Two-Stroke Basics

Quick Tip

Two-stroke symptoms such as hard starting, starting when cold but not when hot, poor idle and rough running could indicate fuel or ignition problems. But, it's a good idea to first check compression. Test the engine cold, close the decompression valve and pull the starter until the gauge stops moving. If readings are below minimum values, check the piston, rings and cylinder before making other repairs. Best to use a threaded gauge that has a check valve.

Source: Partner

Clean Exhaust Port and Muffler

Most manufacturers recommend cleaning carbon deposits from the exhaust port at intervals ranging from 75 to 300 hours. Observe cautions for positioning the piston during cleaning, and for purging carbon particles. Soak the muffler in cleaner and dry. Install a new gasket.

Illustration: Wacker

Spark Plug, Fins And Cylinder Bolts

Because the spark plug is exposed to oil in the fuel, some manufacturers recommend cleaning and gapping the plug every week or at 25-hour intervals, and replacing it at one-month or 100-hour intervals. Keep cooling fins clean to avoid overheating, and check cylinder mounting bolts every week or at 25 hours.

Illustration: Wacker

Air Cleaner

Two-stroke engines use a variety of air-cleaning systems, which should be serviced with strict regard to the manufacturer's recommendations. Typically, systems are either neglected or over-serviced; both approaches are poor economy. If foam filters are to be oiled, use oil specially formulated for the purpose. Don't attempt to clean paper elements, which can be damaged in the process.

Illustration: Wacker

Fuel, Oil, Vents and Filters

When fueling a two-stroke, exercise practical cautions: clean containers; clean fuel of correct octane; and recommended oil type mixed in the specified ratio. Over-oiling causes exhaust-port and muffler deposits, fouls the plug and increases smoke. A bad vent can cause engine shutdown. Replace in-tank and inline filters as recommended.

Illustration: Partner

Starter

To enhance reliability, some manufacturers suggest periodically cleaning the starting mechanism and replacing the starting rope.

Illustration: Wacker

Basic Engine Operation

At the heart of two-stroke engine operation is the fluctuating pressure in its crankcase as the piston moves. As the piston rises in the cylinder, thus creating a larger open volume in the crankcase, a low pressure results. When the piston descends, thus decreasing open crankcase volume, a high pressure results.

The intake of the fuel/air/oil mixture occurs during the low-pressure phase. To allow the mixture to enter the crankcase, some two-stroke engines use a reed valve or poppet valve that is pushed open by atmospheric pressure, working through the carburetor, during the low-pressure phase. But as the piston descends and pressure in the crankcase rises, the valve snaps shut. Other engines may control mixture entry with a rotary valve (driven by the crankshaft) that alternately opens and closes an intake port in sync with the pressure cycle. Most small-displacement, construction-type engines, however, control intake with a "third port" or "piston port" that is opened and closed by the piston skirt in sync with pressure variations. Illustrated here, courtesy of Tecumseh Products, is a third-port-type engine.

Figure A: As the piston rises in the cylinder to compress the fuel/air/oil mixture in the combustion chamber, the piston covers both the third port, which connects the crankcase with the carburetor, and the "transfer ports," which connect the crankcase to the combustion chamber. In addition, the top of the piston is closing the cylinder-wall exhaust ports.

Figure B: The piston reaches the top of its stroke, creating a low pressure in the crankcase. Atmospheric pressure then pushes the fuel/air/oil mixture through the carburetor and into the crankcase via the now-uncovered third port. The spark plug fires and the piston begins its power-stroke descent.

Figure C: As the piston descends, its skirt quickly closes the third port. The top of the piston begins to open the exhaust ports, allowing combustion gases to escape. The descending piston also is causing pressure to rise in the crankcase.

Figure D: When the piston reaches the bottom of its stroke, it uncovers the transfer ports. The high pressure in the crankcase forces the fuel/air/oil mixture through the transfer ports and into the combustion chamber. The turbulent, loop-shaped flow of the intake mixture helps scavenge remaining exhaust gases. (Some engines use a "deflector piston," with a specially shaped dome, to direct the flow of exhaust and intake gases.)