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LAST MONTH I outlined the types of model engines, highlighting performance and design differences. But of all the available types, sizes, and variations of model engines, the most common kind used in trainers today is the two-stroke, .40-cubic-inch-displacement "Ol' Reliable," or "forty."

In this installment I'll cover this type of engine's initial care and feeding, including mounting, break-in, and needle settings. Following this segment I'll cover propellers, glow plugs, fuel, maintenance, and repair. Except for history and propeller sizes, everything I will discuss in these articles will apply to most two-stroke engines from .10 to 2.10 displacement.

The .40 two-stroke has been the most popular RC engine for several decades. A logical outgrowth of CL's most popular engine of the 1950s—the Fox .35—the .40 RC offered increased displacement to compensate for the power that was lost when incorporating a throttle.

The first .40 was familiar to CL pilots who were transferring to RC; remained easy to hand start; was approximately the same physical size, weight, and power as the .35; and offered good fuel economy. These features made the .40 popular then and remain its key advantages to this day.

Today, the old .40 format comes in many displacements (the volume of the cylinder the piston travels). The same-size crankcase (the aluminum engine "block" containing all the moving parts except for carburetion) now varies from the original .40-cubic-inch displacement all the way up to .51 cubic inch.

Naturally, the various .45s, .46s, and .50s produce more power than the .40s, but they use more fuel and require a larger volume of cooling air to operate. These slightly larger-displacement engines usually swing larger-diameter propellers that may cause ground-clearance problems on normal "40-size" aircraft. You may have to adjust the landing-gear length to accommodate them.

The .40s are offered in ringed or aluminum-brass-chrome (ABC) configurations. The original ringed, sometimes baffled, engines feature low fuel consumption and reliable, cool running. The ABC engines are powerful without being temperamental, unless they are solely racing engines—and those are definitely outside this article's scope.

Most, but not all, .40 engines sold today are Schnuerle ported (have extra fuel-intake ports inside the engine) for more power. Whether Schnuerle ported or not, the engine's break-in procedure is determined by its ringed or ABC (also AAC, or aluminum-aluminum-chrome) design.

Before the engine can be properly broken in, it has to be mounted on the airplane or test stand. Mounting on a test stand is easy; just follow the stand manufacturer's directions. Be sure to attach the muffler and tank pressure lines as well.

Almost all of today's .40 two-stroke engines require muffler pressure to the fuel tank to get sufficient fuel into the carburetor. Why? Without muffler pressure the engine must create a vacuum in the fuel feed line to draw fuel from the tank into the carburetor. It does this by drawing air into the carburetor through the venturi opening and then past a small hole (the spray bar) that mixes fuel into the incoming air.

The venturi is that big hole in the carburetor that opens as the throttle is advanced, and the spray bar is the small brass tube inside the venturi. To get enough fuel suction, the incoming air must be moving quickly through the venturi. For proper fuel suction, the volume of moving air is not as critical as its speed.

Before mufflers became common, manufacturers had to make the venturi bore small to increase the incoming air's speed. However, a smaller venturi restricts the total amount of incoming air and therefore reduces power output. Venturi bore size had to be a compromise between power and reliable fuel feed.

The advent of mufflers allowed manufacturers to divert some of the exhaust gases into the fuel tank itself. This rerouting put pressure inside the tank that forced fuel to flow into the carburetor.

While not actually acting as a fuel pump, the addition of muffler pressure meant that venturi suction was no longer the sole source of the engine's fuel feed. As a result, the venturi bore diameter could be made larger without reducing the carburetor's fuel intake.

Making the venturi bore larger increases an engine's power output. Today's engine's larger venturi requires that the muffler be attached every time the engine is run, to ensure that the fuel mixture is "rich" enough (has a high enough fuel-to-air ratio) to lubricate and cool the engine. This is especially important during break-in, whether the engine is mounted on a test stand or in an airplane.

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