Adapting Well Bottom cistern flush, not replacing.

[DIY-JC]

I've got a working (apart from a weep, see another post) cast iron well bottom cistern. I tested to see if the side-exit overflow pipe works and unfortunately, it is too slow to stop the water overflowing through where the flapper is hinged on the side of the tank.
My question is:
Can I cut the top of the outlet pipe underneath the bell to lower than the height of the hinge so the water would flow down the pipe? Or would that destroy the siphoning effect?
I mentioned replacing the innards for a modern internal overflow as shown in other posts to my good lady and lets say, that was a no.

 

[DIY-JC]

Instead of butchering the antique cistern. I'm looking at getting the external overflow to syphon. I think I understand the principle but do I need to
a) Put an elbow pointing downwards on the inside of the cistern to create the siphoning or
b) Put a downward bend on the other outside end of the overflow to create the siphoning or
c) Both.

 

[Hugh Jaleak]

I'd put a valve on the water supply, and slow down the incoming flow to a rate the overflow can cope with. It's rare Ball Valves fails completely anyway, normally would notice a gradual deterioration in operation, rather than sudden failure at full bore.

 

[DIY-JC]

I'd put a valve on the water supply, and slow down the incoming flow to a rate the overflow can cope with. It's rare Ball Valves fails completely anyway, normally would notice a gradual deterioration in operation, rather than sudden failure at full bore.

Cheers Hugh.
I had slowed down the water supply to the point it took 4 minutes to fill. I must admit, when the water got above the hole of the outlet I chickened out at flushed so I didn't fully check if there was a sudden rush of water from the overflow.
Because of the lack of space in the tank, the bell and mechanism taking up a lot of space, I've fitted one of those cup-style filling valves (see attached). I've never used one before so I'm not sure about their reliability so I wanted to ensure the overflow worked.
I'm going to put a slight bend in the pipe to lower the end to give more of a drop in the overflow. There is not much more I can do without drilling another hole in the wall and putting a greater drop in the overflow.

 

[Hugh Jaleak]

Torbeck Valve so you know.

 

[DIY-JC]

Torbeck Valve so you know.

Thanks again Hugh. Every day is an education.