"Carburetor Synchronization" Sport Aviation / Experimenter "Technically Speaking" Article January 2016
Sport Aviation / Experimenter magazine "Technically Speaking" January 2016 Article
Carburetor SynchronizationWith the proliferation of the Rotax 912 80 hp and the Rotax 912S 100 hp engines, the topic of carburetor synchronization has come to the forefront. Until about the 1980s, the popularity of Continental and Lycoming engines dominated the general aviation market, these engines used a single carburetor providing for a single source of air and fuel to the cylinders. The use of dual carburetors was primarily relegated to the area of the two-stroke ultralight market. And, even with these engines, the process of carburetor synchronization was quite simple and reliable. However, with the popularity of the Rotax 9 series engines, it has become important to understand a little bit more about how the induction system works on this amazing little powerhouse. This understanding is important not only from a maintenance standpoint, but from a pilot’s perspective as well.
The Rotax 912 is essentially two engines connected to a single crankshaft and gearbox with both the left and right sides of the engine having their own independent carburetor, ignition, and exhaust system Figure: 1. As you might well imagine, having two engines trying to run a single propeller requires a bit of choreography between the right and left side of the engine in order to make things run smoothly. Most of us, who have spent a considerable amount of time in the air, can remember a time when one of the cylinders on a four-cylinder engine just quit firing, maybe from fouled spark plugs, or a plugged fuel injector. Regardless of the source, if you have ever lost a cylinder, it likely got your attention. Now imagine losing two cylinders. This is nothing short of an all-out assault on your engine and airframe. The shaking can be so violent that the fear of the motor departing the airframe becomes a realistic concern. With an engine like the Rotax 912, which has the right and left side induction systems isolated from each other, you can see the potential hazard with having one throttle wide open and the other at idle. The resulting reaction of the engine would be similar to the scenario of losing two cylinders in our previous example. In fact, we now train pilots differently in a Rotax powered aircraft by teaching them to advance the throttle to full throttle in the event of a violently shaking engine. The reason for this is that on most Rotax powered aircraft the throttles are spring-loaded to the full throttle position. As a result, in the unlikely event of a throttle cable failure, pulling the one remaining throttle cable back to idle when the engine starts to shake just exacerbates the problem. By advancing the throttle to full throttle, it allows the throttle springs to bring both carburetors to the (same) full throttle position. This allows the engine to run smoothly and the aircraft to be flown to the nearest airport where the engine can be shut off for a dead stick landing, a better scenario than losing the engine power entirely. Theoretically, at full throttle the carburetors are perfectly synchronized by the throttle arms hitting the full throttle stops simultaneously.
So we’ve identified that the Rotax 912 is basically two engines running in synchronicity at full throttle. Having one throttle cable adjusted in a slightly different position (let’s say 1/8” of extra cable), compared to the other throttle cable, at full throttle would result in only a miniscule differential in the manifold pressure of the two intake manifolds. However, if the throttle arms are in the idle position 1/8” difference in throttle cable length would result in a massive pressure differential between the two intake manifolds. And as a result, the engine would run extremely rough. At idle a very small adjustment makes a significant change in the pressure differential. And as we open the throttle wider the pressure differential between the two manifolds decreases. The most important synchronization point is at idle and just off idle.
Some of the characteristics associated with mis-synchronized carburetors include: Overall vibration causing wear and tear on the airframe and engine. Rough running at idle, too low of idle speed, including engine stopping during final approach. Excessive wear on the gearbox resulting in an increase in the amount of steel both in the oil filter as well as on the magnetic drain plug. Troubles with the needle and seat within the carburetor seating properly, particularly at idle. This causes excessive fuel in the float bowls and a rich mixture. The rich mixture, in turn, makes the engine run rougher exacerbating the shaking problem.
Rotax provides a fairly comprehensive set of instructions for proper carburetor synchronization in the line maintenance manual, downloadable from the Rotax website (
Once you become familiar with your particular airplane, have the throttle linkage set up correctly, and you understand how it works, the system is rather simple and bulletproof. Most of the problems we see related to carburetor synchronization are simply a lack of understanding about how to properly set up the linkage in relationship to the carburetors, poorly designed throttle systems, or trying to synchronize worn out carburetors that need to be rebuilt. It is a waste of time to be synchronizing the carburetors if they are not set up and working correctly. The 912 and 912S Rotax engines are amazing products. Once you become familiar with the nuances of their maintenance and operation, you can’t help but be impressed by the elegance of the design.