During a busier session…
Honda NSF125F…
Following its owner’s first visit to the work for a long while last Saturday, Keiran had fetched the bike back from our storage container. At this session Keiran, Phil R and Dave worked to try and find parts that had been removed from it some time prior to the Covid Pandemic. Once they had been rounded up an investigation began to work out what was what. A very rough looking petrol tank was found along with a variety of plastic components, which the team fitted together to make something resembling a Honda NSF125F. The fuel tank was going to need a good deal of TLC to get it ready for re-spraying and all the plastic panels would need to be resprayed to get them in a fit state once again. A couple extra panels caused a lot of head scratching. They looked as though they there were part of the fairing ,but no matter which way they were turned, or which side side of the of the bike they were held up again there seemed to be nowhere for them to go. Would a home be found for them in a future session - watch this space.The bike’s Dellorto PHBH26 Carburettor was located but a small number of parts were missing and would need to be sourced. Some bright spark had decided to solder the throttle need valve clip to the needle, meaning that its position couldn’t be alerted so unless it was replaced there was no hope adjusting the mid range fuel air mixture.
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| Honda NS125 - The jigsaw puzzle taking shape. |
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| It's unusual to find an Italian carb on a Japanese bike but this bike was built in Italy.! |
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| The NS125F without its clothing. |
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| Righthand side view of the rather sorry looking NS125F |
What goes on inside your average carburettor...
(Put together with a little help from Copilot.)
Motorcycle carburettor jets control fuel flow at different throttle positions, ensuring optimal air-fuel mixture for combustion. Each jet has a specific role depending on engine speed and load.
Key Jets in a Motorcycle
Carburetor and what they do...
Pilot Jet (Idle Jet)
Function: Controls fuel flow at idle and low throttle
(typically 0–25% throttle).
Operation: Works with the air screw to fine-tune the
mixture for smooth idling and low-speed performance.
Symptoms of issues: with the pilot jet: Poor idling,
hesitation off idle, or stalling.
Main Jet
Function: Regulates fuel delivery at high throttle
openings (usually 75–100%).
Operation: As the throttle opens fully, the vacuum
draws fuel through the main jet into the venturi.
Symptoms of issues with the main jet: Bogging or lack
of power at full throttle; spark plug fouling or lean running.
Needle Jet and Jet Needle
Function of Needle Jet and Jet Needle: Controls fuel
flow from mid-throttle (25–75%).
Operation: The jet needle sits inside the needle jet
and moves up/down with the throttle slide. Its taper affects how much fuel is
allowed through.
Symptoms of issues with the Needle Jet and Jet Needle:
Flat spots or surging during acceleration.
Starter Jet (Choke Jet)
Function: Enriches the mixture during cold starts.
Operation: When the choke is engaged, this jet allows
extra fuel to bypass the normal circuit for easier ignition.
Symptoms of issues with the Starter Jet (Choke Jet):
Hard starting when cold.
Air Jet
Function of the Air Jet: Mixes air with fuel before
it enters the engine.
Operation: Helps atomize fuel for better combustion
and affects mixture strength across throttle ranges.
Symptoms of issues Air Jet: Poor throttle response or
inconsistent performance.
How They Work Together
At idle, the pilot jet dominates. As you open the throttle,
the needle jet and jet needle gradually take over. At full throttle, the main
jet delivers most of the fuel. The starter jet is only active during cold
starts. Air jets fine-tune the mixture across all ranges.
Proper tuning involves balancing these jets to match your
engine’s needs, altitud, temperature, and modifications. If you're experiencing
performance issues, adjusting jet sizes or needle positions can often resolve
them.
For those that are interested the following link takes you
to one of many descriptions of the basics of how a carburettor works, that you
will find on-line on You Tube. It doesn’t tell you everything but it is a good
concise starting point…
Watch this You Tube video for an insight into what goes on in a carburettor.
The engine had plenty of compression, so assuming it also had a healthy spark, it would run just fine once the missing and bodged parts had been sorted out. The team had two extra pieces of bodywork, that they couldn’t find a home for. Both looked like parts of the fairing but neither of them could be found a home. There were also two bits of the fairing missing that would cover up the wiring from the rider’s perspective and these would have to be sourced.
Here is a little background on the Honda NS125F
The Honda NS125F was Honda's bold entry into the competitive 125cc two-stroke sportbike market of the 1980s, particularly targeting the European scene. It debuted in 1985 and was produced through the early 1990s.
Its Origins and Purpose...
First unveiled at the 1984 Bologna Motor Show, the NS125 was Honda’s first serious attempt to challenge European manufacturers like Aprilia and Cagiva in the high-performance 125cc class.
It was designed specifically for the Italian and broader European markets, where 125cc sport bikes were extremely popular due to licensing laws and youth appeal.
Design and Engineering...
The NS125 was available in two versions:
NS125F – the naked version with a small headlight fairing.
NS125R – the fully faired version with sportier aesthetics.
It featured a 124.5cc liquid-cooled two-stroke single-cylinder engine, producing around 25 horsepower.
The engine incorporated Honda’s ATAC system (Automatically Controlled Torque Amplification Chamber), a variable exhaust valve system designed to improve low-end torque. However, this feature was not included on UK models.
The bike used a Dell'Orto carburetor, Marzocchi suspension, and Grimeca brakes and wheels, reflecting its Italian assembly and component sourcing.
Manufacturing and Market...
The NS125 was assembled in Atessa, Italy, to meet local content requirements and appeal to European tastes.
It was sold from 1985 to 1993 in various markets, with the UK receiving it from 1986 onward.
Its styling and performance often led to confusion with the later NSR125, though the two shared no parts.
Styling and Features...
The NS125F came in distinctive color schemes, including red/grey and white/silver.
It had a 6-speed transmission, kick-start only, and a double cradle steel frame.
The bike’s lightweight (around 108–115 kg dry) and nimble handling made it a favorite among young riders and learners.
Legacy...Though eventually overshadowed by the more advanced NSR125, the NS125F remains a cult classic among two-stroke enthusiasts.
Its blend of Italian flair and Japanese engineering made it a standout in its class and era.
Sources: Wikipedia – Honda NS125 MotorcycleSpecs – Honda NS125F Autoevolution – Honda NS Series Honda Classic 2 Strokes Blog
Honda Vigor...
Keiran, Mal and first time attender Dennis worked to get the bike complete. Dennis assisted manufacture metal strips to fix Mal’s shiny new handlebar covers over the ends of the handlebars. They would keep the worst of the wintry weather conditions of his hands. Keiran oversaw the installation of new indicators to get everything flashing. With the aid of the remote fuel tank to ensure sufficient fuel reached the carburettor, Keiran and the team coaxed the engine back into life. Very nice it sounded to.
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| The Honda Vigor complete with it's indicators and handlebar covers. |
Honda CBR1000F...
A set of new seals had been purchased for the two tee-pieces in the fuel feed. The tee pieces divided the fuel feed up between carbs one and two and also carbs three and four. As mentioned previously it was suspected that the fuel leak was coming from one of those seals. Having slid all four carbs apart Graham prised the old seals off the tee-pieces and fitted the new ones. The old ones were so fragile that a couple of them broke when being removed. Unlike when they were removed, the new seals meant that the tee-pieces were now a tight fit when pushed into the carbs. The process of getting the four carbs back into one assembly was a long and frustrating one with several connections having to be made between the carburettors at the same time. It was also necessary to fiddle together the throttle arms and the small springs designed to keep them in place relative to one another. It took a lot of dexterity, a good deal of patience, a little help from Steve and bit of quiet cursing to get them all back into one bank of carbs. Because the possibility of the leak coming from the float bowl seals couldn’t be eliminated, a set of those had been ordered. As they hadn’t yet arrived, testing to see that the carbs were fuel tight would have to wait for another week.
Suzuki GSF1200F Bandit…
Steve Whiston our resident Suzuki Bandit expert spotted that the chrome covers hadn’t been re-installed. After a bit of ferreting around Graham located the covers in a nearby box and re-installed them. Andy buffed them up to a high shine. Whilst putting the covers back on Graham notice, what looked like a vent pipe routed around the air filter housing to drain on the road, that wasn’t connected to anything. Lifting the rear of the fuel tank to investigate, Steve noticed that the pipe needed to be attached to a vent on the fuel tank. Once the tank had been fitted back in place attempts were made to start the bike but despite being charged up the bike’s batter had not got enough oomph to turn the engine over, so a booster pack was employed. Once the fact that the booster pack switched itself off after a period of inactivity had been established the bike’ engine was started and run briefly. Now all that remained was to re-cover the bike’s seat and get it out on the road for a test ride.
Armstrong 500...
Heinz came along and worked to get some of the chassis components back to being fit for purpose. One component that was causing issues was the rear suspension and in particular the lower bush of the right hand shock absorber that was corroded onto its shaft. The club’s induction heater came to the rescue. Using the small coiled element Heinz heated the metal ring from the bush, until it glowed red hot. Then with the aid of water pump pliers and eventually a mole wrench, it was persuaded to start turning on the shaft until it could be slid off. Another small but frustrating hurdle overcome.
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| The offending bush was removed from the spigot above and slightly in front of the chain adjuster. |
