If its definitely not the loom? maybe look at the amal valve? on L8 it takes its power feed from the fuel pump relay (on a factory loom) and on mine i had similar wont start/fuel pump cut out issues caused by water shorting the connector as the amal valve connector isnt that well shielded at the valve from water due to nature of the amal valve pins.

When it was causing issues on mine it would also take out the other engine loom relay as the power supply comes from same source so was getting voltage dips all over and id imagine that wont help with running.