The Yamaha Big Bear 350. A workhorse. A legend. But even the most reliable ATVs can sometimes leave you stranded with a stubborn engine. Starting problems are a common frustration for Big Bear 350 owners, ranging from a simple fix to a more complex diagnosis. Understanding the potential culprits and how to troubleshoot them can save you time, money, and a lot of headaches out on the trail. Let's dive in and get your Big Bear roaring again!

Is it Just a Grumpy Battery? The First Place to Look

Let's be honest, the battery is the most frequent offender when a Big Bear 350 refuses to start. A weak or dead battery simply doesn't have the juice to crank the engine over sufficiently.

  • The Obvious Checks: First, ensure the battery terminals are clean and securely connected. Corrosion can act as an insulator, preventing proper current flow. Use a wire brush to clean the terminals and apply a battery terminal protector to prevent future buildup.

  • Voltage Test: A healthy battery should read around 12.6 volts. Use a multimeter to check the voltage with the ATV turned off. If it's significantly lower (say, below 12 volts), it's likely the battery is the problem.

  • Load Test: A voltage test only tells part of the story. A load test checks the battery's ability to maintain voltage under load (when you're trying to start the engine). Many auto parts stores offer free battery load testing. This is crucial because a battery might show decent voltage, but collapse when you try to crank the engine.

  • Charging Time: If the battery is low, attempt to charge it using a battery charger designed for ATV batteries. Follow the charger's instructions carefully. If the battery refuses to hold a charge, it's time for a replacement. Don't skimp on a good quality battery; it's an investment in reliable starting.

Fuel, Glorious Fuel: Is Your Big Bear Getting Enough?

If the battery is healthy, the next area to investigate is the fuel system. A lack of fuel is another very common reason for starting problems.

  • Fuel Tank Level: Sounds obvious, but make sure you have enough fuel in the tank! Low fuel levels can sometimes prevent the fuel pump (if equipped, some older models rely on gravity) from drawing fuel properly, especially on inclines.

  • Fuel Valve (Petcock): Ensure the fuel valve (petcock) is in the "ON" or "RES" (reserve) position. Sometimes they accidentally get switched off. Also, check the fuel line for any kinks or blockages.

  • Fuel Filter: A clogged fuel filter restricts fuel flow. Locate the fuel filter (usually in the fuel line between the tank and the carburetor) and inspect it. If it's visibly dirty, replace it. These are relatively inexpensive and should be changed periodically as part of regular maintenance.

  • Carburetor Issues (The Most Likely Suspect): The carburetor is responsible for mixing air and fuel in the correct proportions. A dirty or clogged carburetor is a very common cause of starting problems on the Big Bear 350.

    • Pilot Jet Clogging: The pilot jet is a small jet within the carburetor that supplies fuel at idle and low speeds. It's particularly prone to clogging with varnish and debris, especially if the ATV has been sitting for an extended period.
    • Cleaning the Carburetor: The best solution is to remove and thoroughly clean the carburetor. This involves disassembling it, cleaning all the jets and passages with carburetor cleaner and compressed air, and reassembling it. There are many excellent online tutorials and videos that can guide you through this process. If you're not comfortable doing this yourself, take it to a qualified mechanic.
    • Fuel Pump (If Equipped): Some Big Bear 350 models have a fuel pump. If you suspect the fuel pump is the issue, you can test it by disconnecting the fuel line from the carburetor and cranking the engine. Fuel should be pulsing out of the fuel line. If not, the fuel pump may be faulty.

Spark, Spark, Where Art Thou? Ignition System Checks

If you've ruled out battery and fuel issues, it's time to investigate the ignition system. Without a spark, the engine won't fire, no matter how much fuel it's getting.

  • Spark Plug Inspection: Remove the spark plug and inspect it. Look for signs of fouling (black, oily deposits), damage, or excessive wear. A fouled spark plug won't produce a strong spark. Clean the spark plug with a wire brush or replace it with a new one. Always use the correct spark plug specified for your Big Bear 350 model.

  • Spark Test: With the spark plug removed, reattach it to the spark plug wire. Ground the spark plug against the engine block (hold it with insulated pliers to avoid getting shocked!). Crank the engine and observe the spark plug gap. You should see a strong, consistent blue spark. A weak or non-existent spark indicates a problem with the ignition system.

  • Spark Plug Wire and Cap: Check the spark plug wire and cap for damage or corrosion. Ensure the cap is securely attached to the spark plug. A cracked or damaged wire can leak voltage, resulting in a weak spark.

  • Ignition Coil: The ignition coil amplifies the voltage from the battery to create the spark. A faulty ignition coil can cause weak or no spark. Testing the ignition coil requires a multimeter and some technical knowledge. Consult your service manual for instructions.

  • Stator and Pickup Coil: These components generate the electrical signal that triggers the ignition coil. Problems with the stator or pickup coil can also cause starting problems. Testing these components requires specialized tools and knowledge. This is often best left to a qualified mechanic.

Compression: The Silent Killer

While less common than battery, fuel, or spark issues, low compression can also prevent a Big Bear 350 from starting. Compression is the measure of how tightly the air/fuel mixture is squeezed in the cylinder before ignition. Low compression means the engine can't build enough pressure to ignite the mixture.

  • Compression Test: The only way to accurately check compression is with a compression tester. Remove the spark plug and screw the compression tester into the spark plug hole. Crank the engine and observe the reading on the gauge. Consult your service manual for the correct compression specification for your Big Bear 350.

  • Possible Causes of Low Compression:

    • Worn Piston Rings: Piston rings seal the gap between the piston and the cylinder wall. Worn rings allow compression to leak past the piston.
    • Worn Cylinder: A worn cylinder bore can also cause compression loss.
    • Leaking Valves: Valves that don't seal properly will allow compression to leak out.
    • Blown Head Gasket: A blown head gasket can also cause compression loss.

Low compression usually requires a more extensive engine repair.

Other Gremlins to Consider

  • Kill Switch: Ensure the kill switch is in the "RUN" position. It's easy to accidentally bump it.

  • Neutral Switch: The Big Bear 350 may not start if it's not in neutral. Make sure the neutral light is illuminated. A faulty neutral switch can prevent the engine from starting even if it is in neutral.

  • Wiring Harness Issues: Check the wiring harness for any damaged or corroded wires. Damaged wiring can disrupt the electrical signals needed to start the engine. Pay particular attention to ground connections, as poor grounding can cause all sorts of electrical problems.

  • Security System (If Equipped): If your Big Bear 350 has a security system, make sure it's not interfering with the starting process.

Frequently Asked Questions (FAQ)

  • Why won't my Big Bear 350 start after sitting for a long time? Fuel can evaporate and leave behind varnish that clogs the carburetor. Clean the carburetor thoroughly and replace the fuel filter.

  • My Big Bear 350 cranks but won't start. What's the most likely cause? This often points to a fuel or ignition problem. Check the spark plug for spark and the carburetor for fuel flow.

  • How often should I change the spark plug on my Big Bear 350? Consult your owner's manual, but generally, replacing the spark plug every year or every 100 hours of use is a good practice.

  • What kind of fuel should I use in my Big Bear 350? Use fresh, unleaded gasoline with an octane rating recommended in your owner's manual. Avoid ethanol-blended fuels if possible, as they can damage the fuel system over time.

  • Where can I find a service manual for my Big Bear 350? You can purchase a service manual online or from a Yamaha dealer. These manuals provide detailed instructions for troubleshooting and repairing your ATV.

Conclusion

Troubleshooting starting problems on a Yamaha Big Bear 350 can seem daunting, but by systematically checking the battery, fuel system, and ignition system, you can often pinpoint the issue. Remember to start with the simplest checks first and work your way up to more complex diagnostics. Regular maintenance and preventative care will go a long way in keeping your Big Bear 350 running smoothly for years to come.