we have lift off – thanks to Bernoulli…

and a few other things as well.

First, there’s the engine de-coke and re-build. Whilst doing this, I opened the exhaust port a little and opened the exhaust port throat a little more, smoothing the flow as best I could. A new crankshaft seal was fitted on the drive side and I took particular care to seal the small gap above the key way slot, in the primary drive sprocket, as some primary compression can be lost here. The little end bush also seemed a little tight, so the bush was gently eased using 1500 grade wet and dry sandpaper.

Second, I’ve flexibly mounted the carburetor. Now I was quite nervous about cutting the inlet manifold as there’s no going back but I did it anyway and fortunately, it worked out quite well.


The advantages I hope to gain are:

  1. Reduced vibration of the carb that could be causing the float to bounce and flood with petrol; a persistent problem I’ve had,
  2. Reduced heat transfer to the carb from the engine, that I suspect is causing vapour locking , NB: Petrol boils at 95C, however ethanol boils at 78C, so modern fuel are far more prone to this. Sir Walter has always been a bad starter, 10 and 20 minutes after a hot stop.
  3. Increased clearance between the carb and the wheel as the petrol connector nut can touch the wheel, particularly if the wheel bearings are set a little loose. I cut 6mm out of the manifold and left a 3mm gap between the ends.

The finished job looks quite neat, as shown. You’ll see I changed to a different rubber hose as the black one above was too tight. I just hope the clear reinforced hose I used is ethanol proof, or I’ve a breakdown waiting to happen.  I’ve also added some thick black foam rubber around the inlet spigot, so that it’s supported where it goes through the carburetor cover.

Third, I’ve lapped the float needle to the petrol connector using a new method. Previously I’ve spun the needle in a drill whilst holding the petrol connector between my thumb and finger. The problem with this method is the lapped seat may not be axially aligned as it’s done outside the carburetor body. Consequently, it may leak when assembled.

First step is to make a nylon plug on a lathe, that’s a tight slide fit into the float bowl. Also drill a 1.6mm hole in the centre of the plug (on the lathe) to take the float needle, thereby ensuring the 2 diameters are perfectly concentric.

Then fit the petrol connector into the base of the float chamber with its fibre washer and tighten. Next put a dab of fine abrasive paste (Solvol Autosol) on the needle seat and insert it into connector and up through the nylon plug. Finally carefully tighten the chuck of a battery drill onto the exposed tip of the needle and spin it slowly for a minute or so.


Repeat this, cleaning the needle each time, until a good even seat can be seen on the needle. The beauty of this method is that the new seat is perfectly aligned and shouldn’t (I hope) leak.


Fourth, open up the silencer to improve gas flow. This also makes it louder and it’s a scientific fact that something loud goes faster; or so it seems. My silencer is the type that doesn’t come apart (for cleaning), so I simply drilled a 10mm hole in the baffle plate that I could see inside the tailpipe. Yes, crude but effective.


So what’s this got to do with Bernoulli? Well in 1738 Daniel Bernoulli published a book called “Hydrodynamica” (great title), in which he detailed some principles of fluid dynamics.

In a nut shell, he stated that, “if a fluid (liquid or gas) increases its speed, then the pressure drops” and this is one of the fundamental reasons that a 2 stroke engine works. As the products of combustion accelerate down the exhaust port, they cause a drop in pressure that sucks the fresh fuel charge into the cylinder. So it follows that the faster the gases exhaust, the lower the pressure, the greater the suck, the more fuel is drawn in and the more power you get – simple. And that’s what I’ve done to improve Sir Walter, (amongst other things) and IT WORKS!

He can now climb steeper hills without LPA, he’s revving out much, much better and he even sounds fast.

PS, Bernoulli’s principle is why aeroplane wings generate lift, a spinning football bends, ships can’t pass close at sea, jetties always have water beneath them and why a F1 car’s areopackage works, amongst many other things.  Where would we be without Hydrodynamica?



Michelangelo I’m not..

but as the great man said, “Genius is eternal patience” and that’s what’s needed to paint the Cyclemaster engine cover – patience that is; not genius.

So, after a quick rub down with “wet and dry” sandpaper, it’s on with the primer. I used Rust-oleum  for no other reason than it was cheap. I was unsure whether it was suitable for a cellulose based top coat but decided to find out!

For the top cost, I chose a silver that was close as possible to the original Polychromatic grey, then had it mixed at a local motor factors who supplied in an aerosol for a very reasonable £11. It went on well but dried to a flat finish as I’d got paint without lacquer. I did because cellulose lacquer is not resistant to ethanol and it would most likely lift on the petrol tank.

So the next problem was how to paint the red lines on the embosses and the black on the background of the Cyclemaster badge.

I started with the engine cover and found that a magnifying glass and a mascara brush worked well. Yes it’s the first time I’ve used a mascara brush, even though the result looks like I’m a pro!

The foam pad worked well on the raised embossing as it didn’t wipe down the sides like a brush would. However, the red flashes on the petrol tank and carb cover are different as the embosses aren’t flat topped – they are V shaped. So the first job was to mark the lines and I used my vernier set at 4.5mm to make some very small scratches. I then applied 6mm wide masking tape to form the straight sides. Now, the curved ends were a problem and I came up with what I think is a nifty solution; I used a hole punch to make a semi circle on pieces of masking tape.

These were then positioned to close the ends of each flash and the flash painted with a fine brush.

I removed the tape before the paint was dry, to prevent it from bleeding under the tape. Some “experts” recommend leaving it until the paint is dry but I was also concerned the tape may then pull the red off.

The final detailing job, was the black background to “Cyclemaster”. This was a really delicate job that I did with the smallest brush I could find. For some of the detailing around “MADE IN ENGLAND” I used the old cocktail stick trick; for example to apply the dot to the A’s – not easy.

TIP Boyes has a good supply of paints, brushes and masking tape etc.

The final job was lacquering and the big problem here is that it needs to be ethanol proof. The best lacquer for this is two pack and I found an aerosol called Pro2KClear made by Capella Solutions Group. It has a clever little ring pull on the bottom that releases the isocyanate into the paint to start the curing process. This worked really well, the lacquer went on easily and the results speak for themselves.

So far the lacquer looks very durable – scratch resistant and not yet affected by petrol spills.

However, as I said, “I’m no Michelangelo”. But a Cyclemaster is no Sistine Chapel and I’m as happy with the result as the Pope is with his ceiling – job done.

PS TIP Don’t use Humbrol Enamel for the highlights as it wrinkled a little when the lacquer was applied. At one point I thought it was going to have to start again. However it dried OK and the wrinkles can only be seen when examined closely which won’t be a problem when I zoom past at at least 20mph!

More thoughts on the warm starting problem…and some science

After furiously pedalling my warm Cyclemaster to get it to start, I found it began to fire as I rolled the throttle to almost closed. So I’m trying to figure out why, as this could lead me to the solution.

Fuelling control on a 2 stroke normally works like this:

  • Pilot jet – controls idle to around 1/4 throttle
  • Slide cutaway – controls 1/8 to around 1/2 throttle
  • Needle and jet – control from 1/4 to 3/4 throttle
  • Main jet – controls from 1/2 to full throttle

Now the Amal 308 carb is a basic item as it doesn’t have a pilot jet and adjusting screw.  And without a pilot jet, fuelling for warm starting can only be controlled by the cutaway and needle jet. So the next step is to understand how the carburettor works and the answer is the “Venturi Effect”.

The Venturi Effect was discovered by an Italian physicist called Giovanni Battista Venturi and it goes like this:

The Venturi effect is the phenomenon that occurs when a fluid that is flowing through a pipe is forced through a narrow section, resulting in a pressure decrease and a velocity increase.

In a carburetor, the pressure decrease creates suction that causes fuel to be sucked into the passing air where it mixes and gets drawn into the engine. When starting a warm engine with a normal carburettor, the fuel would be drawn through the pilot jet and the pilot jet suction port is positioned just behind the front edge of the throttle slide (engine side). This is the narrowest part, so the point where the air is moving the fastest and the point of maximum suction – see diagram below.


However, the Amal 308 doesn’t have a pilot jet, so fuel can only get drawn from the Needle Jet and here, I think, lies the problem; it’s in the wrong position when trying to start a warm engine! The suction just isn’t enough to raise fuel up the Needle Jet Tube, at low air flow.

Also it gets worse if you open the throttle. This increases the area above the needle jet, so the air velocity decreases and the suction decreases to the point where no fuel is drawn into the air – hence it won’t start. If some fuel is drawn up, the engine doesn’t fire as it’s getting too much air and the mixture is too weak to fire, as the throttle slide is up.


However, as you close the throttle, the air flow is restricted, the mixture gets richer and the engine fires.

I believe this may explain the poor warm starting many Cyclemasters suffer.

It also explains why tilting the carburettor to the left helps; it simply lets a little petrol run into the venturi. And you probably noticed in my “kick-start” video that the engine started on idle, i.e. throttle slide almost down to get down to get maximum suction at the needle jet and limited air.

So, to start your warm engine, I suggest:

  1. mark the idle position on the throttle control and set it just above this point,
  2. tilt the bike and engine to left for a few seconds (experiment with angle of tilt and time),
  3. kick-start or pedal the bike.

and hopefully, it’ll start – before you run out of breath!

Good luck, I hope this helps and please let me know your experience.


For those of a scientific mind the Venturi Effect is governed by the Bernoulli principle. And Bernoulli’s principle is why aeroplanes and birds can fly, why a football follows a curved path when kicked with spin and why ships must maintain a minimum distance when they pass in opposite directions. I can explain if anyone wants to know?