Let’s be honest. Trying to find reliable, clear-cut information on a classic quad like the 1991 Polaris Trail Boss 350 can feel like digging for gold. You’ll find scattered forum posts, blurry diagrams, and conflicting numbers that leave you more confused than when you started.
At FatBoysOffroad, we believe you deserve better. We promise this is the last guide you’ll need. We’ve compiled all the critical 1991 polaris trail boss 350 specs into one comprehensive, easy-to-read resource, packed with the pro tips you need to keep your vintage rig tearing up the trails.
In this deep dive, we’ll break down the engine and transmission, cover the chassis and suspension details, list every essential fluid capacity, and even walk you through common problems. Let’s get your machine dialed in.
Unpacking the Heart of the Beast: 1991 Polaris Trail Boss 350 Engine Specs
The engine is where the magic happens. Understanding its specifications is the first step in proper maintenance and troubleshooting. The 1991 Trail Boss 350L, available in both 2×4 and 4×4 versions, was powered by a surprisingly robust motor for its time.
Engine Type and Displacement
At its core, the Trail Boss 350L uses a piston-port, two-stroke, single-cylinder engine. This design is known for its simplicity, punchy power delivery, and that classic two-stroke sound we all love.
- Engine Type: 2-Stroke Single Cylinder
- Displacement: 346cc
- Bore x Stroke: 80mm x 68mm
- Power Output: Approximately 25-28 horsepower (factory estimate)
Knowing these core numbers is vital when ordering parts like a new piston, rings, or a top-end gasket kit. Always double-check your part numbers against the 346cc engine spec.
Cooling System Explained
The ’91 model featured a liquid-cooling system, a significant upgrade over many air-cooled competitors of the era. This allowed for more consistent operating temperatures and sustained performance, especially on long, slow trail rides.
Ensure your radiator is clean and free of mud and debris. Use a quality 50/50 mix of ethylene glycol antifreeze and distilled water. This is a critical part of your pre-ride check.
Carburetor and Fuel System
Fuel and air delivery is handled by a tried-and-true Mikuni carburetor. This is often the source of running issues on older machines, so knowing its specs is a huge advantage for any DIY mechanic.
- Carburetor: Mikuni VM34SS (34mm)
- Main Jet (Stock): 240
- Pilot Jet (Stock): 35
- Fuel Requirement: 87 Octane Unleaded Gasoline
- Oil System: Automatic Oil Injection (Do not pre-mix fuel if the system is working!)
One of the best 1991 polaris trail boss 350 specs tips we can give is to learn how to clean this carb. A dirty pilot jet is the number one cause of hard starting and poor idling.
Ignition and Starting System
Getting that two-stroke fired up relies on a solid ignition system. The Trail Boss 350 featured both an electric start for convenience and a backup recoil pull-starter for those cold mornings or when the battery is low.
- Ignition: CDI (Capacitor Discharge Ignition)
- Spark Plug: NGK BPR7HS or equivalent
- Spark Plug Gap: 0.028 in (0.7 mm)
- Starting: Electric Start with Recoil Backup
A fresh, correctly gapped spark plug is cheap insurance against starting problems. Keep a spare in your trail kit!
Getting Power to the Ground: Drivetrain and Transmission Details
The engine makes the power, but the drivetrain delivers it. Polaris revolutionized the ATV industry with its automatic transmission, and the 1991 Trail Boss was a prime example of this user-friendly technology.
The Polaris Variable Transmission (PVT)
Instead of a manual clutch and gears, the Trail Boss uses the Polaris Variable Transmission (PVT). This is a type of Continuously Variable Transmission (CVT) that uses a drive belt and two variable-diameter pulleys (a primary/drive clutch and a secondary/driven clutch) to automatically adjust the gear ratio.
- Transmission Type: Automatic PVT
- Gear Selector: F-N-R (Forward, Neutral, Reverse)
- Drive Belt: Polaris Part #3211048 (or equivalent aftermarket)
The benefit of knowing these 1991 polaris trail boss 350 specs is clear: a worn or glazed belt will rob you of power and performance. Inspecting the belt is a key maintenance task.
Drive System (2×4 vs. 4×4)
The 1991 Trail Boss 350L was available in two configurations:
- Trail Boss 350L 2×4: A simple, reliable rear-wheel-drive setup. Lighter and nimbler for casual trail riding.
- Trail Boss 350L 4×4: Featured Polaris’s on-demand four-wheel-drive system, engaging the front wheels when the rear wheels lose traction. This is the model you want for mud, snow, or serious off-roading.
Final Drive: Chain and Sprockets
From the transmission, power is sent to the rear wheels (and front on the 4×4) via a chain drive. This is a high-wear area that requires regular attention.
- Drive Type: O-Ring Chain
- Stock Gearing (Teeth): Typically 12T Front / 34T Rear (confirm on your specific machine)
- Chain Slack: Adjust to approximately 1/2 inch of free play
Keeping your chain clean, lubricated, and properly tensioned is one of the best practices for drivetrain longevity. A dry, rusty chain will wear out your sprockets in a hurry.
Chassis, Suspension, and Brakes: The Foundation of Your Ride
How the machine handles and stops is just as important as how it goes. The Trail Boss was built on a sturdy frame with a suspension setup that was quite advanced for its day.
Frame and Dimensions
These numbers give you a sense of the machine’s size, stability, and capability.
- Dry Weight (approx.): 495 lbs (224 kg) for 2×4 / 541 lbs (245 kg) for 4×4
- Overall Length: 75.5 in (191.8 cm)
- Overall Width: 44.5 in (113 cm)
- Wheelbase: 49.5 in (125.7 cm)
- Ground Clearance: 6.0 in (15.2 cm)
Suspension Setup (Front & Rear)
Polaris used a unique suspension design that provided a comfortable ride over rough terrain.
- Front Suspension: MacPherson Strut with 6.25 inches of travel. This independent setup allows each front wheel to move up and down without affecting the other, soaking up bumps effectively.
- Rear Suspension: Progressive-Rate Swingarm with 8.5 inches of travel. This design gets stiffer as it compresses, providing a soft ride over small bumps and preventing bottoming out on big hits.
Braking System: Stopping Power
The ’91 Trail Boss featured an all-wheel hydraulic disc brake system, controlled by a single lever on the left handlebar. This was a premium feature at the time.
- Brake System: Single-Lever Hydraulic Disc (all wheel)
- Auxiliary Brake: Mechanical Rear Foot Brake
- Brake Fluid: DOT 3 Brake Fluid
Safety First: Always check your brake fluid level and test your brakes before every ride. Spongy brakes are a sign of air in the lines, which must be bled immediately.
Wheels and Tires
Stock tires were designed for general trail use. Many owners upgrade to more aggressive tread patterns depending on their riding style.
- Front Tires: 23 x 8 – 10
- Rear Tires: 22 x 11 – 10
- Bolt Pattern: 4/156
Essential Fluids & Capacities: A Critical 1991 Polaris Trail Boss 350 Specs Care Guide
Using the correct fluids and amounts is non-negotiable for a healthy machine. This section is your cheat sheet for a proper 1991 polaris trail boss 350 specs care guide.
Engine Oil (2-Stroke Injection)
The Trail Boss uses an oil injection system, so you don’t pre-mix the gas. There’s a separate oil tank that feeds the engine automatically.
- Oil Type: Polaris 2-Cycle Injection Oil or equivalent high-quality 2-stroke injector oil.
- Oil Tank Capacity: 2 Quarts (1.9 L)
Never let this tank run dry! Doing so will starve the engine of lubrication and lead to catastrophic failure.
Transmission/Gearcase Fluid
The transmission requires its own specific lubricant to protect the gears and internal components.
- Fluid Type: Polaris AGL (All Gear Lubricant) or equivalent gearcase-specific lubricant. Do not use standard motor oil.
- Capacity: Approximately 16 oz (473 ml) – always fill to the bottom of the check plug hole.
Fuel Tank Capacity
Knowing your fuel capacity helps you plan your rides and avoid a long walk back to the truck.
- Fuel Tank Capacity: 4 U.S. Gallons (15.1 L)
Coolant Requirements
Proper cooling is vital for a liquid-cooled two-stroke engine.
- Coolant Type: High-quality ethylene glycol antifreeze (50/50 mix with distilled water)
- System Capacity: 2.25 Quarts (2.1 L)
Properly disposing of used fluids is a key part of eco-friendly 1991 polaris trail boss 350 specs best practices. Never dump oil or antifreeze on the ground.
Common Problems and Pro-Tips for the ’91 Trail Boss 350
Even the best machines have their quirks. Knowing the common problems with 1991 polaris trail boss 350 specs can save you hours of frustration.
Carburetor Tuning and Cleaning
As mentioned, the Mikuni carb can be finicky, especially with modern ethanol-blended fuels. If your ATV idles poorly, bogs down on acceleration, or is hard to start, a thorough carb cleaning is your first step.
PVT Clutch Maintenance
The drive and driven clutches need to be clean to operate smoothly. Over time, belt dust and dirt can build up, causing jerky engagement or loss of power. Periodically remove the clutch cover and blow out the dust with compressed air.
Electrical Gremlins
On a 30+-year-old machine, electrical connections can corrode. Check and clean major ground points (especially the frame ground from the battery) and use dielectric grease on connectors to prevent issues with lights, the starter, or the 4WD system.
Sourcing Parts for a Classic
Finding parts can be a challenge. While OEM parts are scarce, many aftermarket companies still produce wear items like brake pads, drive belts, bearings, and top-end rebuild kits. Online ATV forums and marketplaces are your best friends for finding used or rare components.
Frequently Asked Questions About the 1991 Polaris Trail Boss 350 Specs
What kind of oil does a 1991 Polaris Trail Boss 350 take?
It takes two main types. For the engine, use a high-quality 2-stroke injector oil in the oil reservoir. For the transmission/gearcase, use Polaris AGL or a comparable gearcase-specific lubricant.
Is the 1991 Trail Boss 350 a 2-stroke or 4-stroke?
The 1991 Polaris Trail Boss 350L is a 2-stroke. This is evident by its oil injection system and the distinct sound and powerband of its 346cc liquid-cooled engine.
How fast can a Polaris Trail Boss 350 go?
In good running condition, a stock 1991 Trail Boss 350 can reach a top speed of approximately 45-50 MPH. Speed will vary based on rider weight, terrain, and the condition of the engine and drivetrain components like the belt and clutches.
What is the value of a 1991 Polaris Trail Boss 350?
The value varies greatly depending on condition. A non-running project might be worth a few hundred dollars. A clean, well-maintained, fully functional model (especially a 4×4) can be worth anywhere from $1,000 to $2,000 or more to the right buyer looking for a reliable vintage quad.
This complete 1991 polaris trail boss 350 specs guide should give you the confidence to tackle any maintenance, repair, or restoration project on your classic Polaris. Knowing your machine inside and out is the key to keeping it reliable for years of future adventures.
Now, grab your tools, use these specs as your roadmap, and give that vintage Trail Boss the attention it deserves. Happy wrenching, and stay safe on the trails!
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