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Combustion Chamber Cleaning of Lycoming aircraft Engines

Hemispherical combustion chamber (rounded)  found in engine such as the Continental 470 and 520 produces fewer combustion chamber deposits. Notice stop spark plug points toward smaller exhaust valve. This prevents detonation by directing the flame front toward the hotter exhaust valve thereby burning the gases in the hot region first before the temperature and pressure rises. Hemi heads have valves angled into the combustion chamber and use two separate rocker arm shafts

Standard combustion chambers, such as this Lycoming 235, produce greater amounts of combustion chamber deposits. Standard combustion chambers have the valves going straight into the combustion chamber and use one rocker arm shaft.

Lycoming introduced the O-235-L2C engine with it's standard combustion chamber about the time that 80 octane gas was becoming hard to get and operators had to use a higher leaded 100 octane fuel. The lead fouling problem was immense. Four possible solutions were introduced:

1. Champion introduced the REM37BY spark plug with its extended electrode design that reduced (some say delayed) lead fouling. 

REM37BY shown on the right

2. Lycoming issued several Service Instructions concerning leaning and operational changes that may reduce lead fouling.

3. Lycoming modified the combustion chamber design by adding small pockets next to the valves. The valve seat is very close to the cylinder wall on this engine (0235 and O290 series) and carbon/lead deposits would build-up on the seat face and cause valve leakage. The engine designation changed from O-235L2C to O-235N2C. The N2C was rated at 2 horsepower less than the O-235L2C.

4. Lycoming introduced a method of cleaning the combustion chamber of carbon and lead deposits. Service Instruction 1418 applicable to all Lycoming engines.

Most mechanics impression of 1418 at the time was "I don't think so" so few have ever done it and now most have never heard of the service instruction or the method described. However, it does work and you don't have to remove the cylinders. Service Instruction 1418 is a method of using walnut shells to blast clean the combustion chamber thru the spark plug ports!

Walnut shells are (or were) often used in engine shops to clean parts that you didn't want to mar the surface by using glass bead. We used walnut shells for years on connecting rods and pistons. If you are interested you can download  SI1418 above.

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