The Velosolex engine uses crank case pressure to drive a small diaphragm fuel pump to lift fuel from the tank to the Carburetor.
This page shows how to remove, check and rebuild the fuel pump.
Symptoms of a pump problem can be lack of fuel at the engine or bubbles seen in the fuel return pipe and an engine reluctant to idle.
A full fuel pump rebuild kit should include the diaphragm, back plate, glass valve balls and ball seats.
With the flywheel and engine covers removed the fuel pump can been seen mounted on the front of the crank case, behind the brass contact for the front light.
Both fuel pumps pipes can be removed with a 9mm spanner.
The pump can then be removed by undoing the lower hex and upper slotted screws.
The valve innards can be be accessed by undoing the threaded stud in the top of the pump. Importantly, the copper washer underneath must be kept.
The top glass ball should drop out of the valve assembly.
If the valve seats are difficult to remove, they can be pulled using a small wood screw.
The valve stack is shown above.
The new valve parts need to be pushed into place in the reverse order. The lower valve seat should be orientated as shown above so the castellations are not blocking the hole in the back of the pump.
The valve seats should be pushed gently into place with a rounded rod. In this case a large roofing nail rounded on the grinder.
After the stack of valve parts have been inserted the stud needs to be replaced ensuring the washer is present to allow correct "ball lift" within the valve.
The new pump backing plate should be orientated with the dished/concave side facing forwards, ie: towards the pump. Importantly it needs to be clipped onto the central stud on the engine casing and this can be done with a light of a screwdriver handle.
Another fail mode to check for is an air leak around the metal disc on the front of the pump. This can be sealed with chemical metal (shown here) or epoxy resin, although replacement pumps are readily available, which is a better long term solution.
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