The older engines, could be in a tractor, or car, truck, boat, just about anything, used an "updraft carburetor". The main reason was to keep the carb low, so no need of a fuel pump.
The manufactures wanted to keep everything as simple as possible. They actually wanted their product to last and be trouble free, and be field serviceable. Hard to imagine that concept today!
As for how the intake works, if you'll trace the flow, you'll see why there is only 6 ports instead of 8.
Before we start, keep something in mind. The moving gasses, whether they be fresh air with suspended gasoline vapor going in under a partial vacuum, or flaming hot, still burning exhaust gasses coming out under extreme pressure, they both have velocity. They follow the law that "an object in motion tends to stay in motion, an object at rest tends to stay at rest". The design of a good manifold will take that law and use it to the best advantage to make the engine as efficient as possible.
Starting with the exhaust, each port coming from the engine dumps into a common collector, or the body of the manifold. The collector is constantly pulsing with start-stop bursts of hot, still expanding gasses. The object is to use these pulses of energy to create a draft effect that actually creates a vacuum on the adjoining ports, helping to draw out as much of the trailing end of the exhaust stroke as possible. Exhaust manifolds are somewhat effective at this, but the effect favors some cylinders over others. That is the reason behind performance engines using "headers". Real headers are constructed of equal length tubes, converging into a common collector. That way each tube draws close to the same vacuum at the end of each exhaust cycle, making for a more equal distribution of charge in each cylinder.
Perfectly clear, right? It gets better!
The intake. Looking at the intake, notice there are only 2 ports, which means 2 cylinders have to share each port. Doesn't sound very efficient does it? Well, look at it this way, only one intake valve is open at a time, so in reality the common port only has to supply one cylinder at a time, so no need for a separate port for each cylinder. But... Think about the exhaust and the "scavenging" effect, this is just the opposite, the "ram" effect of the moving air/fuel mix. It's actually better to keep the flow moving in a steady stream instead of stopping it every time the valve closes. This way, when one valve closes, the one next to it opens, ready to receive the already moving flow instead of having to start the flow every time the valve opens. That also helps smooth out the flow through the carburetor. carbs like smooth, steady flow, not choppy start-stop flows.
Even V design engines use this effect. Most down draft intakes have paired runners that divide the flow into the most efficient, smoothest directions.
Of course there are many different designs. Not all manifolds use paired runners, some are straight in, a runner for each cylinder. That still gives a ram effect, but a choppy flow. Some carbs can handle it, fuel injection likes it.
Way more than I can explain here or even imagine that I understand. All kinds of theories, but the dyno tells the final result, that and the emissions test...
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Today's Featured Article - Timing Your Magneto Ignition Tractor - by Chris Pratt. If you have done major engine work or restored your tractor, chances are you removed the magneto and spark plug wires and eventually reached the point where you had to put it all back together and make it run. On our first cosmetic restoration, not having a manual, we carefully marked the wires, taped the magneto in the position it came off, and were careful not to turn the engine over while we had these components off. We thought we could get by with this since the engine ran perfectly and would not need any internal work. After the cleanup and painting was done, we began reassembly and finally came to t
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