[Bluebell]

(A general warning for all as well as)
Fritz. If your old mechanical pump is as bad as it sounds, you should by pass it completely (I assume you are pumping through it)
If your diaphragm is old it won't handle the new fuel. One of the reasons a mechanical pump fails is because the diaphragm fails. If your diaphragm leaks, an electric pump pushing through it can fill your sump with petrol. Apart from the possibility of a big bang, the petrol will wash the oil from the engine, and cause an expensive seizure.
If you are running an electric pump I would recommend a complete bypass of the mechanical pump. The reason being that it is human nature to neglect that which we consider unnecessary , more so than we would if we were relying on it.
I have known of pumps failing like this when they were the only pump involved. The pump delivered enough petrol to the carb, while leaking into the sump, until the engine seized.