The 2-Stroke Internal Combustion Engine Troubleshooting
A two-stroke cycle or “two-cycle” engine produces one power stroke
for every revolution of its crankshaft. Intake and compression occur during the
piston’s “up” stroke; power and exhaust occur during the “down” stroke. Six distinct processes occur during every
revolution of the crankshaft, and a loss or weakness of even one of these processes
will affect the other five.
Piston Ported Induction
1. Intake
As the piston moves up toward the cylinder head, a low pressure (vacuum) is created in the crankcase. As the moving piston uncovers the intake port, atmospheric pressure pushes a fresh charge of fuel-air mixture into the crankcase. The mixture accomplishes two functions before it is moved to the combustion chamber:
■ Oil suspended in the mixture coats all internal engine parts including cylinder walls, crankshaft, and bearings.
■ The atomized fuel mixture absorbs heat as it enters the crankcase, lowering the engine’s operating temperature.
2. Compression
As the piston continues to move upward and cover the cylinder ports, the combustion chamber is sealed and compression begins. The air-fuel mixture introduced during the previous revolution’s transfer cycle is now trapped and becomes compressed between the piston and cylinder head.
1. Intake
As the piston moves up toward the cylinder head, a low pressure (vacuum) is created in the crankcase. As the moving piston uncovers the intake port, atmospheric pressure pushes a fresh charge of fuel-air mixture into the crankcase. The mixture accomplishes two functions before it is moved to the combustion chamber:
■ Oil suspended in the mixture coats all internal engine parts including cylinder walls, crankshaft, and bearings.
■ The atomized fuel mixture absorbs heat as it enters the crankcase, lowering the engine’s operating temperature.
2. Compression
As the piston continues to move upward and cover the cylinder ports, the combustion chamber is sealed and compression begins. The air-fuel mixture introduced during the previous revolution’s transfer cycle is now trapped and becomes compressed between the piston and cylinder head.
3. Ignition
As the piston approaches the top of the cylinder, the spark plug fires and ignites the fuel-air mixture. To compensate for increases in engine rpm, spark timing is advanced electronically.
4. Power (combustion)
As the piston approaches the top of the cylinder, the spark plug fires and ignites the fuel-air mixture. To compensate for increases in engine rpm, spark timing is advanced electronically.
The fuel-air mixture expands rapidly as it burns, forcing the piston down the
cylinder. Piston movement is transferred
to the crankshaft through the connecting rod, rotating the crankshaft.
5. Exhaust
Expanding gasses continue to force the piston downward, exposing the exhaust port. Most of the spent combustion gasses are expelled through the exhaust port.
6. Transfer
The piston’s downward movement covers the intake port and uncovers the transfer port, while simultaneously compressing the fresh fuel-air mixture drawn into the crankcase during Step 1. As the transfer port is uncovered, the fresh mixture swirls rapidly into the cylinder.
5. Exhaust
Expanding gasses continue to force the piston downward, exposing the exhaust port. Most of the spent combustion gasses are expelled through the exhaust port.
6. Transfer
The piston’s downward movement covers the intake port and uncovers the transfer port, while simultaneously compressing the fresh fuel-air mixture drawn into the crankcase during Step 1. As the transfer port is uncovered, the fresh mixture swirls rapidly into the cylinder.
As the fresh fuel-air mixture enters the cylinder, it also helps
to push or scavenge any remaining exhaust gasses out through the exhaust port.
The burning mixture expands, forcing the piston down and
compressing the fresh mixture in the crankcase. As the piston exposes the
transfer port, crankcase pressure forces fresh mixture into the combustion
chamber and helps push the remaining exhaust gasses through the exhaust port.
1.Intake
As the piston moves up toward the cylinder head, low pressure (vacuum) created
in the crankcase allows atmospheric pressure to open the reed valve and push a
fresh charge of fuel-air mixture into the crankcase. The fuel-air mixture accomplishes two functions
before it is moved to the combustion chamber:
■ Oil suspended in the mixture coats all internal engine parts including cylinder walls, crankshaft, and bearings.
■ The atomized fuel mixture absorbs heat from the crankcase, lowering the engine’s operating temperature.
■ Oil suspended in the mixture coats all internal engine parts including cylinder walls, crankshaft, and bearings.
■ The atomized fuel mixture absorbs heat from the crankcase, lowering the engine’s operating temperature.
2.Compression
As the piston continues to move upward and cover the cylinder ports, the combustion chamber is sealed and compression begins. The air-fuel mixture introduced during the previous revolution’s transfer cycle is now trapped and becomes compressed between the piston and cylinder head.
3. Ignition
As the piston approaches the cylinder head, the spark plug fires and ignites the fuel-air mixture. To compensate for increases in engine rpm, spark timing is advanced electronically.
As the piston continues to move upward and cover the cylinder ports, the combustion chamber is sealed and compression begins. The air-fuel mixture introduced during the previous revolution’s transfer cycle is now trapped and becomes compressed between the piston and cylinder head.
3. Ignition
As the piston approaches the cylinder head, the spark plug fires and ignites the fuel-air mixture. To compensate for increases in engine rpm, spark timing is advanced electronically.
Reed Valve Induction System (reed valve open)
4. Power (combustion)
The fuel-air mixture expands rapidly as it burns, forcing the piston down the cylinder. Piston movement is transferred to the crankshaft through the connecting rod, rotating the crankshaft.
5. Exhaust
Expanding gasses continue to force the piston downward, exposing the exhaust port. Most of the spent combustion gasses are expelled through the exhaust port.
6. Transfer
The piston’s downward movement compresses the fuel-air mixture drawn into the crankcase during Step 1, causing the reed valve to close. As the moving piston uncovers the transfer port, the fresh mixture swirls rapidly into the cylinder. As the fresh fuel-air mixture enters the cylinder, it also helps to push or scavenge any remaining exhaust gasses out through the exhaust port.
The fuel-air mixture expands rapidly as it burns, forcing the piston down the cylinder. Piston movement is transferred to the crankshaft through the connecting rod, rotating the crankshaft.
5. Exhaust
Expanding gasses continue to force the piston downward, exposing the exhaust port. Most of the spent combustion gasses are expelled through the exhaust port.
6. Transfer
The piston’s downward movement compresses the fuel-air mixture drawn into the crankcase during Step 1, causing the reed valve to close. As the moving piston uncovers the transfer port, the fresh mixture swirls rapidly into the cylinder. As the fresh fuel-air mixture enters the cylinder, it also helps to push or scavenge any remaining exhaust gasses out through the exhaust port.
Troubleshooting
Mechanical failure of any carburetor is far less common than
problems arising from contaminated fuel, improper adjustment, or operator abuse. Carburetor operation is directly affected by the
quality of air and fuel entering the carburetor! Before troubleshooting or adjusting
any carburetor, inspect fuel and air filters for cleanliness, operation, and proper
installation.
1.Is the fuel tank filled with clean, fresh fuel of the proper
grade and mixture?
2. Check for spark.
3. Compression at least 100 psi or above?
4. Is the air filter clean and properly installed?
2. Check for spark.
3. Compression at least 100 psi or above?
4. Is the air filter clean and properly installed?
5. Are the spark arrestor and muffler clean and properly
installed?
6. Inspect fuel filter, and check fuel lines for leaks/deterioration. Pressure-test fuel system for leaks.
7. Inspect the impulse tube or passage for condition and operation. Pressure-test crankcase as required.
6. Inspect fuel filter, and check fuel lines for leaks/deterioration. Pressure-test fuel system for leaks.
7. Inspect the impulse tube or passage for condition and operation. Pressure-test crankcase as required.
8. Inspect the tank vent. Clean or replace as required
9. Reset carburetor mixture adjustments.
10. Pressure test the carburetor for pop-off and reseat values.
9. Reset carburetor mixture adjustments.
10. Pressure test the carburetor for pop-off and reseat values.
