There’s a special kind of pride that comes with keeping a classic machine alive and kicking. If you own a 1991 Polaris Trail Boss 350L 4×4, you know exactly what we’re talking about. It’s a relic from an era of simpler, tougher ATVs, but keeping one running in top shape requires a bit of know-how and a willingness to get your hands dirty.
You’ve probably found that information on these vintage quads can be scattered and hard to find. We promise this guide is different. We’re going to give you the expert, field-tested advice you need to diagnose problems, perform critical maintenance, and truly understand your machine.
In this ultimate owner’s guide, we’ll dive deep into the heart of your Trail Boss. We’ll cover its iconic features, the most common problems and their fixes, a bulletproof maintenance schedule, and some smart, sustainable practices to keep you riding for years to come.
What Makes the 1991 Polaris Trail Boss 350L 4×4 a Trail Legend?
Before we turn wrenches, let’s appreciate what you’ve got. The Trail Boss line was a game-changer for Polaris, and the 1991 350L 4×4 was a serious workhorse and trail machine for its time. It wasn’t just another quad; it was a statement.
The heart of this beast is its 346cc liquid-cooled two-stroke engine. Liquid cooling was a premium feature back then, allowing for more consistent power and engine longevity compared to its air-cooled rivals. This single-cylinder motor provided snappy, responsive power that makes it a blast to ride even today.
Another key feature is the legendary Polaris On-Demand 4×4 system. With the flip of a switch, you could engage the front wheels, giving you true four-wheel-drive traction to pull you out of mud, snow, or sketchy situations. It was simple, effective, and a major reason these ATVs became so popular.
Benefits of Owning a Vintage Trail Boss
- Simplicity: No complex computers or fuel injection systems here. The mechanical systems are straightforward, making it an excellent platform for the DIY mechanic to learn on.
- Durability: These machines were built with heavy-gauge steel frames and robust components. With proper care, they can outlast many modern ATVs.
- Parts Availability: You’d be surprised! Many aftermarket companies still produce wear-and-tear items like bearings, brake pads, and carburetor kits for this popular model.
Common Problems with the 1991 Polaris Trail Boss 350L 4×4 (And How to Fix Them)
Every classic has its quirks. This section is your go-to reference for troubleshooting. Understanding these common problems will save you a ton of time and frustration, turning a weekend-killer into a quick fix.
Two-Stroke Engine Woes: Carburetor and Fuel System
The two-stroke engine is powerful but finicky if not maintained. Most “engine” problems on this machine trace back to fuel or air delivery. The stock carburetor is a Mikuni VM34, a solid carb that’s easy to work on.
The Problem: The engine bogs down, won’t idle, or is hard to start. This is almost always a carburetor issue, often caused by ethanol fuel gumming up the tiny passages (jets) inside.
The Fix:
- Turn off the fuel petcock and carefully remove the carburetor.
- Disassemble the carb on a clean bench. Take photos as you go if you’re new to this.
- Use carburetor cleaner spray and compressed air to thoroughly clean the main jet, pilot jet, and all small orifices in the carb body. Never use a metal wire to clean jets, as it can damage them.
- Check the fuel lines for cracks and replace them if they look brittle. Also, ensure the fuel filter is clean.
The Infamous Chain Drive System
Unlike modern shaft-driven ATVs, your Trail Boss uses a concentric chain drive. This system keeps chain tension consistent as the suspension cycles, but it requires regular attention.
The Problem: The drive chain is loose and slapping, or the rear axle seems to have play in it. The eccentric bearing housing (which holds the axle and allows for chain adjustment) can seize up from rust and neglect.
The Fix:
- Loosen the two large pinch bolts on the rear bearing carrier.
- Use a large spanner wrench or a punch and hammer to gently rotate the entire housing. Rotating it will either tighten or loosen the chain.
- Aim for about 1/2 to 3/4 inch of slack in the middle of the chain.
- If the housing is seized, you’ll need to use plenty of penetrating oil (like PB Blaster), heat, and patience. This is one of the most challenging jobs on this ATV, so be prepared.
4×4 Engagement Issues
When you flip that 4×4 switch, you expect it to work. If it doesn’t, the problem is usually in one of three areas.
The Problem: The 4×4 indicator light comes on, but the front wheels aren’t pulling. Or, the light doesn’t come on at all.
The Fix:
- Check the Hub Fluid: The front hubs use a special fluid (Polaris Demand Drive Fluid is recommended) to engage the magnetic clutches. If the fluid is low or contaminated, they won’t work. Drain and refill.
- Test the Coils: Each front hub has an electromagnetic coil. You can test for resistance with a multimeter. An open circuit means the coil is bad and the hub needs to be rebuilt or replaced.
- Inspect the Wiring: Trace the wires from the hubs up to the 4×4 switch on the handlebars. Look for any breaks, corrosion, or loose connections.
Your Essential 1991 Polaris Trail Boss 350L 4×4 Care Guide
An ounce of prevention is worth a pound of cure, especially with a 30-year-old machine. Following this simple 1991 polaris trail boss 350l 4×4 care guide will prevent most of the common problems we just discussed.
Routine Fluid Checks and Changes
Fluids are the lifeblood of your ATV. Keep them clean and topped off.
- Two-Stroke Oil: If your oil injection system is still working, keep the reservoir full with a quality 2-stroke injector oil. Many owners choose to bypass the injector and run premix fuel (a 40:1 ratio is a safe starting point) for peace of mind.
- Transmission Fluid: The transmission/gearcase requires regular changes. Use a fluid specifically designed for ATV gearcases.
- Front Hub Fluid: As mentioned, check this yearly and use the correct Polaris Demand Drive Fluid.
- Brake Fluid: The single-lever brake system uses DOT 3 or DOT 4 brake fluid. Bleed the system annually to keep it free of moisture and air bubbles.
Air Filter and Drivetrain TLC
Simple mechanical checks are crucial for reliability.
Air Filter: A two-stroke engine is extremely sensitive to airflow. A dirty air filter will choke its performance and can lead to engine damage. Clean your foam air filter after every dusty ride with filter cleaner and re-oil it with proper foam filter oil.
Drivetrain: Keep your drive chains clean and lubricated with a quality chain lube. While you’re down there, use a grease gun to hit all the zerk fittings on the suspension A-arms and pivot points. This simple step from our 1991 polaris trail boss 350l 4×4 guide prevents squeaks and premature wear.
Sustainable and Eco-Friendly 1991 Polaris Trail Boss 350L 4×4 Practices
Being an off-roader means being a steward of the land. You can enjoy your classic quad while minimizing your environmental impact. Adopting a few sustainable 1991 polaris trail boss 350l 4×4 habits is easy.
The most eco-friendly thing you can do is keep your vintage machine running! By maintaining and repairing your Trail Boss, you’re preventing it from ending up in a landfill and reducing the demand for manufacturing new vehicles, which has a huge carbon footprint.
When performing maintenance, consider these eco-friendly 1991 polaris trail boss 350l 4×4 tips:
- Use Biodegradable Oils: Look for biodegradable two-stroke oils and chain lubes. These products break down naturally and are less harmful to the environment if spilled.
- Proper Fluid Disposal: Never dump used oil or coolant on the ground. Collect all waste fluids and take them to your local auto parts store or municipal recycling center for proper disposal.
- Wash Responsibly: Wash your quad on a grassy area away from drains or waterways to allow the dirt and grime to be filtered by the soil, or use a commercial car wash with a proper water reclamation system.
Frequently Asked Questions About the 1991 Polaris Trail Boss 350L 4×4
What kind of oil does a 1991 Polaris Trail Boss 350L take?
It requires several fluids. For the engine, use a high-quality 2-stroke injector oil. For the transmission, use Polaris AGL or a comparable gearcase lubricant. The front 4×4 hubs require Polaris Demand Drive Fluid for proper operation.
How do I engage the 4×4 on this model?
On the right-hand side of the handlebars, there is a red rocker switch. With the ATV in gear and moving slowly, push the switch to the “ON” position. An indicator light on the dash pod should illuminate, and the front wheels will engage automatically when the rear wheels lose traction.
What is the correct fuel-to-oil ratio if I bypass the oil injector?
A safe and common ratio for premixing fuel is 40:1. This translates to 3.2 ounces of 2-stroke oil per 1 gallon of gasoline. It’s always best to mix it in a separate, approved gas can before filling the ATV’s tank.
Are parts still available for this ATV?
Yes, surprisingly so! While you won’t find every part at the dealer, a huge aftermarket exists. Retailers like Dennis Kirk, Rocky Mountain ATV/MC, and even Amazon carry common maintenance items like brake pads, carburetor rebuild kits, bearings, cables, and sprockets.
Your 1991 Polaris Trail Boss 350L 4×4 is more than just an old ATV; it’s a piece of off-road history. It represents a time when machines were simple, rugged, and built to last. By following these tips and giving it the consistent care it deserves, you’re not just a rider—you’re a custodian of a classic.
Embrace the simplicity, learn its systems, and don’t be afraid to get your hands dirty. The reward is a reliable, capable, and incredibly fun machine that will turn heads on the trail for decades to come. Now grab your tools, show that classic some love, and we’ll see you out there!
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