Troubleshooting for 2-Cycle Bike Engines
Step-by-step instructions for the DIY Mechanic!
Poor starting scenario 1:
Hold in the clutch. Let out the clutch. Keep pedaling while the engine turns over and you can hear the piston moving up and down in the cylinder. The bike does not produce any power.
- Make sure all nuts and bolts are snug per maintenance instructions.
- Check the fuel.
- Check the fuel valve. Down and in-line is on. Sideways is off. (Always turn the fuel valve off when not in use to avoid accidental spills.)
- Check the position of the choke. Down for normal running and warm starting. Up for just a second during cold starts.
- Visually inspect that fuel is in the fuel line. All that is needed is a trace amount. Pressing the 'tickle' button several times with the fuel valve open can help with this.
- Check to make sure that all of the electrical connections leading from the magneto are connected. Blue leads to blue, black leads to black as the wires enter the CDI box from the magneto. Make sure that a proper connection is being made in the spark plug boot. Inspect and/or replace the spark plug. Correct spark plug gap is 0.038".
- Follow the white wire as it leads from the magneto up to the kill switch. On certain models, the kill switch may be at fault grounding out against the handlebar. Disconnect the kill switch at the handlebar and attempt to restart the bike.
Poor starting scenario 2:
Hold in the clutch. Let out the clutch. Keep pedaling and the bike drags to a silent halt. The engine does not turn over.
- Clutch free play. A small amount of free play is needed in the clutch lever. Too much and the bike won't start. Too little and the bike won't start. Small adjustments can be made in the barrel end adjuster at the clutch lever, while major adjustments are done at the clutch arm cable stop at the engine.
- Check that the engine chain is not bound around the output sprocket inside of the engine. An improperly adjusted chain tensioner can cause the drive chain to bind within the engine.
- Remove the right side clutch plate and inspect the condition of the 2 gears, the small main pinion gear and the larger clutch gear, for wear.
- Remove the 4 screws on top of the cylinder head. Carefully remove the cylinder head and head gasket. With one hand securely hold the cylinder from moving. With the other hand, gently push down on the top of the piston. Inspect the cylinder wall for scarring or abrasion.
Weak spark or no spark:
To check if you are receiving power to the spark plug, follow these simple steps.
- Remove the spark plug from the cylinder head.
- Re-attach the spark plug to the spark plug cap.
- Lay the spark plug on the cylinder head so that the metal of the plug is touching any metal portion of the cylinder head. DO NOT HOLD THE SPARK PLUG OR BOOT BY HAND.
- With the clutch out, push the bike forward or turn the rear wheel. A bright spark should be visible.
If no spark is visible:
- Double check the connection of the wires, particularly the blue to blue and black to black.
- Check that no particles are lodged between the side and center electrodes of the spark plug.
- Try replacing the spark plug or cleaning it by running sand paper through the side and center electrodes and resetting the gap to 0.038".
- Re-test for spark with the kill switch pressed in. Try feathering the kill switch. Disconnect the white wire completely allowing the wire to hang free, then re-test the engine.
- Magneto Testing - Take a voltmeter or multimeter and adjust it to the Ohms setting at 20k. Ohms measures the resistance across a circuit. Check voltmeter across the following wires:
- Blue wire to the White wire of the magneto. The resistance should be 0.25 to 0.40. New magnetos read 0.31.
- White wire to Black wire should read 0.0.
- Blue wire to Black wire should read close to 0.31.
If your reading is far off from this, then the magneto is to blame for bad spark.
- CDI Testing - Run the voltmeter at the same 20k setting. Check voltmeter across the following wires:
- From the inside of the spark plug boot to the Black wire of the CDI. Your reading should measure 2.0 - 2.7. A new CDI will read 2.3.
- Spark Plug Boot to the Blue wire there should be no change, or 1.0.
- Blue wire to Black wire there should be no change, or 1.0.
If the resistance is different than specified, this simple test will tell you where your problem lies.
Consistently running at full RPM is about the worst thing that can be done an engine long term. Always running at full throttle does not leave enough time for the oil mixed in the gas to circulate around the engine and lubricate the cylinder wall and lower engine bearings.
If you are going to run at high RPMs, do a plug check. Pull the spark plug once in a while and see what color it is. If your plug is tan, white, dry and clean, then you need to either slow down, not ride for so long, and/or increase the oil/gas ratio. If your plug is black and moist, then you're doing well. Even better is if a little black drop occasionally falls from the exhaust tip.
If your spark plug is black, dry, and sooty, then it may be time to clean the combustion chamber. "Decarbonisation" was a common maintenance procedure in the days when British motorcycles roamed the streets and leather clad rockers owned the roads. You need to keep a clean environment in the combustion chamber. With high mile heavy use (5,000 miles+), we do see a build up of carbon beneath the cylinder head and on the combustion chamber. If the carbon buildup is thick, this barnacle-like substance can cause a piston seizure if any small piece breaks off and lodges between the piston and cylinder wall.
To "Decarbonise" an engine, remove the 4 nuts on top of the cylinder head, remove the cylinder head and metal head gasket. Bring the piston to the top of its stroke. Using sand paper or a soft wire wheel, remove the carbon from the top of the piston crown and the bottom of the cylinder head. Be careful not to score any of the inner surfaces.
FOREIGN PARTICLES ARE THE ENEMY! Clean your air filter often.