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Users Manual
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very hard to see, so if it has been used for some time – change
it. When you change the pump membrane, always change the
gasket too. The membrane shall always be placed nearest to the
carburettor housing, otherwise the valves will not work. Then take
a last check of the channel between the crankcase and the carbu-
rettor.
Now check the pumping action again. If it still doesn’t work, check
the needle valve (see below) in the carburettor, which could be
stuck. The pump pushes fuel towards the needle valve, but can’t
open it. Let us look at the other side of the carburettor. If you take
off the steel cover you will find another membrane, a lever with
a spring and a needle valve. The needle valve is held in closed
position by the spring working through the lever. The lever rests
against the main membrane (which can be recognized by the
small rivet in the middle of the membrane). The lower side of the
main membrane and a cavity of the carburettor housing form a
small fuel reservoir. When air is rushing through the carburettor,
the fuel under the main membrane is sucked out. The fuel lever
under the main membrane is now reduced. The membrane is
moving downwards and forces the lever to open the needle valve.
Now more fuel comes in from the pump side of the carburettor.
The membrane moves upwards and the lever can close the valve.
This is how the carburettor keeps a steady level of fuel.
From the chamber under the membrane the fuel goes through the
H and L needle valves to the venturi of the carburettor. The chan-
nels are so thin that the fuel does not pass through unless it is
sucked. The design and location of the small holes through which
the fuel can enter into the venturi are of great importance for the
characteristic of the carburettor.
Now for some service hints. Let start with the needle valve. First
take off the steel cover and the main membrane with its gasket.
Then loosen the little screw under the membrane. Now it is pos-
sible to dismount the needle valve. Don’t loose the spring.
Check the needle valve. The conical rubber end must be smooth.
If you can see impressions – change the valve. Check the bot-
tom of the hole that the needle valve is sitting in. It shall also be
smooth. When you have cleaned everything, start to assemble
the needle valve with its lever and spring. The spring and the
lever must not be deformed (you shall have some as spare parts).
When you have assembled the needle valve, the arm of the lever
that is in contact with the membrane shall be at the same level as
the adjacent level of the carburettor.
