Herald 250 Carburettor overhaul on Saturday February 24th

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Composed by Anna Charalambous-Green


Since the early 1900's Britain had been innovating in motorcycle engineering and design. Starting commercially in 1896, the Coventry based Excelsior Motor Company, began the production of the first publicly available motorcycle. Five years later, in 1901, Royal Enfield produced their first motorcycle – complete with a 239cc engine that drove the rear wheel using a belt. In 1902, Norton would also join the scene, as well as the British Small Arms Company (BSA) in 1910.


In 1902, again in the Coventry area, Triumph launched its first commercial motorcycle, The Triumph H or 'Trusty Triumph' was the world's first modern motorcycle and the factory started out producing 500 units annually.

Three years later, in Redditch - West Midlands area, Royal Enfield produced the Royal Enfield Bullet. The longest lived motorcycle design in history.

British motorcycle manufacturers and designers were world leaders, with BSA purchasing Triumph in the 1950s and becoming the largest manufacturer in the world.

However, in 1959, Honda took this title and still holds it to this day.

Basic trade figures from the mid -1950's onwards showed that foreign governments were backing the 'unrestricted development ' of motorbikes, the most basic form of motorised working class transport. Pouring money into the science and engineering behind producing the smaller engine capacity machines, 250cc and gradually moving up the market with larger capacity engines.

Imports of these innovative Japanese and Italian bikes shot up in the 1960s. The numbers imported in the 50's was negligible compared to 111,700 bikes coming into the country in 1968.

These imports had the benefit of new design and technology which the British bikes had lapsed behind on, due in part to a marketing strategy that was concentrating, at the time, on short term profitability rather than longer term innovation.

Fast forward in time to 2010 and the Herald Motorcycle Company, based in Cambridgeshire.

This company picked up on the astute financial moves that many automotive companies were making at this time, by subcontracting their manufacturing to China.

Twenty to thirty years prior to this, and the early imports of Chinese machines that broke down quickly and for which you couldn't get spares, damaged the reputation of the Chinese market massively. However, after huge investment in the market, Chinese technology made leaps and bounds in the automotive sector. So much so that Western companies started to lower their costs by getting components made in Asia. In 2005 BMW signed an agreement with Chinese manufacturer Loncin, who have been making complete engines for the German giant since 2007.

The manufacturing giant, Lexmoto started working with Chinese technological firms to ensure that products met European quality expectations and that good spares and technical support were readily available.

Herald and Mutt have taken this concept a stage further, by reengineering the basic Chinese machines and adding a retro classic look to the smaller models. This and the basic fuel economy of the 125cc and 250cc models make them appealing to the commuter biker in terms of efficiency and cost effectiveness. 

Herald have scored big time with these smaller models in China itself, with the emissions regulations in some Chinese cities making it illegal to register any bike over 250cc. 

And so on that note, I think its worth a plug for the British motorcycle lobbyists the National Motorcycle Council (NMC) and the British Motorcycle Federation (BMF) to mention that if all drivers were encouraged to move to two wheel vehicles instead of four, we could make inroads in tackling climate change. Motorcycles are low emission vehicles from the manufacture to the running of the machine. And it would tackle the mounting problems faced in parking in urban areas. We should take a leaf out of the Chinese' book and move to lower capacity, two wheel vehicles in my humble opinion. Also, if all drivers were to take their tests on two wheel vehicles prior to taking their tests in four wheel vehicles, I believe we would raise the standards of driving, road awareness and consideration for other road users.

Enough said!

Moving onto the workshop session on the 24th February 2024, where my 2016 Herald Rambler had a Masterclass in Carburettor cleaning and tuning by Graham Sines.

The 2016 model, preferred by me and used regularly as a commuter vehicle, was the last model with a carburettor fitted, prior to the Euro 4 regs:

"What really makes you smile is the fact that [this bike] has a fuel tap and a single carburettor. Next year [2017] the model will have to meet Euro4, and ABS will need to be fitted alongside more eco-friendly fuel injection, but the carburettor is certainly a major part of the 2016 Herald’s charm."


The first thing a good mechanic does when faced with a running problem with the machine, is diagnose the problem. 

I had come to the conclusion that the carburettor needed cleaning due to the fact that the bike stuttered to a halt if the choke wasn't out for at least four miles of riding. Towards the end of the four miles, the engine was running fast and the choke could be pushed back in.

So the first thing that Graham did was look for holes or splits in the tubing surrounding the carburettor to see if air was entering the system.

Graham started to take the bike apart, looking carefully at the tubing that ran from the engine to the carburettor. This is part of the engine’s emission reduction system. Instead of venting crankcase pressure into the atmosphere, gases escaping from the crankcase are vented into the air filter box when high levels of vacuum open a diaphragm valve. The first thing to notice was the air intake in front of the engine which creates a vacuum when the throttle is closed. Any air in the system at the point of closing the throttle, is funnelled down the tubing from the air intake and directed into the air filter. 

We examined the rubber hosing that directed the air into the air filter, and it was intact, with no corrosion. 

A further consideration was that if the rubber diaphragm in the emission control valve was damaged or corroded, then air would be entering the system. So instead of air being directed through the air filter, it could be directed into the carburettor causing too much air to be going into the system at this point.

In order to test the theory as to whether air is entering the system, it is useful to blow into the pipe to see if the diaphragm is leaking. We did this and it was impossible to force air through it - so the diagnosis was that this system was functioning as it should. 

At this stage, we took the whole carburettor off the bike in order to examine it further.





We looked at the air vent tube at the base of the carburettor which allows air to go into the float chamber which will let the fuel level go up and down without creating a vacuum above it. As well as the vent there is a further opening in the float bowl. It has a pipe attached to it to allow any excess fuel in the chamber to drain from it onto the road.

The 2016 Herald Rambler has a Constant Vacuum Carburettor. There is a slide (or vacuum piston). When the bike is idling (the throttle closed), the fuel is sucked out of the fuel bowel via the pilot jet and is mixed in with the air before going into the cylinder. The whole process is powered by the low pressure the piston creates inside the cylinder. When we use the throttle, the butterfly valve inside the carburettor opens up and the low pressure effects the top of the vacuum chamber. The diaphragm is suctioned up, and with it, the slide and the jet needle are raised. As the jet needle is raised, its tapered shape allows more and more fuel through the main jet, more air is allowed through the carburettor, proportionally to the quantity of fuel. Pulling the choke lever adds an extra quantity of fuel into the chamber, enriching the mix and allowing an easier start in a cold engine. 




Old fashioned carburettors had the slide fixed to the throttle cable. The problem with that is that when you open the throttle wide, the vacuum drops and ceases to draw fuel out of the jet, and the engine stalls! With a constant vacuum carburettor, as the engine speeds up, the flow of air is constantly balanced with the flow of fuel.

We noted that there was quite a bit of muck around the pilot air jet. This is a small brass jet near the carburettor opening furthest from the engine. Its purpose is to control the flow of air mixing with the fuel as it passes through the idle (or pilot) jet. 

We had a look inside the carburettor. We checked the rubber diaphragm to see if it was completely intact. It was 👍🏼

We took out the emulsifying or needle jet. The small holes in the jet are there to bleed air into the fuel. Quite often, these holes get blocked up, making the mixture richer.

We checked the pilot jet by squirting carb cleaner through it. This was fine.




We checked the other jets (Main jet, needle jet, choke jet), in this way too.






And then it was time to put the carburettor back together!

By using the new work bench space in the workshop, which has been moved to a mezzanine level with excellent natural light, we had put all screws and washers onto a white sheeted surface in an order, so that we knew exactly how many screws and washers needed to go back in and in what order.

This done, we moved back to the bike, which was still on the bike lift and began the process of reattaching the carburettor. 

It is important to ensure that there is just enough 'play' between the throttle cable and the carburettor; too tight, and the throttle opens when you move the handle bars! 

Off the ramp now and outside for the tuning!





TUNING THE CARBURETTOR:

My Herald is a noisy article - which to me is great as, in theory, 'Loud Pipes Save Lives' .

But the tuning up of a carburettor is an exact science, and one akin to fine tuning a musical instrument, as, if you are not using a gauge, you are listening hard to the tick-over sound in order to literally 'tune the bike'.

The reason this is SO important is the following:

Fuel-efficiency
Better power delivery
Eliminate engine vibration
Solve heating issues

First;

Prepare your bike engine.

Run the bike for a few minutes to warm up the engine. Listen to how it is running. The engine can sound fast and stressed if the mixture is too rich (e.g. if there is too much fuel as opposed to air in the mix). Rev it up a bit - really annoy them neighbours :) Not really...the annoying thing! :) But do rev it!

Second;

Once your engine is warm, find the right screw.

There should be:

-Air Screw: Controls the air mixture.
-Fuel Screw: Controls the fuel mixture.
-Idle speed screw: Controls how much the throttle is open and thus the speed at which the engine is ticking over.  

Third:

Start making adjustments. For example if adjusting an air screw…

Turning it clockwise will make the mixture richer because you are decreasing the amount of air being mixed with the fuel. Turning it anticlockwise will make the mix weaker because more air can flow past the screw and mix with the fuel.

Fourth:

Find the middle ground. By adjusting the air screw you should be able to find the balance between a rough and revving engine. Try turning the air screw in until the speed of the engine starts to fall. Then start unscrewing the screw. As you turn the screw you will notice that the engine speed will start to rise then begin to fall as the mixture starts to become too weak. Turn the screw back to the point where the engine is running at its fastest. Fine tuning like this will involve minor/half turns of the screw to find a normal revving engine sound.

The above instructions relate to tuning an airscrew. Sometimes you will find yourself working with a fuel screw. As a fuel screw restricts the flow of fuel at low engine speeds, adjusting it has the opposite effect to air screw. Turning it clockwise will restrict fuel flow making the mixture weaker and turning it anticlockwise will add more fuel, making the mixture richer. 

I cannot emphasis how important this process is on a bike with carburettors (e.g. a bike that one can joyously fettle with, rather than expensively plug-in!). 

The bike was a veritable joy to ride home following the workshop session on the 24th February and Graham's Masterclass in carburettor cleaning and tuning.

I flew home with the wind beneath my exhaust pipe.

Bloomin' WONDERFUL!

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