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I am thinking you are asking about the pulse type fuel pumps that are commonly used on small engines. They are a small diaphragm pump. The principle they work on is that as the piston goes up and down in the cylinder there is a corresponding fall and rise in the crankcase pressure. (Actually the manufacturers try to create a small vacuum at all times to keep the engine from blowing oil out the seals, but still there is a rise and fall in the pressure, even though it is less than atmospheric pressure all the time.) They use the changes in pressure in the crankcase to operate that small diaphragm pump. It has check valves (usually rubber flaps) that allow the pump to force a tiny bit of fuel toward the carburetor every time the pressure goes up in the crankcase. I have put a pressure gauge on them before, I have never seen one develop more than two psi, even with the carburetor line completely pinched off. You can install one of these pumps on an engine that did not originally have a pump. If the engine is apart, just drill and tap for a 1/4 nipple in the crankcase well above the oil level. You want your hose to the pump to not have any dips in it because if it fills with oil the oil will dampen the pulses and the pump will quit working. It is best if the pump is mounted high and the pulse hose runs up to it.
Briggs and Stratton connects their single cylinder fuel pumps to the dipstick tube. It has the nipple for the pulse line molded into it. This might be an option for your replacement engine, those dipstick tubes shouldn't be very expensive. The other thing that sometimes happens is that even at 2 psi, sometimes the pump can blow fuel past the needle valve and cause the carburetor to flood. Single cylinder Briggs and Stratton engines with carburetors set up for a fuel pump usually use an inlet needle seat with a noticeably smaller inlet hole for the needle to seat against when compared to a similar carburetor intended only for gravity feed fuel delivery. Because the needle seat hole is smaller the inlet needle can hold back the flow of fuel and also, because it has a small pressure, there is still enough fuel available to operate the engine under all loads.
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