2 Cylinder Onan Engine Troubleshooting (3 Expert Fixes)
WARNING: Working on small engines like Onan’s two-cylinder models involves inherent risks. Improper handling of fuel, electricity, and mechanical components can lead to serious injury or even death. Always disconnect the spark plugs before working on the engine and ensure the fuel lines are secured. If you’re not comfortable with engine repair, seek a qualified mechanic.
2 Cylinder Onan Engine Troubleshooting: 3 Expert Fixes
As someone who’s spent countless hours tinkering with, rebuilding, and relying on small engines, especially those robust Onan twins, I can tell you they’re generally reliable workhorses. But like any engine, they can develop quirks. Over the years I’ve faced my share of head-scratching problems with these engines, often in the most inconvenient situations – like mid-winter, needing to get the snowblower running to clear a path to the woodpile. So, I’m going to share three common issues and my go-to fixes. These fixes are based on my experiences and observations of Onan engines.
Understanding the Onan Two-Cylinder Engine
The Onan two-cylinder engine, known for its durability and use in generators, welders, and other heavy-duty equipment, is a staple for many. These air-cooled engines operate on a four-stroke cycle, delivering consistent power. Understanding their basic operation helps in troubleshooting.
- Four-Stroke Cycle: Intake, Compression, Combustion, Exhaust.
- Air-Cooled System: Relies on airflow over the cylinder fins to dissipate heat.
- Ignition System: Usually a points-based or electronic ignition, providing the spark for combustion.
- Carburetion: Mixes air and fuel for combustion.
- Lubrication: Oil is critical for cooling and lubrication of internal components.
Fix #1: The Stubborn Starting Problem
The Scenario: You turn the key, the engine cranks, but it just won’t fire up. This is a common frustration, and often stems from issues with the fuel or ignition systems. I remember one time, a client called me out to his remote cabin because his generator wouldn’t start. He’d been relying on it for heat, and it was a chilly situation. After a few checks, the problem was simpler than he’d feared.
The Culprits:
- Fuel Delivery:
- Clogged Fuel Filter: A dirty fuel filter restricts fuel flow. I’ve seen filters so clogged with debris that the engine was completely starved.
- Stale Fuel: Gasoline degrades over time, especially when mixed with ethanol. After about 30 days, fuel can start to go bad, leading to poor combustion. Ethanol fuels can absorb moisture from the atmosphere. This moisture can cause the fuel to phase separate, where the ethanol and water separate from the gasoline.
- Carburetor Issues: Blocked jets or a faulty float valve in the carburetor can disrupt the air-fuel mixture.
- Ignition System:
- Faulty Spark Plugs: Spark plugs can become fouled with carbon or oil, preventing them from sparking.
- Weak Ignition Coil: A weak coil may not produce a strong enough spark to ignite the fuel-air mixture.
- Faulty Points/Condenser (If Equipped): Worn or misadjusted points can disrupt the ignition timing.
The Fix:
- Fuel System Check:
- Inspect the Fuel Filter: Remove the filter and check for blockage. Replace if necessary. I recommend using a clear inline fuel filter, allowing you to visually inspect the fuel condition.
- Drain the Fuel Tank: Remove old fuel and replace it with fresh gasoline. Add a fuel stabilizer to prevent future degradation. For long-term storage, drain the fuel completely.
- Clean the Carburetor: This is often the trickiest part.
- Disassembly: Carefully disassemble the carburetor, noting the position of each component.
- Cleaning: Use carburetor cleaner and compressed air to clear any blockages in the jets and passages. I use a set of carburetor cleaning wires to ensure all passages are clear.
- Reassembly: Reassemble the carburetor, replacing any worn gaskets or seals.
- Ignition System Check:
- Inspect Spark Plugs: Remove the spark plugs and check for fouling or damage. Clean or replace as needed. The spark plug gap should be within the manufacturer’s specification, typically 0.030 inches.
- Test the Ignition Coil: Use a multimeter to test the coil’s primary and secondary resistance. Compare the readings to the manufacturer’s specifications.
- Check Points/Condenser (If Equipped): Inspect the points for wear or pitting. Clean or replace as needed. Adjust the point gap to the manufacturer’s specification (typically 0.020 inches). A faulty condenser can also cause weak spark.
Data Points and Statistics:
- Fuel Degradation: Gasoline can lose up to 5% of its octane rating after just one month of storage.
- Spark Plug Fouling: Carbon fouling can reduce spark plug efficiency by up to 70%.
- Carburetor Jet Size: Main jet sizes typically range from 0.020 to 0.040 inches, depending on the engine model.
Case Study:
I once worked on an Onan engine that refused to start after sitting idle for six months. The fuel was stale, and the carburetor jets were completely blocked with varnish. After thoroughly cleaning the carburetor and replacing the fuel, the engine started on the first pull.
Fix #2: The Overheating Nightmare
The Scenario: Your Onan engine runs for a while, then starts to overheat, losing power and potentially causing severe damage. Overheating is a serious issue that needs to be addressed promptly. I had this issue with a generator powering a remote logging operation. The engine was overheating, causing the generator to shut down. After some troubleshooting, I discovered the problem was a combination of a dirty air filter and a clogged cooling fan.
The Culprits:
- Restricted Airflow:
- Dirty Air Filter: A clogged air filter restricts airflow, causing the engine to run lean and overheat.
- Obstructed Cooling Fins: Debris buildup on the cylinder fins prevents proper heat dissipation.
- Faulty Cooling Fan: A damaged or malfunctioning cooling fan reduces airflow over the engine.
- Internal Engine Issues:
- Low Oil Level: Insufficient oil reduces lubrication and cooling, leading to overheating.
- Improper Timing: Incorrect ignition timing can cause the engine to run hot.
- Lean Fuel Mixture: A lean fuel mixture burns hotter, increasing the risk of overheating.
The Fix:
- Airflow Check:
- Inspect the Air Filter: Remove the air filter and check for dirt or debris. Clean or replace as needed. Foam air filters can be washed with soap and water, then oiled lightly.
- Clean Cooling Fins: Use compressed air to blow out any debris from the cylinder fins. I use a stiff brush to dislodge stubborn dirt.
- Inspect Cooling Fan: Check the cooling fan for damage or obstructions. Ensure it is securely mounted and rotating freely.
- Internal Engine Check:
- Check Oil Level: Ensure the oil level is within the recommended range. Use the correct type and viscosity of oil, as specified by the manufacturer.
- Verify Ignition Timing: Use a timing light to check the ignition timing. Adjust as needed, following the manufacturer’s specifications.
- Adjust Fuel Mixture: If the engine is running lean, adjust the carburetor to richen the fuel mixture. Refer to the carburetor’s adjustment procedure.
Data Points and Statistics:
- Air Filter Restriction: A clogged air filter can reduce airflow by up to 50%.
- Oil Viscosity: Using the wrong oil viscosity can increase engine temperature by 10-15 degrees Celsius.
- Cooling Fin Efficiency: Clean cooling fins can improve heat dissipation by up to 30%.
- Engine Temperature: Operating temperature for Onan engines typically ranges from 250-350°F.
Case Study:
I worked on an Onan engine powering a water pump that was constantly overheating. The problem was a combination of a dirty air filter and a clogged cooling fan. After cleaning the air filter and cooling fan, the engine ran at a normal temperature. I also advised the owner to relocate the pump to a more shaded area to reduce the ambient temperature.
Fix #3: The Mystery Misfire
The Scenario: Your Onan engine starts and runs, but it misfires, causing it to run roughly and lose power. Misfires can be caused by a variety of issues, making them challenging to diagnose. I once had a misfire issue with an Onan engine in a wood splitter. The engine would start, but it ran very rough and lacked power. After checking the usual suspects, I discovered the problem was a cracked intake manifold.
The Culprits:
- Ignition System:
- Faulty Spark Plug Wires: Damaged or corroded spark plug wires can cause a weak spark.
- Cracked Distributor Cap (If Equipped): Cracks in the distributor cap can cause arcing and misfires.
- Weak Ignition Coil: A weak coil may not provide a strong enough spark to ignite the fuel-air mixture consistently.
- Fuel System:
- Clogged Fuel Injector (If Equipped): A clogged fuel injector can disrupt the fuel spray pattern, causing misfires.
- Vacuum Leaks: Vacuum leaks can disrupt the air-fuel mixture, leading to misfires.
- Engine Mechanical Issues:
- Low Compression: Low compression in one or more cylinders can cause misfires.
- Valve Problems: Bent or burned valves can cause misfires.
The Fix:
- Ignition System Check:
- Inspect Spark Plug Wires: Check the spark plug wires for damage or corrosion. Use an ohmmeter to test the resistance of each wire. Replace if necessary.
- Inspect Distributor Cap (If Equipped): Check the distributor cap for cracks or damage. Replace if necessary.
- Test Ignition Coil: Use a multimeter to test the coil’s primary and secondary resistance. Compare the readings to the manufacturer’s specifications.
- Fuel System Check:
- Inspect Fuel Injectors (If Equipped): Check the fuel injectors for clogs or damage. Use a stethoscope to listen for the injectors clicking.
- Check for Vacuum Leaks: Use a spray bottle with soapy water to check for vacuum leaks around the intake manifold and vacuum lines. Look for bubbles forming when the engine is running.
- Engine Mechanical Check:
- Perform Compression Test: Use a compression tester to check the compression in each cylinder. Compare the readings to the manufacturer’s specifications. Low compression indicates a potential issue with the piston rings or valves.
- Inspect Valves: If the compression test indicates a valve problem, remove the cylinder head and inspect the valves for damage.
Data Points and Statistics:
- Spark Plug Wire Resistance: Spark plug wire resistance should typically be less than 10,000 ohms per foot.
- Compression Ratio: Compression ratios for Onan engines typically range from 7:1 to 9:1.
- Vacuum Leak Size: Even a small vacuum leak (0.020 inches) can cause a noticeable misfire.
Case Study:
I worked on an Onan engine powering a generator that was misfiring badly. After checking the ignition and fuel systems, I performed a compression test. One cylinder had significantly lower compression than the other. After removing the cylinder head, I discovered a burned exhaust valve. Replacing the valve and reassembling the engine resolved the misfire.
Preventing Future Problems: My Proactive Maintenance Tips
Preventive maintenance is key to keeping your Onan engine running smoothly. Here are some of my top tips:
- Regular Oil Changes: Change the oil and filter at the recommended intervals. I use a high-quality synthetic oil for better protection.
- Air Filter Maintenance: Clean or replace the air filter regularly. A clean air filter improves engine performance and fuel economy.
- Fuel Stabilizer: Add fuel stabilizer to the fuel tank, especially when storing the engine for extended periods.
- Spark Plug Replacement: Replace the spark plugs at the recommended intervals.
- Inspect Fuel Lines: Check the fuel lines for cracks or leaks. Replace as needed.
- Clean Cooling Fins: Keep the cylinder fins clean to ensure proper cooling.
- Check for Vacuum Leaks: Regularly check for vacuum leaks around the intake manifold and vacuum lines.
- Store Properly: Store the engine in a dry, protected area. Cover the engine to prevent dust and moisture from entering.
Tool Calibration Standards:
- Torque Wrench: Calibrate your torque wrench annually to ensure accurate tightening of bolts.
- Timing Light: Verify the accuracy of your timing light before using it to check ignition timing.
- Compression Tester: Check the accuracy of your compression tester by comparing it to a known pressure source.
Safety Equipment Requirements:
- Safety Glasses: Always wear safety glasses when working on engines to protect your eyes from debris.
- Gloves: Wear gloves to protect your hands from oil, grease, and sharp edges.
- Hearing Protection: Wear hearing protection when running engines to prevent hearing damage.
- Ventilation: Work in a well-ventilated area to avoid inhaling harmful fumes.
- Fire Extinguisher: Keep a fire extinguisher nearby in case of fuel spills or fires.
By following these troubleshooting steps and maintenance tips, you can keep your Onan two-cylinder engine running reliably for years to come. Remember, safety first, and when in doubt, consult a qualified mechanic. Good luck, and happy tinkering!
