Cleaning the decompression port on 2 stroke engines

by Had Robinson

Most paramotors have a small hole called a decompression port (DCP) that goes from the cylinder wall into the exhaust port.  This provides modest relief to the starting systems from the normally high compression of paramotor engines.  Moving the piston past top-dead-center can be a next to impossible task for a pilot on an engine without a flash-type starter or electric start.  Even with the flash or an electric start, the great forces can quickly wear out these systems.  Engines smaller than 100cc generally do not have a DCP.

The problem is that the hole will become clogged with combustion deposits.  If AVGAS is used, the DCP will become clogged much more often – this is one of the downsides of AVGAS.  Ethanol-free premium gasoline has almost zero deposits so cleaning the DCP does not have to be done as often.  Regardless of the fuel used, other gasoline additives such as Sta-Bil or fuel system cleaners may leave deposits so it pays to remove the cylinder head every 100 hours of runtime or less to be sure the DCP is clear.

Each manufacturer has a slightly different way of creating the port.  The Polini, for example, drills a 2.6mm hole straight down the cylinder wall into the exhaust port.  This hole is then connect with another hole that is drilled from an angle through the cylinder wall about 25mm down from the top of the cylinder.  The Simonini drills just one hole from the cylinder side into the exhaust port.

Thankfully, it is possible to clean the DCP without removing the cylinder.  The cylinder head, however must always be removed.  If is not necessary to remove the exhaust manifold unless it is leaking and you want to reattach it.  It is also a good time to remove any combustion deposits that are on the top of the piston and on the cylinder head.

Special tools needed:

1. Remove the cylinder head.  This may also require removing cooling shrouds and other peripheral equipment.

2. If the cylinder is still on the engine, it is very important to move the piston to just beneath the DCP in the cylinder wall.  Put copious amounts of oil around the outside of the piston.  The oil will trap the deposits that resulted from cleaning the DCP (and the top of the piston, as needed).  YOU DO NOT WANT THESE DEPOSITS GETTING INTO THE INTAKE PORTS ON THE CYLINDER!  The deposits can migrate to the oil ports for the main bearings and CLOG them.  After the DCP is cleaned, rotate the crankshaft so that the piston goes up to the top.

3. Locate the DCP hole in the cylinder wall.  Remove the exhaust system and, looking into the exhaust manifold, locate the DCP hole in the top of the exhaust port.  If it is Polini, remove the head gasket and you will see the DCP opening in the outside top of the cylinder wall right beneath where the gasket lay.  This hole intersects the hole in the cylinder wall – there are two separate holes that must be cleaned.

4. With the correct sized bit in the pin vise, remove the deposits from the DCP holes.  Use compressed air to carefully clean them.

In the photo below of a Polini Thor engine, the red arrow points to the DCP in the cylinder wall.  The black arrow points to the bore hole that connects the DCP in the cylinder wall to the hole in the exhaust port (not visible but just beyond the green arrow.  Your motor may have only one hole from the cylinder wall to the exhaust port.

typical decompression port on a 2 stroke engine

A pin vise is used to first clean the hole to the exhaust port.  Use the same setup to clean the short hole from the cylinder wall to the exhaust port hole.  Note the quantity of (harmless) lead phosphate deposits that were cleaned from the exhaust port.  This engine has over 160 hours on it, used AVGAS, and was overdue for a cleaning and replacement of the upper connecting rod bearing.

typical decompression port on a 2 stroke engine

The pin vise.  They can be ordered for a few $$$'s off of eBay.  The vise comes with four collets (2 pieces that are reversible) that can hold almost any sized bit under 3mm.  The extra double collet is inside the upper part of the vise.

typical decompression port on a 2 stroke engine

5. This is also a good time to clean the deposits from the top of the piston with a razor blade.  The cylinder head should also be cleaned with a Dremel type tool with a steel brush.  Check that the piston rings move freely in the lands.  If not, remove them and clean the lands.

6. Rotate the crankshaft so that the piston goes back down to JUST UNDER the DCP port.  You will notice that the oil will have collected the deposits which can be easily wiped up and removed with a rag.  You may want to use a vacuum cleaner to remove any loose deposits sitting on the head of the piston.

That's it.  Reassemble the engine using NEW gaskets.  Do not reuse head or cylinder gaskets.  The slightest leak in these gaskets will affect performance at the high end.  For additional information on reassembling the top end of a paramotor, go to this page on cylinder and piston notes.

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