How to check the condition of the decompression port

by Had Robinson

Why does the decompression port (DCP) become clogged?  When the power stroke of the engine starts, the piston moves down and the DCP (which is an inch or so down from the top of the cylinder) is exposed.  At the same time, some of the high pressure burned gases are vented through the port to the exhaust.  The DCP does cause some decrease in the efficiency of the engine but its benefits far outweigh any loss.  As the engine runs over many hours, the burned gases will slowly deposit carbon/lead oxides inside the length of the port.  Depending on what type of fuel is used, the port may be blocked completely after approximately 100 hours of operation or more (unleaded fuels) or 50 hours (AVGAS).

One of the important advantages of unleaded fuels is that they do not coat engine parts with deposits anywhere near as quickly as do leaded fuels like AVGAS.  However, fuels with ethanol greatly increase fuel system maintenance, especially with regards to corrosion of the metal parts of the carburetor, premature hardening of the flexible materials in the fuel system, and hotter running temperatures.  According to the 2 stroke engine oil manufacturers, ethanol causes oil to deteriorate which is why mixed ethanol fuels must be used quickly.  It is worth the effort to find premium ethanol-free unleaded gasoline.  See our fuel/oil page for more info on this subject.

When the decompression port (DCP) becomes clogged the forces necessary to start the engine greatly increase.  Various systems, such as flash or electric starters, may hide the condition of the DCP.  However, the net effect of a clogged port will be to wear starter systems out prematurely.  This is to say nothing about wearing out the pilot when he attempts to start the engine with a pull starter that is not the flash type.

This technique does not require the removal of the cylinder head which is usually done in order to visually examine the condition of the port.

  1.  Remove the exhaust system so that the exhaust port and manifold are exposed.  You must be able to hear the air going in/out the DCP, a hissing sound.
  2. What we are attempting to do is listen to the air escaping from the DCP as the piston moves upward.  When the piston begins its compression stroke, it closes the intake port and then the exhaust port on the cylinder.  As it continues to move up, the pressure created is vented through the DCP into the exhaust port.  As the piston continues its movement upward, it will cover the DCP and the gases will begin to be fully compressed by the piston and ready for the spark to ignite the mixture.
  3. Look into the exhaust port and rotate the crankshaft (CCW when looking at the engine from the rear) until the piston just moves past and closes the exhaust port.  At this point, the DCP is open and further movement of the piston will force air through it and out the exhaust port.  It is a distinct hiss that is easy to recognize.
  4.  When the piston is in the right position, it is best is to rapidly rotate the crankshaft back and fourth approximately 20 degrees.  Removing the starter to get access to the flywheel is the easiest way if the engine is clutched.  Alternately, slowly pull the starter through a few revolutions but the pilot has to be a bit more attentive to when the piston closes the intake port on the upstroke.  Unclutched engines simply require the pilot to grasp the propeller and rotate it back and fourth.
  5. Here is a video of the process.  In the first part of the video, the crankshaft is rotated quickly back and fourth and the hissing is very distinct.  In the second part of the video, the piston is advanced so that the port is closed.  Any movement of the piston at this point does not generate any sound.

As the port begins to clog up, the hissing will decrease and more force will be required to move the piston.  When the port is cleaned and the engine is reassembled, pilots would do well to perform this test in order to get an idea of what an unclogged port sounds like.  The good news is that once the port starts to clog, it tends to clog quickly.

Paramotor engines are finicky and delicate, principally because they must have high output and be lightweight.  This combination adds complexity that we must address.

Turkey Vulture