Cleaning the decompression port on 2 stroke engines
by Had Robinson
updated December 19, 2020
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 100cc or smaller generally do not have a DCP.
After 25-50 hours the DCP can become clogged with combustion deposits. If AVGAS is used, the DCP will become clogged much sooner, the only significant downside of using 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 50-100 hours of runtime to be sure the DCP is clear. That is, routine maintenance for the upper connecting rod bearing must also be done at about the same time. All engines will experience DCP clogging but not at the same time.
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 iconnects with a second 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. 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:
- drill bit of the correct size for your engine's DCP (These holes are different on each engine. Unfortunately most manuals do not specify the needed size.
- pin vise (or drill chuck)
- replacement cylinder head gasket (recommended)
1. Remove the exhaust system and THEN remove the cylinder head. This may also require removing cooling shrouds and other peripheral equipment. The exhaust system must be removed in order to remove the DCP deposits that will accumulate in the port. All 2 stroke engines recycle the fuel rich exhaust stream that leaves the cylinder and any deposits in the tuned pipe of the exhaust will be sucked back into the engine and, possibly, cause damage. Removing the cylinder head first will likely allow the cylinder to be disturbed/moved when the exhaust system is removed. This may cause the cylinder base gasket to leak. It is not necessary to remove the cylinder in order to clean the DCP.
2. If the cylinder is still on the engine, 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. Some DCP's are a single hole that goes from the cylinder wall to the exhaust port. Others (e.g. the Polini) have (2) holes. One goes from the top of the cylinder straight down to the exhaust port and then there is an intersecting hole from the cylinder wall to the first hole.
4. With the correct sized bit in the pin vise, remove the deposits from the DCP holes.
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.
A pin vise is used to first clean the hole to the exhaust port (if there is one) then use the same setup to clean the short hole from the cylinder wall to the first hole.
The pin vise. They can be ordered for a few $$$'s from 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.
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. If the cylinder head is removed, check that the piston ring(s) moves freely in the lands. If not, remove them and clean the lands. If the piston ring(s) is badly stuck, the damage to the cylinder wall will be obvious and the top end of the engine should be replaced.
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. Use a vacuum cleaner to remove any loose deposits sitting on the head of the piston.
7. Lower the piston just below the exhaust port and use air to blow the loose deposits out of the exhaust port. Be careful not to blow the deposits back into the cylinder.
That's it. Reassemble the engine using NEW gaskets. It is not a good idea to reuse the cylinder head gasket because the slightest leak will affect engine performance. If the cylinder was not removed or disturbed, the cylinder base gasket will probably be OK. For additional information on reassembling the top end of a paramotor, go to this page on cylinder and piston notes.