings...

Vegetable oil is not a good idea. It will evaporate out, as it runs past the hot parts, like the top ring and edge of the piston crown etc., and leave the heavier fractions to bake on as a varnish - gumming up your rings and bores.

Shaving off weight? It will be of some value - but most of your improvements will come from keeping it well tuned and perhaps rewinding the alternator to 12V and perhaps going with a high energy sparking system - AND - building a "1/2" to "full fairing" for it.

http://thekneeslider.com/images/2009/12/mg-v8c1.jpg

https://greasengasoline.files.wordpr...-400-copy1.jpg

In reality, you really cannot extract heaps of power from a small engine, and SOME benefit can be obtained from lowering the final drive gear ratio, a little. The trade offs begin, in having so little power to begin with, that any real variation too the equation of RPM <=> TORQUE, means that while it may go at say 40% less fuel with a full fairing, when the desired road speed is set to run at the motors highest power output (optimum position on the torque curve), that any increase to the resistance to forward motion, such as head winds, hills and cross winds, will immediately strain that lofty ideal, and require shifting gears, increasing power etc., to keep the engine at it's optimum position on the torque curve.

My good friend Phil Irving wrote a book, "Tuning for Speed", which with some intelligent reinterpretation, can be rephrased as "Tuning for optimum efficiency" (not necessarily for going fast).

I am assuming that this is a 2 stroke, in that the factory engineers, may have designed a very good engine, that is both of a reasonable power output, while remaining relatively frugal to begin with.

While tuning it to produce a lot more power, may seem a good idea, it's a false economy, both in terms of engine life and the obtainment of spares, as well as supplying more fuel to produce more power.

However, tuning an engine to be of a modest power output and to be very frugal, in combination with lowering the bike and riders Coefficient Of Drag, and greatly extending the engines life span, requires a magnitude of intelligent planning and design, beyond fitting a good racing carb, reed valves and a highly tuned exhaust for a high revving engine.

[Tugger]

Hmmmmm doubts and misgivings...

Vegetable oil is not a good idea. It will evaporate out, as it runs past the hot parts, like the top ring and edge of the piston crown etc., and leave the heavier fractions to bake on as a varnish - gumming up your rings and bores.

Shaving off weight? It will be of some value - but most of your improvements will come from keeping it well tuned and perhaps rewinding the alternator to 12V and perhaps going with a high energy sparking system - AND - building a "1/2" to "full fairing" for it.

http://thekneeslider.com/images/2009/12/mg-v8c1.jpg

https://greasengasoline.files.wordpr...-400-copy1.jpg

In reality, you really cannot extract heaps of power from a small engine, and SOME benefit can be obtained from lowering the final drive gear ratio, a little. The trade offs begin, in having so little power to begin with, that any real variation too the equation of RPM <=> TORQUE, means that while it may go at say 40% less fuel with a full fairing, when the desired road speed is set to run at the motors highest power output (optimum position on the torque curve), that any increase to the resistance to forward motion, such as head winds, hills and cross winds, will immediately strain that lofty ideal, and require shifting gears, increasing power etc., to keep the engine at it's optimum position on the torque curve.

My good friend Phil Irving wrote a book, "Tuning for Speed", which with some intelligent reinterpretation, can be rephrased as "Tuning for optimum efficiency" (not necessarily for going fast).

I am assuming that this is a 2 stroke, in that the factory engineers, may have designed a very good engine, that is both of a reasonable power output, while remaining relatively frugal to begin with.

While tuning it to produce a lot more power, may seem a good idea, it's a false economy, both in terms of engine life and the obtainment of spares, as well as supplying more fuel to produce more power.

However, tuning an engine to be of a modest power output and to be very frugal, in combination with lowering the bike and riders Coefficient Of Drag, and greatly extending the engines life span, requires a magnitude of intelligent planning and design, beyond fitting a good racing carb, reed valves and a highly tuned exhaust for a high revving engine.

[johnathanf1lm]

Its a 4 stroke. Ik you guys keep saying areo mods but i like the bike for the looks. If i wanted it to be areo i would just buy a small sport bike. That and lets be honest a 125 for around town wont really see a major improvent for areo anyway due to the added wieght. And yes the cost of doing engine work with lighten or titanium would cost a pretty penny and wont increase the power output by a lot. But when the bike has about 10 hp any little bit more power using the same amount of fuel will go a long way

[Tugger]

Buy the hard cover book.

Obviously it's no 350Kmh road racing monster.

Run it in as per the manufacturers instructions.

Olive Oil is fine to run your engine in on, provided you change it regularly. like every 300 - 500Km.

And you use the bike as a daily commute....


But if your around town speed, is say 60Kmh (40 MPH), where you will be spending most of your time at, this is where you want the best breathing and the most power to be at - the most amount of power for the least amount of fuel.

Set up three things.

write down the rpm at this sign posted speed. does it have much hard pull beyond this speed? And after the proper run in period, or determine the top speed as per manual / rpm limit.

This is going to give you an idea of where to best to tune for maximum potential power, relative to the natural characteristics of the engine


Make a tuned length exhaust pipe - about 40 cm longer than the straight pipe in your exhaust.

Bend it and fit it.

Find the jets that fit the carburettor, buy about 4 of them, in increments of one size larger than stock.

Find the adjustment range of the needle

Figure out how to tune the engine to run rich, by going up 2 jet sizes lift the neddle 2 notches and fit a high volume of air flowing oiled foam.

A tuned length straight pipe exhaust has a resonant frequency, cut off say 20mm at a time until the motor pulls it's hardest at 60Km - and this peak starts at say 50Kmh and fades out at say 70Kmh...

Find or fit or make a suitable silencer - allow and adjust for the extra length, then tune in the carburettor... by the spark plug colour method - light tan to slightly brown.

Like I said - buy the book.

Phil Irving, Tuning for speed.