The electric pump should be AFTER the mechanical if you use both. Otherwise, should the mechanical pump diaphragm rupture, the electric pump will start filling your engine crankcase with fuel.
We ran IDI engines like this for years. We eventually just used electric pumps and blocked off the mechanical pump opening with a fabricated plate - but a big block Chevy mechanical fuel pump blockoff plate from a speed shop works perfectly.
Personally, I carry a spare electric pump, just in case. And spare fuel filters.
The huge advantage to an electric pump is that you do not have to waste battery amperage and overheat the starter purging air from the fuel lines after a filer change or running out of fuel. Cranking 440 cubic engines at high compression just to get fuel flow from a mechanical pump is a complete waste of time, energy and wear, when there is an easy alternative.
On a V belt driven IDI, another advantage is that you get more belt adjustment range on the vacuum pump, allowing the use of belts with slightly longer dimensions.
Always have the filter the last item on the line before the injection pump. You do not ever want debris from a worn or failing lift pump to enter the injection pump.
Ok so order would be ,from tank, tank switch, check valve, electric pump, mechanical lift pump, filter, hpop/ip.
Right?
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