Bob, Briggs has had two "suction" carburetors (fuel tank below carburetor, so no gravity feed). One of them was called VacuJet: that one had no fuel pump at all, just used venturi suction to lift the fuel from the tank to the venturi. It worked fairly well on the shallow fuel tanks on the vertical crankshaft engines, but was poor to unacceptable on the deep fuel tanks often used on the horizontal crankshaft engines. The other type was the PulsaJet, which had a little fuel pump in the diaphragm between the carburetor and fuel tank: the diaphragm was operated by intake suction pulses. The PulsaJet worked with either shallow or deep tanks and was a better device than the VacuJet, but the diaphragm had to be handled carefully because its built-in flap valves were sensitive.
Both the VacuJet and the PulsaJet were made in both metal (aluminium I think) and plastic (Minion) forms. In the third generation engines (second part of the model number was a 2, because they started with zero on the first generation) both VacuJet and PulsaJet had an automatic choke. Leo's carburetor is an automatic choke type. In my experience the choke works extremely well - but it does make reassembly of carburetor to fuel tank a bit trickier, since you have to preload the diaphragm.
For information, once we establish that Leo's cooling system is OK, the muffler will be next then the tappets will be the third thing to check. By the time he's through those three areas, he'll be more experienced and we'll be able to look at more complex stuff.
