I've been wondering, what exactly is this thing called a vacuum fuel pump? I've heard it's used in carbureted engines. I can't quite grasp how that vacuum pressure system actually kicks in. Are there fuel droplets getting mixed in along the way? It all seems a bit confusing—I've never actually seen one in action...
What is a vacuum fuel pump and how does it work?
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Man, that vacuum fuel pump is actually a pretty simple but clever system, bro. It was used in carbureted engines back in the day when there was no electronic injection or anything like that. What does it do? It sucks fuel into the engine, just like a pipette but powered by vacuum.
How does it kick in? When the engine runs, the vacuum (negative pressure) created in the intake manifold moves the diaphragm at the back of the pump. This diaphragm then operates a rod, sucking fuel from the tank and pumping it into the carburetor. It's a fully mechanical system—no electricity needed. I’ve had it work plenty of times myself, especially in my old Seat 133, and it never gave me trouble. If you ask if it leaves any droplets behind, yeah bro, carburetors always have a little leakage, but the vacuum pump mostly handles that too.
Man, when I first heard about it, I was like, "A vacuum fuel pump? What's that?" Actually, I had seen it working in my dad's old SEAT 133 back in the day. I think it was in the '90s. When the engine wasn't running, you'd pump it by hand, and then the vacuum system would send the fuel to the carburetor. It was a completely mechanical system, no electricity involved.
Let me explain how it works briefly: In carbureted engines, the manifold vacuum (the negative pressure in the intake manifold) is used as the driving force. The so-called pump is actually a diaphragm mechanism. The vacuum created during the intake stroke pulls the diaphragm towards it, and a spring connected to it ensures the fuel is drawn in. So, when the engine is running, fuel flows continuously thanks to the vacuum. There might be some droplets now and then, especially if the system is old or the gaskets are worn out. Back in my SEAT, when the engine would choke a bit, there was probably a leak in the vacuum line, you could smell unburnt fuel.
In carbureted systems, without a vacuum pump, starting the engine would be nearly impossible. In fuel-injected systems, this system doesn't exist; they use electric pumps. It's a completely different principle. If you're dealing with a carbureted vehicle, I'd recommend checking the vacuum line and pump thoroughly, bro. Otherwise, the engine will misfire or not start at all.