91 Polaris Trail Boss 350 – Your Complete Guide To Maintenance

Let’s be honest. That old 91 Polaris Trail Boss 350 sitting in your garage or barn is more than just an ATV; it’s a tough-as-nails piece of off-road history. You know it’s built like a tank, but you also know that a machine over three decades old comes with its own set of quirks and needs a little extra love to stay reliable.

You’ve come to the right place. We promise this isn’t just another spec sheet. This is your hands-on, grease-under-the-fingernails guide to keeping that classic quad alive and kicking. We’re here to give you the expert insights and practical steps to not only fix its common issues but to make it a dependable partner for years of trail adventures.

In this complete 91 polaris trail boss 350 guide, we’ll walk you through essential pre-ride checks, troubleshoot the most frequent problems you’ll encounter, explore simple performance mods, and share some pro-level tips to transform your vintage rig into a trail-taming beast. Let’s get those wrenches turning.

The Enduring Legacy: Why This ATV is a Classic Worth Keeping

Before we dive into the nuts and bolts, it’s worth appreciating what makes the 91 Polaris Trail Boss 350 so special. This era of ATVs was all about simplicity and durability. Whether you have the liquid-cooled 350L 2-stroke or the air-cooled 4-stroke model, the engineering is straightforward, which is fantastic news for the DIY mechanic.

The primary benefits of 91 polaris trail boss 350 ownership are clear: they are incredibly easy to work on, parts are surprisingly still available through aftermarket suppliers, and they deliver a pure, unfiltered off-road experience that modern, computer-controlled machines can’t replicate.

Understanding Your Model: 2-Stroke vs. 4-Stroke

It’s crucial to know which engine your Trail Boss has, as maintenance procedures differ significantly.

The 350L (2-Stroke): This is the peppy, oil-injected model. It’s lighter, has a distinct “ring-ding” sound, and requires 2-stroke injector oil in its separate reservoir. It’s known for a snappy powerband.
The 350 4×4 (4-Stroke): This model has a more traditional engine with engine oil in the crankcase that you check with a dipstick. It delivers smoother, more predictable torque, ideal for work and technical trails.

Your Essential 91 Polaris Trail Boss 350 Care Guide: The Pre-Ride Ritual

The single best thing you can do for your vintage quad is a thorough pre-ride inspection. It takes five minutes but can save you hours of frustration on the trail and prevent costly damage. These are non-negotiable 91 polaris trail boss 350 best practices.

Fuel and Fluids: The Lifeblood

Old machines are particularly sensitive to fluid levels and quality. Don’t skip these checks.

Check the Fuel: Is the tank full? More importantly, is the fuel fresh? Gas that’s been sitting for months can gum up the carburetor. If in doubt, drain it and start with fresh, non-ethanol fuel if possible.
Inspect the Oil: For 2-stroke models, ensure the injector oil reservoir is topped off with a quality 2-stroke injector oil. For 4-stroke models, pull the dipstick, wipe it, re-insert it fully, and check the level. It should be between the ‘add’ and ‘full’ marks.
Look for Leaks: Do a quick visual scan under the engine and around hoses for any drips of oil, fuel, or coolant (on the 350L model).

Chain, Brakes, and Tires: Control and Safety

These components connect you and your machine to the trail. Their condition is paramount for safety.

Tire Pressure: Check all tires with a low-pressure gauge. They should typically be around 3-5 PSI, but check the tire sidewall for specific recommendations.
Brake Function: Squeeze the brake levers and press the foot brake. They should feel firm, not spongy. A spongy feel indicates air in the hydraulic lines, which means it’s time to bleed the brakes.
Chain Tension: The drive chain should have about a half-inch of slack. Too tight, and it puts stress on bearings; too loose, and it can fall off. Adjust as needed and lubricate it with a quality chain lube.

Tackling Common Problems with the 91 Polaris Trail Boss 350

Every old machine has its personality and a few common ailments. Here’s how to 91 polaris trail boss 350 troubleshooting when things go wrong. Don’t worry; most issues are simple fixes you can handle with basic tools.

Problem 1: “It Cranks But Won’t Start!”

This is the most common complaint. Remember the three things an engine needs: spark, fuel, and air.

Check for Spark: Get a spark plug socket and remove the spark plug. Reconnect it to the plug wire, and hold the threaded part against a metal part of the engine. Crank the engine over. You should see a strong, blue spark. No spark? The issue could be a bad plug, coil, or CDI box.
Check for Fuel: Is the fuel petcock turned on? Can you smell gas? A quick check is to carefully spray a very small shot of starting fluid into the air intake. If it fires up for a second and dies, you have a fuel delivery problem, likely a clogged carburetor.
Check for Air: Remove the seat and airbox cover. Is the air filter filthy? A clogged filter can choke the engine. Clean or replace it.

Problem 2: Bogging Down or Losing Power

If your Trail Boss starts fine but bogs down when you give it gas, the carburetor is almost always the culprit. Over time, the tiny passages inside, called jets, get clogged with varnish from old fuel.

The fix is a thorough carb cleaning. You’ll need to remove the carburetor, disassemble it carefully in a clean workspace, and spray out all passages and jets with carburetor cleaner. Pay special attention to the main jet and pilot jet. A rebuild kit with new gaskets is a great idea while you have it apart.

Problem 3: The Infamous Polaris Variable Transmission (PVT)

The automatic PVT clutch system is brilliant but needs maintenance. If you notice poor acceleration, slipping, or a jerky take-off, your clutches or drive belt need attention.

Remove the plastic clutch cover. The drive belt should be clean and free of glazed or cracked spots. Blow out any dust and debris from the primary (front) and secondary (rear) clutches with compressed air. A worn belt is a common cause of poor performance and is an easy part to replace.

Sustainable Off-Roading: Eco-Friendly Tips for Your Vintage ATV

Owning a classic machine is a great way to recycle and reuse, but we can do more. Adopting a few eco-friendly 91 polaris trail boss 350 habits ensures our trails stay beautiful for generations to come.

Proper Fluid Disposal: Never dump old oil, coolant, or brake fluid on the ground or down a drain. Collect it in a sealed container and take it to your local auto parts store or municipal recycling center. They almost always accept it for free.
Use Biodegradable Products: When possible, opt for biodegradable chain lube and cleaners. These products break down naturally and have less impact on the environment you’re out there to enjoy.
Tread Lightly: This is a core tenet of responsible off-roading. Stay on designated trails, avoid creating new paths, and cross streams only at designated crossings. This prevents erosion and protects delicate ecosystems. This is the cornerstone of a sustainable 91 polaris trail boss 350 lifestyle.

Frequently Asked Questions About the 91 Polaris Trail Boss 350

What kind of oil does a 91 Polaris Trail Boss 350 use?

This depends entirely on your model. If you have the 350L (2-stroke), you need to fill the reservoir with a quality 2-stroke injector oil. Do not mix it in the gas! If you have the 4-stroke model, you’ll use a 4-stroke ATV/motorcycle oil like a 10W-40 in the engine crankcase.

How fast can a 91 Polaris Trail Boss 350 go?

In good running condition, you can expect a top speed of around 45-50 MPH. Speed isn’t its main strength; its durability and trail capability are where it truly shines.

Are parts still available for this model?

Yes, surprisingly so! While you might not find everything at a Polaris dealer, a huge number of aftermarket companies produce parts. eBay, Amazon, and specialized ATV parts websites are fantastic resources for everything from carburetor rebuild kits and brake pads to plastics and engine components.

Final Thoughts: Keep That Classic Trail Boss Roaring

Your 91 Polaris Trail Boss 350 is a gateway to adventure and a fantastic platform for learning the art of mechanics. It’s a machine that rewards a little bit of care with unwavering reliability.

Embrace the routine maintenance, don’t be afraid to get your hands dirty, and tackle problems one step at a time. The satisfaction of keeping this classic ATV running perfectly is a reward in itself.

Now, go through that pre-ride checklist, fire it up, and hit the trails. Stay safe, have fun, and enjoy the ride!

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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.