So, you’ve got a 1991 Polaris Trail Boss 350L. Whether it’s a recent barn find or a trusty steed that’s been in the family for decades, you know this machine is a classic piece of off-road history. It represents a simpler time in ATV design, with a gutsy two-stroke engine and a rugged, no-frills attitude.
But keeping a vintage quad running strong comes with its own unique set of challenges. You’ve probably wondered how to tackle that finicky carburetor, what that weird noise is, or just how to give it the care it deserves.
You’re in the right place. At FatBoysOffroad, we live for this stuff. This comprehensive guide promises to be your go-to resource, packed with expert advice from years in the garage and on the trail. We’ll preview everything from essential maintenance and troubleshooting common problems to simple performance mods that make a real difference. Let’s get your hands dirty and keep that Trail Boss ripping.
Understanding Your Classic: What Makes the 1991 Polaris Trail Boss 350L Tick?
Before you grab a wrench, it helps to know what you’re working with. The 1991 Polaris Trail Boss 350L isn’t like the complex, computer-controlled machines of today. It’s a testament to straightforward, effective engineering.
This ATV is built around a liquid-cooled (that’s the “L” in 350L) 346cc two-stroke engine. Unlike a four-stroke, a two-stroke completes its power cycle in just two piston strokes, giving it that snappy, responsive feel and signature “braap” sound. This also means it requires oil to be mixed with the gasoline for lubrication, which this model does automatically via an oil injection system.
Key Features and Quirks
Understanding the core components is the first step in any good 1991 polaris trail boss 350l guide. Knowing these will help you diagnose issues faster.
- Two-Stroke Powerplant: This engine is the heart of the machine. It’s powerful for its size but requires specific care, particularly regarding fuel/oil mixture and spark plug health.
- Polaris Variable Transmission (PVT): This is an early version of the automatic CVT (Continuously Variable Transmission) that Polaris is famous for. It uses a belt and clutches to deliver smooth power without shifting gears.
- Chain Drive: Power gets to the solid rear axle via a chain. This is a simple, effective system, but it requires regular cleaning, lubrication, and tension adjustments.
- MacPherson Strut Front Suspension: This design provides a decent amount of travel and is relatively simple to maintain, but it’s a common wear point on older machines.
- Single-Lever Hydraulic Brakes: One lever on the left handlebar and a foot pedal control all the brakes. This system is effective but needs to be bled properly to work well.
The Essential 1991 Polaris Trail Boss 350L Care Guide
Consistent maintenance is the secret to reliability. A little time in the garage prevents a lot of frustration on the trail. This 1991 polaris trail boss 350l care guide covers the non-negotiables for keeping your quad in top shape.
Fluid and Filter Checks
Fluids are the lifeblood of your machine. Check them before every ride.
- Two-Stroke Oil: The oil injection tank is usually under the seat or front fender. NEVER let it run dry. If it does, the engine will seize. Use a quality two-stroke injector oil specifically designed for air or liquid-cooled engines.
- Transmission Oil: The transmission/gearcase has its own oil. Check the level via the dipstick or fill plug. We recommend changing it once a season with Polaris AGL or a compatible gearcase lubricant.
- Coolant: Since you have the 350L model, check the radiator and overflow bottle levels. Use a 50/50 mix of ethylene glycol-based antifreeze and distilled water.
- Air Filter: A clean air filter is crucial for engine health. Pull the foam filter out, wash it in warm, soapy water, let it dry completely, and then re-oil it with proper foam filter oil. Do this after every few rides, especially in dusty conditions.
Chain and Drivetrain Maintenance
A neglected chain will rob you of power and can be a serious safety hazard. Follow these 1991 polaris trail boss 350l best practices for chain care.
First, check the tension. With the ATV in neutral on level ground, find the midpoint of the chain between the sprockets. It should have about 1/2 to 3/4 inch of up-and-down play. To adjust, loosen the rear axle carrier bolts and use the adjuster bolts to move the axle forward or back. Make sure to adjust both sides evenly!
After setting the tension, clean the chain with a stiff brush and chain cleaner. Wipe it dry and apply a quality chain lube made for O-ring or X-ring chains while slowly rotating the rear wheels.
Diagnosing Common Problems with Your 1991 Polaris Trail Boss 350L
Even with great care, a 30-year-old machine will have issues. Here’s how to troubleshoot the most common problems with 1991 polaris trail boss 350l models.
Engine Won’t Start or Runs Poorly
This is the number one complaint. The cause is almost always one of three things: fuel, spark, or compression.
Fuel System Woes:
- Clogged Carburetor: The Mikuni VM34SS carb is notorious for getting gummed up, especially with modern ethanol fuel. If the ATV has been sitting, the pilot jet is likely clogged. A thorough carb cleaning is the first step. You’ll need carb cleaner, small brushes, and compressed air.
- Fuel Filter: Check the in-line fuel filter. If it’s dark or full of debris, replace it. It’s a cheap and easy fix.
- Fuel Pump: The vacuum-operated fuel pump can fail. To test it, disconnect the fuel line from the carb, place it in a jar, and crank the engine. You should see fuel pulsing out.
Spark Issues:
- Old Spark Plug: Start with a fresh NGK BPR7ES spark plug. Check that it’s gapped correctly (usually around 0.028″).
- Weak Spark: To check for spark, remove the plug, reconnect it to the plug wire, and ground the threads against the engine block. Crank the engine and look for a strong, blue spark. If it’s weak, orange, or non-existent, you could have a bad plug cap, ignition coil, or CDI box.
Brakes Feel Spongy or Weak
Spongy brakes mean there’s air in the hydraulic lines. You need to bleed the system. You’ll need a clear tube, a small wrench (usually 8mm or 10mm), and a bottle of fresh DOT 3 brake fluid.
Start at the caliper furthest from the master cylinder. Attach the tube to the bleeder screw and put the other end in a jar with some clean fluid. Have a friend slowly pump the brake lever a few times and then hold it down. While they hold, crack open the bleeder screw just enough for fluid and air bubbles to escape. Close it before they release the lever. Repeat until no more bubbles come out, then move to the next caliper.
How to Boost Performance: Simple Mods for Your Trail Boss
Looking for a little more grunt? You don’t need to rebuild the engine to feel a difference. Here are a few simple ways and how to 1991 polaris trail boss 350l mods can wake your machine up.
Tires and Traction
The single biggest improvement you can make is a good set of tires. The old, worn-out stock tires are likely hard as a rock. A modern set of trail tires like the Maxxis Bighorn 2.0 or ITP Mud Lite will transform the handling and traction, giving you one of the best benefits of 1991 polaris trail boss 350l ownership: conquering tough terrain.
Clutch and Carburetor Tuning
For the DIY mechanic, a clutch kit is a great next step. A kit from a brand like EPI or Dalton Industries comes with different springs and weights for your clutches. This allows you to tune the PVT to engage at a different RPM, improving low-end torque for climbing or top-end speed for open trails.
If you’ve added a less restrictive air filter or an aftermarket exhaust, you’ll need to rejet the carburetor to match. A jet kit comes with different-sized main and pilot jets to help you dial in the perfect air/fuel ratio, preventing a lean condition that can damage your engine.
Sustainable & Eco-Friendly Riding: Best Practices for the Trails
Owning a classic ATV also means being a responsible steward of the trails. A few simple habits can make a huge difference.
A core part of a sustainable 1991 polaris trail boss 350l experience is preventing leaks. Regularly check your fuel lines, oil lines, and gearcase for any drips or cracks. A small leak on the trail can contaminate soil and water sources.
For a more eco-friendly 1991 polaris trail boss 350l ride, consider using a high-quality, biodegradable two-stroke oil. These oils are designed to burn cleaner and break down more easily in the environment. And always, always stay on designated trails to prevent erosion and protect wildlife habitats.
Frequently Asked Questions About the 1991 Polaris Trail Boss 350L
What kind of oil and gas should I use?
Use 87 octane unleaded gasoline. For the oil injection system, use a quality two-stroke injector oil rated for liquid-cooled engines. For the transmission, use Polaris AGL Synthetic Gearcase Lubricant or a compatible 75W-90 gear oil.
Is the 1991 Polaris Trail Boss 350L a 2×4 or 4×4?
The 1991 Trail Boss 350L is a two-wheel drive (2×4) model. It sends power only to the rear wheels via a solid axle and chain drive.
How fast does a 1991 Polaris Trail Boss 350L go?
In good running condition, you can expect a top speed of around 45-50 MPH. The real strength of this machine, however, is its snappy low-end and mid-range torque on the trails, not its top speed.
Are parts still available for this ATV?
Yes, surprisingly so! Many common wear items like brake pads, bearings, chains, sprockets, and carburetor rebuild kits are readily available from aftermarket suppliers. Major components like plastics or engine cases can be harder to find and may require searching on eBay or ATV salvage forums.
We hope these 1991 polaris trail boss 350l tips help you keep your classic quad running like a champ. It’s a rewarding machine to own and work on, and with a little bit of knowledge and effort, it will provide years of fun.
Remember to always put safety first—wear a helmet, work in a well-ventilated area, and never take shortcuts. Now get out there, stay safe, and have a blast on the trails!
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