Fuel problem

[NoviceM]

I have had several breakdowns resulting from fuel pump failing. As usual, a bang with a spanner got pump working again.
But, it got to think about a short term temporary solution to 'get me home'.
To this end, as a test, I temporarily rigged up 1 litre plastic motor bike fuel tank about 10 inches above level of SU carb.
Pipe from this tank was connected to inlet of carb in place of normal pipe from fuel pump (this supply pipe was blocked off and fuel pump electrical supply temporarily disconnected)
This arrangement (a bit Heath Robinson at this stage) worked well...with one exception. The carb bowl overflowed a little, obviously a problem!.
When the temporary arrangement was returned to normal, the bowl did not overflow under su fuel pump pressure.
The question/issue is this.......I am surprised that gravity at a head of 10 to 12 inches develops greater pressure than the su fuel pump? Does this seem possible/logical ?

[svenedin]

I did a quick internet search and the output pressure of a low pressure SU fuel pump is 1.5 Lb/sq inch according to Burlen.

A head of 12 inches is 0.433 Lb/sq inch so much lower than the output pressure of a properly functioning SU LP fuel pump.

Perhaps you have a faulty needle valve in the float chamber as well as a worn out SU fuel pump? If your pump has been failing for some time perhaps the needle valve fault has been masked. Another suggestion is that the SU fuel pump delivers fuel in as pulsatile manner with each stroke of the diaphragm. Maybe that creates a jerky action of the float the helps the needle valve to fully close. Just my thoughts.......

Stephen