This article was written by Dave for Motorcycle Dealer Magazine.
The last eighteen months have seen a huge increase in the number of machines
going through our workshop with the same problems. One or more of the
following symptoms:- poor starting, erratic tick-over, rough running and
flooding. All caused by muck in the carburettors.
So where is this muck coming from? It ranges from individual rust flakes to a
fine rusty mud. Not unusual in itself, but why the sudden increase in
incidents? Is a local filling station supplying dirty fuel? No, the owners
used different stations over a wide area and many use premium brands. Is there
a similarity in the machines affected? Well, yes, most are over three years
old but there is no pattern in machine size or makes. Two-stroke and
four-strokes are equally 'infected'.
It took some lateral thinking to trace the source of the problem.
The last two years has also seen an increase in the market share of
high-detergent fuels, designed to clean your engine as you drive. These were
first introduced by the premium brands, such as Shell, but are now standard at
many supermarket and other discount outlets.
How can something designed to clean, end-up making it dirty? Well the mechanism
works like this…
The detergent action of the fuel is not limited to the combustion chamber. The
fuel is very aggressive in cleaning all surfaces it comes into contact with,
including the fuel tank, lines, and carbs. Any particles of rust and other
debris (paint flakes etc.) will be loosened from the inside of the tank and
carried with the fuel to the control valve and jets.
This theory has been confirmed by discussions with the fuel experts at Shell,
who confirm the action of the fuel and have run into similar problems in
storage and supply. Any petrol station converting to high-detergent fuels is
advised to fit new filters to the pumps and to check/change them more
frequently than usual.
Here is the solution to the problems with the bikes. Strip and clean the
carburettors, fuel tap etc. Flush the tank to rinse away any loose particles
and fit a good quality in-line filter. This solution has worked on every
machine we've tried; some following weeks of recurring problems.
Why go to all this trouble? Why not just avoid high detergent fuels? Well the
stuff does work. Over time the action of the detergent will clean out the
entire fuel system and the engine internals, head, valves, piston etc. Because
of its aggressive cleaning action it also has a corrosion inhibitor which will
prevent further rusting of the fuel tank, which not only causes the rust flakes
but often results in dangerous leaks.
So why does this problem only affect motorcycles, over three years old, and not
cars? Since the fuel is supplied (hopefully) clean and filtered, the problem
will only occur on machines which have a build-up of rust deposits in the tank.
The deposits mainly arise as a result of condensation forming on the inside of
the tank when it is less than full (which is why it is better to store a
machine with a full tank, than a half empty one). There is also a small amount
of water in each gallon of petrol supplied. The older the machine, the more
likely that the build-up has become significant.
So what about cars? Well until very recently all road machines in this country
had to have metal (in most cases, steel) tanks. Many cars have had plastic
tanks for years. The temperature variations that apply to a motorcycle tank
are greater than for a car tank: it is changes in temperature and humidity
which cause condensation. However, the main reason I suspect bikes are more
likely to have problems is simply the relative size of the components
concerned. The average car has an engine capacity of between 1000 and 2000cc,
with a single carb. Most bikes have one carb per cylinder, so the average bike
has only 100 to 300cc per carb, regardless of engine size. The jets and other
carb components are significantly smaller and more susceptible to blockages.
Where cars are pump fed, most bikes rely on gravity. This increases the chance
of debris sticking fuel control valves.
This last bit is all conjecture, but is seems to fit. Just to prove me wrong,
a colleague's Triumph Spitfire (the car) had exactly the same problems
recently. Cleaning the carb and fitting an in-line filter seem to be a
permanent fix. Maybe we will hear more about this from the classic car guys.
In the meantime we advise all our customers, with older machines, to fit
in-line filters. This way they get the benefit of modern fuel technology,
while avoiding the problems.
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