Out of curiosity I've pulled apart an old type 4 fuel pump to see if can understand how it works. type 1 pumps are pretty much the same as far as I can tell. The lower section, pic 4, with the large diaphragm seems obvious enough. As it moves up and down, and fuel is sucked in, two non return valves ensure the fuel is delivered into the chamber above, pic 2, and then on to the carbs. Above this chamber is the non return diaphragm in pic 1. This is held down by the top plate in pic 3 and a spring, and a valve underneath covers the outlet in the centre of pic 2. My assumption is that when fuel enters the chamber in pic 2, the pressure forces the non return diaphragm up and opens the valve so fuel can flow out. What I don't get is what happens when the float valves close on the carbs. The fuel can't leave the pump so what happens to it? How does the pump manage not to blow itself up with a build up of pressure?
Same question occurred to me. Don’t know exactly, but I’ve read somewhere that these kind of diaphragm pumps don’t mind pumping against a dead end. They just don’t do much until things improve.
I think the arrangement in the pump I bought at Slough for a fiver had a separate demand valve regulator in the top of the pump to control fuel pressure. Basically back pressure on a diaphragm closes a small valve when the force created by fuel pressure exceeds that on a spring trying to open the valve It was cheap because one of the castings was warped so it didnt seal and dripped judging by all the mastic all over it . After some consideration of the problem, a few minutes with a file and the gasket could sit flat again. However somebody had jammed a 10 mm bit of wire in one of the petrol passages and it had made a hole somewhere random. So I havent dared / bothered trying it out. The pumping stroke is when the pushrod drops, so its just the spring against the back pressure of the downstream system. If that stops taking fuel, the spring travels a shorter distance before being stopped by the pressure rising. If it were powered on the rod rising it would break pretty quickly as the rod is forced up against the pump and back pressure in the pump. .
Aha, I think that last part makes good sense. The push rod pulls the main diaphragm down which sucks in fuel. It returns only under spring pressure so yes it's that that stops the pump doing any work. It's not at the non return valve end. I had a mental fiver on you knowing the answer Mike.
