That’s both a sentiment shared by numerous mechanics at their wit’s end, and a statement of fact. It is, after all, how they work. A carburetor is essentially the simplest way of moving fuel from a tank, atomizing it, and spraying it into the engine- and since this is ultralight aviation (and the most light of the ultralights) that means that we’re essentially stuck with them by default.
The breed of carburetor most of our machines use is a diaphragm type- this is a type of carburetor pioneered in the motor tools world. Chainsaw-men wanted a fuel delivery system that would run in any orientation, even upside-down, and they wanted it lightweight. The diaphragm carburetor was the logical and effective answer. It featured only a few moving parts and was very simple, and the fact that it had no floats to worry about meant that it could, indeed, work in any position.
Where do we come in? Well, we’re taking a carburetor that was designed to run on a chainsaw, and putting it onto a much larger displacement engine. Worse still- chainsaws have their fuel reservoir horizontally right next to the carburetor, and the fuel only has to travel a few inches. Those same carburetors on PPGs are asked to suck their fuel vertically, against gravity, and they’re asked to suck it through a foot or more of fuel line. They’re well capable of the task, as evidenced by their ubiquitousness in the industry, but these design limitations mean that, here more than ever, they are very sensitive to maintenance and need to be kept at near perfect condition. Especially since failures in the fuel delivery system often lead to the dreaded, engine-killing, cylinder-scoring lean condition.
What are the other options? Well, there are other varieties of carburetor- for example, float bowl types are arguably more bulletproof. But, the float doesn’t handle g-forces well and they have no internal fuel pump, which on paramotors necessitates an inconvenient fuel tank location above the engine, or a seperate vacuum-driven pump which adds complexity. Fuel Injection systems light and simple enough for our application are just being pioneered, but their cost remains prohibitive to many consumers and the added complexity of sensors and ECMs adds multiple failure points, despite significant gains in fuel economy and ease-of-start.
So, the position of the industry remains that diaphragm carburetors are the best fit for us. How do we, the end users, maintain them and make sure that they keep our engines running healthily?
Preventative maintenance is an important step. The diaphragms of the diaphragm carburetor themselves are a wear part- any material that flexes thousands of times per minute while being exposed to gasoline will eventually wear out. When they do, the fuel will stop flowing as well as it did. Since the air-fuel mixture settings are fixed, a fuel flow deficiency will lead to a lean condition. Internal filter screens will need to be checked on a regular basis, especially if dubious-quality fuel has been used recently. Clogged or obstructed filters will lead to reduced fuel flow, and a lean condition. The needle rides on a spring that needs to be tuned to a set, precise weight value to make the needle release at the correct pressure. Over time, the spring can stiffen, making the “pop off” pressure too high. Having the pop off pressure too high will cause, you guessed it, a lean condition.
The point i’m trying to get across here is that most problems with the fuel delivery system result in not enough fuel reaching the combustion chamber, not too much. As much as we wish it was the other way around.
These lean conditions, by the way, kill your engines in a savage twofold assault. One, on a two stroke engine, the fuel is mixed with the engine oil. A lack of fuel means a lack of lubrication, and our little engines make such impressive power by spinning really, really quick. Your average, modern engine design can top out at over 9,000 RPM. That means that lubrication is very important and a lack thereof can be catastrophic. Perhaps even more catastrophic, however, is concept number two- that oxygen causes fire to burn hotter.
A cutting torch uses two gases- one is acetylene. This is a flammable gas, but it’s almost disappointing when it burns by itself- a dull orange flame that spews out acrid smoke. Add the second gas (Oxygen) and you get an intense jet of blue flame. Add a bit more oxygen on top of that and apply it to red-hot metal and it’ll slice through an inch of steel like butter. The point of the analogy is to demonstrate that we need oxygen- of course, there can be no combustion without it- but not too much. If the engine’s Stoichiometric ratio (which, for anyone keeping score, is about 14.7 parts air to 1 part fuel) is thrown off too far in the direction of too much air, the engine will begin running drastically hotter. Since it’s a common property of metals to expand when heated, this will typically cause the piston to swell until it’s too big for it’s bore, which will seize it and cause irreparable damage to the piston and cylinder wall. If it’s really hot, it’ll just melt a hole in the top of the piston, which will throw a lot of metal into the crankcase. The crankcase which contains sensitive bearings that are relatively expensive and difficult to replace. You can see where this is going.
And, for all that scariness, the carburetor is what’s supposed to be keeping that ratio where it needs to be. Do all in your power to keep the carburetor happy. Check it’s components and replace them on a timeframe, regardless of if they seem fine. Ensure your fuel delivery system is free of air leaks- clamps tight, lines intact. This is a key component to many hours of happy flying.