Old Bike Australasia: Little Ripper - 1971 Honda CB125S

07 April 2015
Story and photos: Jim Scaysbrook

The old saying, “familiarity breeds contempt,” could apply to the CB125S, because it went about its business so unobtrusively, and with such utter reliability, that its fundamental needs often went neglected. Tappets went un-adjusted, as did points gap and ignition timing, and oil went unchanged. Occasionally, when the supply of lubricant became totally exhausted or converted to paste by engine waste products after many tens of thousands of miles, the overhead camshaft, which ran directly onto the cylinder head casting, would nip up. Because the rear chain on the CB was totally enclosed, and out of sight and mind, it would run and remain dry for ages, wearing the teeth off the sprockets. You just could not kill these things. The SL model of course got an even harder life, flogged remorselessly around trails and on countless rural properties, where it was expected to perform its duties in kelpie-like, uncomplaining and totally stoic fashion.

Stunning simplicity. If the British had built an engine like this they’d still be in business. 

The CB125 remained in the Honda range for fourteen years, with a few changes along the way. A front disc brake was added in 1974 (the CB125S2), and the engine enlarged marginally (from 122cc to 124cc via 0.5mm larger bore) in 1976. The battery/coil/points system gave way to CDI in 1980. Production actually ceased after 1982, but after a year’s hiatus, two small batches (with 12 volt electrics) were produced for USA-only in 1984 and 1985.

Restoring a CB125S

This 1971 model came my way after spending 20 years in an outback shed. The extremely grubby disassembly process took place whenever I had a spare hour. Almost inevitably, the kickstarter had been electric-welded to the shaft, so it was out with the angle grinder for that one. Eventually, the two engine side covers surrendered and the innards were laid bare for the first time since they vacated Japan in 1971.

New speedo and tacho set was sourced via the internet.

On the right hand side lies the centrifugal oil filter, and after a typical fight I managed to get the three screws out to remove the end cover. “Hello, there’s another cover under here,” I thought, but this was in fact a solidified mass of alloy-impregnated oily gunk that had set rock-hard and totally occupied the cavity that was supposed to be home to clean, free-flowing oil. After removing the flywheel from the other side of the engine, the reason became clear. The cam chain was so completely clapped out that it had thrashed around and worn away two alloy bosses on either side of the bottom sprocket, which, surprisingly, was in reasonable condition, unlike the top sprocket, which was practically devoid of teeth. Despite the fact that there was no lubricant left in the engine save for half a cup of dense black sludge, the camshaft had not grabbed hold of its plain bearings, and although two of the three piston rings were broken, the piston itself and the bore were quite OK.

Both fork legs were rusted solidly into the lower triple clamps - the rubber-mounted headlight brackets hold water perfectly. When attempting to remove the petrol tap, the entire bottom fell out of the tank, revealing that it had been roughly fibre-glassed together at some stage of its existence. Fortunately, a replacement was not difficult to find. From the beginning of this adventure, I was determined to re-use as many of the original components as possible, save for the stuff that just wears out, such as the cam chain and its tensioners, rings, gaskets and seals. Much of the new stuff came from Bert Kingston’s BK Performance in Brisbane. Bert has one of the world’s largest stock of NOS Honda parts, and is certainly worth a call for any Honda restoration.

Standard Honda rear shocks look puny and have performance to match.

Of course, there were some parts that simply could not be repaired or procured locally, and here I discovered that there is a thriving after-market business dealing in these bikes (and many other Japanese lightweights) throughout Asia, where literally millions of the bikes have been sold over the past half century. One of the best is a Taiwan web site called Vintage Avenue, which has an incredible inventory of parts for tiddlers from all four Japanese manufacturers.

The ‘125’ sidecover stickers came via the web, as did a complete set of cables, an air filter element, and the two little rubber mudflaps for the front and rear mudguards. Some parts, including the handlebars and switchgear, came up OK with just hand polishing, but one item that needed major surgery was the seat. This was entrusted to Tony O’Connor at Eldorado in Adelaide. The final assembly of the engine was done by Trevor Love of Surfside Motorcycle Garage at Brookvale.

Fully enclosed chain case ensures plenty of chain life provided you remember to lubricate it.

A new life

There are no real tricks to assembling a motorcycle as inherently simple as this one – it certainly went back together with far greater harmony than it had come apart! And so came the moment of truth – would it start? Of course it would, second kick in fact. The first test run revealed nothing more serious thank a few minor tweaks needed, and instantly reminded me of just how sweet these little Hondas are. The power is adequate without being overly impressive, but it has little weight to shift and actual motors along quite well. The gearbox is typically Japanese as it snicks through the range in silent harmony. The brakes also do their job well – the front brake is actually very effective, and the front suspension seems to soak up most road ripples without a problem. On the other hand, the rear suspension ain’t much chop. The puny looking Honda units were under-sprung and virtually devoid of dampening when new, and they certainly haven’t improved with age.

Sarah Heatherington puts the newly-refurbished single through its paces.

It’s no coincidence that more and more Japanese ultra-lightweights are appearing on the classic rally local scene. The bikes are relatively cheap to restore and generally require little intricate knowledge, workshop prowess or special tools. Handbooks and parts lists are readily available.

Specifications: 1971 Honda CB125S
Engine: 4 stroke single cylinder, single overhead camshaft
Bore x stroke: 56 mm x 49.5mm = 122cc
Compression ratio: 9.0:1
Ignition: battery & coil
Carburation: 22mm Keihin
Power: 12hp@9,000 rpm.
Torque: 8.9kW@8,000 rpm
Transmission: gear primary, chain final drive, 5-speed gearbox with wet clutch.
Suspension: telescopic oil-damped front forks, swinging arm rear.
Wheels/tyres: Front 2.75 x 18, rear 3.00 x 18.
Brakes: single leading shoe drum brakes front and rear.
Weight: 114kg
Top speed: 112 km/h
 

 

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