1991 Polaris Trail Boss 350: The Ultimate Diyer'S Restoration & Care

There’s a special kind of pride that comes with keeping a classic machine running strong. If you own a 1991 Polaris Trail Boss 350, you know exactly what we’re talking about. It’s more than just an ATV; it’s a piece of off-roading history that represents a simpler, tougher era of trail riding.

But keeping a vintage quad in prime condition presents its own unique set of challenges. You’ve likely searched for reliable information, only to find scattered forum posts and vague advice. We promise to change that. This is the definitive guide you’ve been looking for.

In this comprehensive FatBoysOffroad deep-dive, we’ll cover everything from its iconic features and a crucial pre-ride checklist to troubleshooting the most common problems you’ll face. We’ll also provide a step-by-step maintenance schedule and even explore some modern upgrades to keep your classic roaring for years to come.

Why the 1991 Polaris Trail Boss 350 Remains an Off-Road Legend

Before we wrench, let’s appreciate what makes this machine special. The Trail Boss line was a game-changer for Polaris, and the 1991 350L 4×4 model, in particular, hit a sweet spot. It was one of the pioneers of features we now take for granted.

Understanding the benefits of a 1991 Polaris Trail Boss 350 helps you appreciate its quirks. It’s not about raw speed; it’s about reliability, simplicity, and a direct connection to the trail.

Key Features That Defined an Era

The 346cc Two-Stroke Engine: This air-cooled, oil-injected two-stroke motor is the heart of the beast. It’s known for its snappy power delivery and relatively simple design, making it a great engine for DIY mechanics to learn on.
On-Demand 4WD: Polaris’s early four-wheel-drive system was revolutionary. The ability to switch into 4WD on the fly gave riders unprecedented traction and control when the trail got nasty.
MacPherson Strut Front Suspension: This setup provided a plush ride for its time, absorbing bumps and ruts effectively. While not as sophisticated as modern double A-arm systems, it’s durable and easy to service.
Full Floorboards: A signature Polaris feature that offered superior foot protection and comfort compared to the simple footpegs found on many competing models of the day.

Your Pre-Ride Inspection: A Non-Negotiable 1991 Polaris Trail Boss 350 Care Guide

Before you hit the trails, a thorough pre-ride check is the most important habit you can develop. For a vintage machine, it’s not just a suggestion—it’s essential for safety and preventing minor issues from becoming major breakdowns. Think of this as one of the most important 1991 polaris trail boss 350 tips we can offer.

Grab a flashlight and spend 10 minutes going through this checklist. We call it the T-CLOCS method, a classic acronym from the motorcycle world that works perfectly here.

T – Tires & Wheels: Check tire pressure (typically 3-5 PSI, check your sidewall). Look for cracks, punctures, or bubbles. Grab each wheel and check for side-to-side play, which could indicate a bad wheel bearing.
C – Controls & Cables: Squeeze the brake levers. They should feel firm, not spongy. Test the throttle—it must snap back crisply when released. Check the 4WD switch and shifter for smooth operation.
L – Lights & Electrics: Test the headlight (high and low beam), taillight, and brake light. Check the kill switch to ensure it shuts the engine off. Look for any frayed or exposed wires.
O – Oil & Fluids: This is critical for the two-stroke engine. Check the two-stroke oil injection tank and top it off with a quality TC-W3 rated injection oil. Check the transmission fluid level using the dipstick. Also, check the brake fluid reservoir.
C – Chassis & Suspension: Visually inspect the frame for any cracks, especially around welds. Check your chain for proper tension (about 1 inch of slack) and look for any tight spots or rust. Push down on the fenders to test the suspension’s movement.
S – Stand: Not applicable here, but it’s part of the acronym! Instead, fire it up. Let it idle for a minute. Listen for any strange noises or rattles before you head out.

Solving Common Problems with the 1991 Polaris Trail Boss 350

Every machine has its quirks, and after 30+ years, your Trail Boss has likely developed a few. Here’s a breakdown of the most common problems with the 1991 Polaris Trail Boss 350 and how to start diagnosing them like a pro.

Problem 1: It Won’t Start or is Hard to Start

This is the big one. A no-start condition almost always comes down to three things: fuel, spark, or compression. Here’s how to check them.

Check for Spark: Get an inline spark tester—it’s a cheap and invaluable tool. It connects between your spark plug and the plug wire. When you crank the engine, the tester should light up brightly. No spark? The issue could be a bad spark plug, plug wire, ignition coil, or CDI box. Start with the easiest and cheapest: a new NGK BPR7ES spark plug.
Check for Fuel: Is the fuel petcock turned on? Is there fresh gas in the tank? Old gas is a common culprit. If the gas is good, your carburetor is the next suspect. It might be clogged from sitting.
Check for Compression: A two-stroke engine needs good compression to run. You’ll need a compression tester. A healthy 350L engine should have around 120-130 PSI. Anything below 100 PSI suggests it’s time for a top-end rebuild (piston and rings).

Problem 2: The Engine Bogs Down Under Load

If your Trail Boss idles fine but sputters or dies when you give it throttle, it’s almost always a fuel delivery issue. This is a classic symptom of a dirty carburetor.

The Mikuni VM34 carburetor on these machines is simple but sensitive to dirt and ethanol-clogged jets. Learning how to clean a 1991 Polaris Trail Boss 350 carburetor is a rite of passage. You’ll need carb cleaner, a soft brush, and small wires or jet cleaning tools to clear the tiny passages in the pilot and main jets.

Problem 3: The 4WD Isn’t Engaging

The electromagnetic coils in the front hubs are a common failure point. First, check the simple stuff: inspect the wiring going to the hubs for any breaks or corrosion. Check the fuse box. If that looks good, you can test the coils themselves with a multimeter for proper resistance. A failed coil will need to be replaced.

Key Maintenance Jobs You Can Tackle at Home

Following a solid maintenance schedule is the core of this 1991 Polaris Trail Boss 350 guide. These are the jobs that prevent problems before they start and ensure your ATV is always ready for the trail.

Changing the Transmission / Gearcase Fluid

This is an easy but often-neglected task. The transmission and chain case share fluid on these models.

Warm up the engine for a few minutes to get the fluid flowing.
Locate the drain plug on the bottom of the gearcase. Place a drain pan underneath and remove the plug.
While it drains, remove the fill plug/dipstick on the top of the case to allow air in.
Once drained, clean and reinstall the drain plug.
Refill with Polaris AGL (or a compatible gearcase lubricant) through the dipstick hole until it reaches the full mark on the stick. Do not overfill.

Cleaning the Air Filter

A clean air filter is vital for engine health. The stock foam filter is reusable and should be cleaned after every few rides, especially in dusty conditions.

Remove the seat and airbox lid to access the filter.
Carefully remove the filter from its cage.
Wash it thoroughly in a bucket of warm, soapy water. Squeeze it out—never wring or twist it, as this can tear the foam.
Rinse with clean water until the water runs clear. Let it air dry completely.
Once dry, saturate it with a quality foam air filter oil. Squeeze out the excess until it’s tacky but not dripping. Reinstall.

Sustainable & Eco-Friendly Riding: Modern Best Practices for a Vintage ATV

Owning a classic doesn’t mean we can’t be modern-minded riders. A few simple steps can make your time on the trail more responsible. This is how you create a sustainable 1991 Polaris Trail Boss 350 experience.

One of the best eco-friendly 1991 Polaris Trail Boss 350 practices is fluid management. When you change your transmission oil, take the used oil to an auto parts store or local recycling center. Never dump it on the ground or in the trash.

Consider using biodegradable chain lube and two-stroke injection oil. These products are designed to break down more naturally in the environment, reducing your impact. Finally, always follow Tread Lightly! principles: stay on designated trails, cross streams at designated points, and pack out everything you pack in.

Frequently Asked Questions About the 1991 Polaris Trail Boss 350

What kind of gas should I use in a 1991 Polaris Trail Boss 350?

Use 87 octane unleaded gasoline. Because it’s an older machine, it’s highly recommended to use ethanol-free gasoline if you can find it. If not, add a fuel stabilizer to every tank to prevent the ethanol from gumming up the carburetor, especially if the ATV will sit for more than a couple of weeks.

What is the top speed of a 1991 Polaris Trail Boss 350?

In good running condition, a stock 1991 Trail Boss 350L 4×4 can reach a top speed of around 45-50 MPH. Its strength isn’t top speed, but rather its low-end torque for trail riding and work.

How much is a 1991 Polaris Trail Boss 350 worth?

Value depends heavily on condition. A non-running project machine might only be worth a few hundred dollars. A clean, well-maintained, all-original model could fetch anywhere from $1,200 to $2,500 or more to the right collector or enthusiast who appreciates its classic status.

Wrenching on your 1991 Polaris Trail Boss 350 is more than just maintenance; it’s about connecting with a classic piece of off-road machinery. Take your time, use the right tools, and don’t be afraid to get your hands dirty. The satisfaction of keeping this legend alive and roaring down the trail is a reward in itself.

Now get out there, stay safe, and have fun on the trails!

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Thomas Corle

Founder & Automobile expert at FatBoysOffroad

Automobile expert and off-road enthusiast, sharing in-depth knowledge of vehicles, gear, and adventures. Helping readers make smart decisions and enjoy the thrill of off-roading safely and confidently.