Stihl MM55 Tiller Won't Start (5 Expert Fixes For Woodcutters)

Did you know that a neglected tiller, left sputtering and refusing to start, can delay spring planting and cost small farmers and gardeners thousands of dollars in lost yield? I’ve seen firsthand the frustration a stubborn engine can cause. In this article, I’ll share my expert solutions to get your Stihl MM55 tiller back in action. As a woodcutter and small-scale farmer myself, I understand the value of reliable equipment. Over the years, I’ve learned to diagnose and fix common issues that plague these machines. Let’s get started and turn that frustration into a successful start-up!

Stihl MM55 Tiller Won’t Start: 5 Expert Fixes for Woodcutters

A non-starting tiller can be a major setback, especially when you’re eager to prepare your garden or woodlot for planting. The Stihl MM55 is a versatile and reliable machine, but like any piece of equipment with a small engine, it can sometimes be temperamental. Through my experience, I have identified the most common causes of starting problems and developed a systematic approach to troubleshooting. Let’s explore these fixes, drawing upon my years of experience with small engines and wood processing.

1. Fuel System Issues: The Prime Suspect

The fuel system is the first place I check when a small engine refuses to start. It’s the lifeline of your tiller, and even a minor disruption can bring everything to a halt.

Old or Contaminated Fuel:

The Problem: Gasoline degrades over time, especially when mixed with ethanol. Ethanol attracts moisture, leading to phase separation, where the ethanol and water separate from the gasoline, leaving a gummy residue that clogs the carburetor and fuel lines.

My Experience: I once left my tiller unused over the winter with fuel still in the tank. Come spring, it wouldn’t start. The fuel had turned into a thick, varnish-like substance.

The Fix:

Drain the Tank: Completely drain the fuel tank. I recommend using a siphon to remove the old fuel without spilling it.
Inspect the Fuel: Look at the drained fuel. Is it cloudy, discolored, or does it have a strange odor? These are signs of contamination.
Replace with Fresh Fuel: Use fresh, high-quality gasoline with an octane rating recommended by Stihl (usually 89 or higher). I prefer using non-ethanol fuel whenever possible to prevent future problems.
Fuel Stabilizer: Add a fuel stabilizer to the fresh fuel. Stabilizers prevent fuel degradation and protect the fuel system. I’ve had great success with brands like STA-BIL and Sea Foam.

Data Point: According to a study by the EPA, gasoline can start to degrade in as little as 30 days, especially when exposed to air and temperature fluctuations.

Clogged Fuel Filter:

The Problem: The fuel filter prevents debris from entering the carburetor. Over time, it can become clogged with dirt, rust, and fuel residue.

My Experience: I’ve seen filters so clogged that they completely restricted fuel flow. The engine would start briefly, then die.

The Fix:

Locate the Fuel Filter: The fuel filter is usually located in the fuel tank, attached to the fuel line. Consult your Stihl MM55 owner’s manual for the exact location.
Inspect the Filter: Remove the filter and inspect it. If it’s visibly dirty or clogged, it needs to be replaced.
Replace the Filter: Use a new fuel filter that is specifically designed for the Stihl MM55.
Clean the Fuel Lines: While the filter is removed, inspect the fuel lines for cracks or clogs. If necessary, replace them as well. I use a small piece of wire to gently clean the lines.

Data Point: A clogged fuel filter can reduce fuel flow by as much as 50%, leading to poor engine performance and starting problems.

Blocked Carburetor:

The Problem: The carburetor mixes fuel and air for combustion. Small passages within the carburetor can become clogged with varnish and debris, especially from old fuel.

My Experience: I’ve spent hours cleaning carburetors. It’s a delicate process, but essential for a smooth-running engine.

The Fix:

Carburetor Cleaner: Start by spraying carburetor cleaner into the carburetor’s intake. Let it sit for a few minutes to dissolve deposits.
Disassemble the Carburetor: If the cleaner doesn’t work, you may need to disassemble the carburetor for a thorough cleaning. Refer to your Stihl MM55 service manual for detailed instructions.
Clean Each Part: Use carburetor cleaner and small brushes or wires to clean each part of the carburetor, including the jets and passages. Pay close attention to the tiny openings.
Reassemble the Carburetor: Carefully reassemble the carburetor, ensuring all parts are properly aligned.
Carburetor Adjustment: After reassembly, you may need to adjust the carburetor’s idle and mixture screws for optimal performance.

Data Point: A study by the Equipment Engine & Marketing Association (EEMA) found that over 70% of small engine problems are related to fuel system issues, with carburetor problems being the most common.

2. Ignition System Problems: Spark is Key

If the fuel system is functioning correctly, the next thing I check is the ignition system. Without a spark, there’s no combustion, and your tiller won’t start.

Faulty Spark Plug:

The Problem: The spark plug ignites the air-fuel mixture in the cylinder. Over time, the spark plug can become fouled, worn, or cracked, preventing it from producing a strong spark.

My Experience: I’ve seen spark plugs with cracked insulators, completely worn electrodes, and covered in carbon deposits.

The Fix:

Inspect the Spark Plug: Remove the spark plug and inspect it. Look for cracks, damage, or excessive carbon buildup.
Clean the Spark Plug: If the spark plug is only slightly fouled, you can try cleaning it with a wire brush.
Check the Gap: Use a spark plug gap tool to ensure the gap between the electrodes is correct. The proper gap for the Stihl MM55 is typically around 0.020 inches (0.5 mm), but consult your owner’s manual for the exact specification.
Test for Spark: Reconnect the spark plug to the spark plug wire and ground the spark plug against the engine block. Pull the starter cord. You should see a strong, blue spark. If the spark is weak or nonexistent, the spark plug needs to be replaced.
Replace the Spark Plug: Replace the spark plug with a new one that is specifically designed for the Stihl MM55.

Data Point: A worn or fouled spark plug can reduce engine power by as much as 30% and increase fuel consumption by 10%.

Defective Ignition Coil:

The Problem: The ignition coil provides the high voltage needed to create a spark at the spark plug. If the ignition coil is defective, it won’t produce enough voltage, and the engine won’t start.

My Experience: Diagnosing a faulty ignition coil can be tricky, but it’s a common cause of starting problems.

The Fix:

Test the Ignition Coil: Use a multimeter to test the ignition coil’s primary and secondary windings. Refer to your Stihl MM55 service manual for the proper testing procedure and resistance values.
Check the Air Gap: Ensure the air gap between the ignition coil and the flywheel is correct. This gap is crucial for proper ignition timing. The correct air gap for the Stihl MM55 is typically around 0.010-0.014 inches (0.25-0.35 mm).
Replace the Ignition Coil: If the ignition coil fails the multimeter test or the air gap is incorrect, replace it with a new one that is specifically designed for the Stihl MM55.

Data Point: The ignition coil is one of the most frequently replaced components in small engines, accounting for about 15% of all repairs.

Faulty Kill Switch:

The Problem: The kill switch is designed to stop the engine by grounding the ignition system. If the kill switch is faulty or stuck in the “off” position, it will prevent the engine from starting.

My Experience: I’ve encountered kill switches that were corroded or damaged, causing them to malfunction.

The Fix:

Inspect the Kill Switch: Check the kill switch for damage or corrosion.
Test the Kill Switch: Use a multimeter to test the kill switch’s continuity. In the “on” position, the kill switch should have continuity. In the “off” position, it should not have continuity.
Disconnect the Kill Switch: If the kill switch is suspected to be faulty, disconnect it from the ignition system. If the engine starts with the kill switch disconnected, the kill switch needs to be replaced.
Replace the Kill Switch: Replace the kill switch with a new one that is specifically designed for the Stihl MM55.

Data Point: A faulty kill switch can prevent an engine from starting even if all other components are functioning correctly.

3. Compression Issues: The Heart of the Engine

Compression is essential for proper combustion. If the engine doesn’t have enough compression, it won’t start.

Worn Piston Rings:

The Problem: The piston rings seal the gap between the piston and the cylinder wall, preventing combustion gases from escaping. Over time, the piston rings can wear down, reducing compression.

My Experience: I once worked on an old engine with completely worn piston rings. It had virtually no compression and wouldn’t even sputter.

The Fix:

Compression Test: Use a compression tester to measure the engine’s compression. Insert the compression tester into the spark plug hole and pull the starter cord several times. The compression reading should be within the range specified in your Stihl MM55 service manual.
Oil Test: If the compression is low, add a small amount of oil to the cylinder through the spark plug hole. Retest the compression. If the compression increases after adding oil, the piston rings are likely worn.
Replace the Piston Rings: Replacing the piston rings is a major repair that requires disassembling the engine. If you’re not comfortable with this type of repair, I recommend taking the tiller to a qualified mechanic.

Data Point: Low compression can reduce engine power, increase fuel consumption, and cause the engine to run rough.

Leaking Valves:

The Problem: The valves control the flow of air and exhaust gases into and out of the cylinder. If the valves are not sealing properly, compression will be lost.

My Experience: I’ve seen valves that were bent, burned, or covered in carbon deposits, preventing them from sealing.

The Fix:

Valve Inspection: Inspect the valves for damage or carbon buildup.
Valve Lapping: If the valves are slightly leaking, you can try lapping them to improve the seal. Valve lapping involves using a lapping compound to grind the valves against the valve seats.
Valve Replacement: If the valves are severely damaged, they need to be replaced. This is a major repair that requires disassembling the engine.

Data Point: Leaking valves can significantly reduce engine compression and cause the engine to run poorly.

Blown Head Gasket:

The Problem: The head gasket seals the cylinder head to the engine block. If the head gasket is blown, compression will be lost, and the engine won’t start.

My Experience: I’ve seen head gaskets that were cracked, burned, or completely disintegrated.

The Fix:

Inspect the Head Gasket: Look for signs of a blown head gasket, such as oil or coolant leaks around the cylinder head.
Compression Test: A compression test can also help diagnose a blown head gasket.
Replace the Head Gasket: Replacing the head gasket is a major repair that requires disassembling the engine.

Data Point: A blown head gasket can cause significant engine damage if not addressed promptly.

4. Starting Mechanism Issues: Pulling Won’t Cut It

Sometimes, the problem isn’t with the engine itself, but with the starting mechanism.

Broken Starter Cord:

The Problem: The starter cord is used to turn the engine over and start it. If the starter cord is broken, you won’t be able to start the engine.

My Experience: I’ve had starter cords break on me in the middle of a job. It’s frustrating, but easy to fix.

The Fix:

Replace the Starter Cord: Replace the broken starter cord with a new one that is the correct length and diameter.
Inspect the Starter Pulley: While you’re replacing the starter cord, inspect the starter pulley for damage or wear. If necessary, replace the starter pulley as well.

Data Point: A broken starter cord is a common problem with small engines, especially those that are used frequently.

Damaged Recoil Spring:

The Problem: The recoil spring retracts the starter cord after you pull it. If the recoil spring is damaged, the starter cord won’t retract, and the engine won’t start.

My Experience: I’ve seen recoil springs that were broken, stretched, or corroded.

The Fix:

Replace the Recoil Spring: Replace the damaged recoil spring with a new one that is specifically designed for the Stihl MM55.
Lubricate the Recoil Spring: Lubricate the recoil spring with a light oil to prevent it from rusting and to ensure smooth operation.

Data Point: A damaged recoil spring can make it difficult or impossible to start the engine.

Stuck Flywheel Brake:

The Problem: The flywheel brake is designed to stop the engine quickly when you release the kill switch. If the flywheel brake is stuck, it will prevent the engine from starting.

My Experience: I’ve encountered flywheel brakes that were corroded or damaged, causing them to stick.

The Fix:

Inspect the Flywheel Brake: Check the flywheel brake for damage or corrosion.
Lubricate the Flywheel Brake: Lubricate the flywheel brake with a light oil to prevent it from rusting and to ensure smooth operation.
Adjust the Flywheel Brake: If the flywheel brake is stuck, you may need to adjust it to ensure it is not contacting the flywheel when the kill switch is in the “on” position.

Data Point: A stuck flywheel brake can prevent the engine from starting even if all other components are functioning correctly.

5. Mechanical Issues: Beyond the Engine

Sometimes, the problem lies outside the engine itself.

Clogged Air Filter:

The Problem: The air filter prevents dirt and debris from entering the engine. A clogged air filter restricts airflow, which can cause the engine to run poorly or not start at all.

My Experience: I’ve seen air filters so clogged that they completely blocked airflow.

The Fix:

Inspect the Air Filter: Remove the air filter and inspect it. If it’s visibly dirty or clogged, it needs to be cleaned or replaced.
Clean the Air Filter: If the air filter is made of foam, you can wash it with soap and water. Allow it to dry completely before reinstalling it. If the air filter is made of paper, you can try tapping it gently to remove loose dirt.
Replace the Air Filter: If the air filter is heavily soiled or damaged, replace it with a new one that is specifically designed for the Stihl MM55.

Data Point: A clogged air filter can reduce engine power by as much as 20% and increase fuel consumption by 10%.

Obstructed Exhaust System:

The Problem: The exhaust system removes exhaust gases from the engine. If the exhaust system is obstructed, it can prevent the engine from starting.

My Experience: I’ve seen exhaust systems that were clogged with carbon deposits, nests, or other debris.

The Fix:

Inspect the Exhaust System: Check the exhaust system for obstructions.
Clean the Exhaust System: Remove any obstructions from the exhaust system. You may need to use a wire brush or other tools to remove carbon deposits.
Replace the Muffler: If the muffler is severely clogged or damaged, replace it with a new one.

Data Point: An obstructed exhaust system can cause the engine to overheat and run poorly.

Incorrect Valve Clearance:

The Problem: The valve clearance is the gap between the valve stem and the rocker arm. If the valve clearance is incorrect, the valves may not open or close properly, which can prevent the engine from starting.

My Experience: I’ve seen engines with valve clearances that were too tight or too loose, causing them to run poorly or not start at all.

The Fix:

Check the Valve Clearance: Use a feeler gauge to check the valve clearance. The correct valve clearance for the Stihl MM55 is specified in your owner’s manual.
Adjust the Valve Clearance: If the valve clearance is incorrect, adjust it according to the instructions in your service manual.

Data Point: Incorrect valve clearance can cause significant engine damage if not addressed promptly.

Wood Anatomy and Properties: Understanding Your Material

Understanding the wood you’re processing is crucial, whether you’re tilling soil for planting trees or preparing firewood. Different wood species have varying properties that affect everything from how easily they split to how much heat they produce when burned.

Hardwood vs. Softwood: A Tale of Two Structures

The terms “hardwood” and “softwood” are often misleading. They don’t necessarily refer to the actual hardness of the wood. Instead, they describe the type of tree the wood comes from.

Hardwoods: Typically come from deciduous trees (trees that lose their leaves in the fall). They have a more complex cell structure, making them denser and generally harder than softwoods. Examples include oak, maple, ash, and birch.
Softwoods: Typically come from coniferous trees (trees that have needles and cones). They have a simpler cell structure, making them less dense and generally softer than hardwoods. Examples include pine, fir, spruce, and cedar.

Data Point: Hardwoods generally have a higher BTU (British Thermal Unit) rating than softwoods, meaning they produce more heat when burned. Oak, for example, has a BTU rating of around 27 million per cord, while pine has a rating of around 20 million per cord.

Moisture Content: The Key to Seasoning

Moisture content is the amount of water in wood, expressed as a percentage of the wood’s dry weight. Freshly cut wood can have a moisture content of 50% or higher. Seasoning wood involves reducing its moisture content to around 20% or less.

Why Season Wood?

Easier to Burn: Dry wood ignites more easily and burns more efficiently.
More Heat: Dry wood produces more heat than wet wood.
Less Smoke: Dry wood produces less smoke, reducing creosote buildup in your chimney.
Less Creosote: Creosote is a flammable substance that can cause chimney fires.

Data Point: Burning unseasoned wood can reduce the efficiency of your wood stove by as much as 50% and increase creosote buildup by 70%.

Wood Density: Affecting Splitting and Burning

Wood density is the mass of wood per unit volume. Denser woods are generally harder to split but produce more heat when burned.

High-Density Woods: Oak, maple, and hickory are examples of high-density woods.
Low-Density Woods: Pine, poplar, and basswood are examples of low-density woods.

Data Point: High-density woods take longer to season than low-density woods.

Logging Tool Selection and Maintenance Best Practices

Selecting the right logging tools and maintaining them properly is essential for safety, efficiency, and productivity.

Chainsaw Selection: Power and Precision

Choosing the right chainsaw depends on the size and type of trees you’re cutting.

Small Chainsaws: Ideal for limbing and cutting small trees (less than 12 inches in diameter).
Medium Chainsaws: Suitable for felling medium-sized trees (12-20 inches in diameter).
Large Chainsaws: Necessary for felling large trees (over 20 inches in diameter).

Data Point: According to the Consumer Product Safety Commission (CPSC), there are approximately 30,000 chainsaw-related injuries each year in the United States. Proper training and safety equipment can significantly reduce the risk of injury.

Chainsaw Maintenance: Keeping Your Saw Sharp

Regular chainsaw maintenance is crucial for safety and performance.

Sharpening the Chain: A sharp chain cuts faster, smoother, and with less effort. I sharpen my chainsaw chain every time I refuel.
Cleaning the Air Filter: A clean air filter ensures proper airflow to the engine.
Checking the Chain Tension: Proper chain tension prevents the chain from derailing.
Lubricating the Chain: Chain lubrication reduces friction and wear.

Data Point: A dull chainsaw chain can increase cutting time by as much as 50% and increase the risk of kickback.

Splitting Tools: Manual vs. Hydraulic

Splitting wood can be done manually with an axe or maul, or hydraulically with a log splitter.

Axes and Mauls: Manual splitting tools are best for small-scale firewood preparation.
Hydraulic Log Splitters: Hydraulic log splitters are more efficient for splitting large quantities of wood or for splitting large, difficult-to-split logs.

Data Point: A hydraulic log splitter can split a cord of wood in as little as 2 hours, while it can take 8-10 hours to split a cord of wood manually.

Firewood Seasoning Techniques and Safety Considerations

Seasoning firewood properly and handling it safely are essential for efficient burning and preventing accidents.

Stacking Firewood: Maximizing Airflow

Properly stacking firewood is crucial for seasoning it effectively.

Elevate the Wood: Stack the wood on pallets or other supports to keep it off the ground.
Stack Loosely: Leave space between the logs to allow for airflow.
Cover the Top: Cover the top of the stack with a tarp or other waterproof material to protect the wood from rain and snow.
Orient the Stack: Orient the stack so that it is exposed to the prevailing winds.

Data Point: Properly stacked firewood can season in as little as 6 months, while improperly stacked firewood can take a year or more to season.

Firewood Safety: Protecting Yourself and Others

Handling firewood can be dangerous if proper safety precautions are not taken.

Wear Safety Glasses: Protect your eyes from flying debris.
Wear Gloves: Protect your hands from splinters and cuts.
Wear Steel-Toed Boots: Protect your feet from falling logs.
Lift with Your Legs: Avoid back injuries by lifting with your legs, not your back.
Be Aware of Your Surroundings: Watch out for tripping hazards and other potential dangers.

Data Point: According to the National Safety Council (NSC), back injuries are the most common type of injury associated with firewood handling.

Project Planning and Execution: From Tree to Fireplace

Planning your wood processing projects carefully can save you time, money, and effort.

Assessing Your Needs: How Much Wood Do You Need?

Determine how much firewood you need based on your heating needs.

Calculate Your Heating Needs: Estimate how much wood you’ll need to heat your home for the winter.
Consider Your Wood Stove’s Efficiency: Different wood stoves have different efficiencies.
Factor in the Wood’s BTU Rating: Different wood species have different BTU ratings.

Data Point: A typical home in a cold climate requires 3-5 cords of firewood per year.

Developing a Plan: Step-by-Step Approach

Create a detailed plan for your wood processing project.

Identify the Trees to Cut: Select trees that are dead, dying, or diseased.
Fell the Trees Safely: Use proper felling techniques to avoid accidents.
Limb and Buck the Trees: Limb the trees and cut them into manageable lengths.
Split the Wood: Split the wood into pieces that are appropriate for your wood stove.
Stack and Season the Wood: Stack and season the wood properly.

Data Point: Planning your wood processing project can reduce the time it takes to complete the project by as much as 20%.

By systematically addressing these potential issues, you’ll be well-equipped to diagnose and fix your Stihl MM55 tiller, getting it back to work in no time. Remember to consult your owner’s manual for specific instructions and safety precautions. With a little patience and the right knowledge, you can overcome these challenges and enjoy the benefits of a well-maintained machine.

Conclusion: Back to Tilling

Getting your Stihl MM55 tiller running again is a matter of systematic troubleshooting. Start with the fuel system, move to the ignition, check compression, examine the starting mechanism, and finally, look for mechanical issues. Remember to consult your owner’s manual for specific instructions and safety precautions. With a little patience and the right knowledge, you can overcome these challenges and enjoy the benefits of a well-maintained machine. Now, get back to tilling and make the most of your woodcutting and gardening endeavors!