Chainsaw Troubleshooting: Hard Starting, No Power, Oil Leaks and Overheating — Diagnosed in Order

Chainsaw troubleshooting for four common faults: hard starting, no power, oil leaks and overheating. What each symptom means, and the order to check things in.

How to troubleshoot without guessing

The three questions

Every petrol engine needs three things to run, and they arrive in a fixed order:

1. Spark — the ignition makes a strong, well-timed spark at the plug.
2. Fuel — the right fuel/air mixture reaches the cylinder.
3. Compression — the piston and rings can squeeze it.

Test them in that order, because that is the order of cheapness. A plug costs less than a carburettor diaphragm; a diaphragm costs less than a piston and cylinder. Owners who start by turning carburettor screws spend an afternoon chasing a fault that a new spark plug would have fixed.

The fork: engine or cutting attachment?

Before you touch the engine, answer one question: does the problem change when the bar and chain come off?

• Remove the bar and chain, then run the saw. If it revs cleanly and holds its note with the bar off, but bogs when the chain is fitted, the fault is in the cutting attachment — a dull chain, a tight chain, a worn sprocket, a dragging brake or a slipping clutch. That is the cheapest possible outcome, and it is the outcome in a surprising number of "no power" cases.
• If it behaves the same with the bar off, the fault is in the powerhead. Continue with the engine-side checks.

Four free tools

You do not need a workshop to diagnose most of this:

• Your nose. Smell the fuel. Fresh mix smells sharp; stale fuel smells flat, like varnish or old paint thinner. A plug that smells strongly of petrol means flooding.
• Your eyes. Read the spark plug. Its colour tells you whether the saw is running rich, lean or correctly, and carbon on the exhaust screen tells you the same story at the other end of the engine.
• Your hand on the starter. A saw with healthy compression has a hard, springy resistance and a distinct "bump" as it comes up on the stroke. A saw that pulls over easily and spins is telling you the top end is tired.
• A flat surface and a clean rag. Most leaks are found by cleaning the saw properly, not by looking harder.

Safety before you start

Everything in this guide is done with the engine off, the spark plug cap pulled off, and the saw on a flat surface. Two specific warnings worth stating plainly:

Fault 1: It will not start

First, separate the three kinds of "will not start"

They point at completely different causes:

The three kinds of will-not-start, and where each one points

What actually happens	What it means	Where to look
Dead. No fire at all, no attempt to run.	No spark, or no fuel reaching the cylinder at all.	Spark test, then fuel delivery.
Fires on choke, dies the moment you release the choke.	The engine is getting fuel but not enough — it is running lean.	Fuel filter and line, tank vent, carburettor diaphragm, air leak.
Runs for a few minutes, then dies. Restarts later.	Fuel supply or ignition breaking down with heat.	Tank vent (does it restart when you loosen the fuel cap?), ignition coil, overheating.

Then work the cold-start path in order

1. Is the fuel fresh, and is it mixed correctly?

This is the first check, not the last, because it is the most common cause and the cheapest to fix.

• Fuel mix is perishable. Ethanol-blended petrol should be used within about 30 days of mixing, and the clock starts when you mix it, not when you next open the can.
• Most current saws specify 50:1 — 2.6 fl oz of two-stroke oil per US gallon, or 20 mL per litre. Older and budget saws often want 40:1, 32:1 or 25:1. Your own manual or the sticker on the housing is the only authority.
• Use petrol rated 89 octane or higher with no more than 10% ethanol. Never E15 or E85.
• Straight petrol with no oil at all is the fastest way to destroy a two-stroke. If there is any chance the tank was filled from an unmixed can, stop and drain it rather than pulling the starter again.

Fresh, correctly mixed fuel from a labelled can eliminates roughly half of all no-start faults before you have taken a single panel off.

2. Is it your starting procedure?

More saws are "faulty" because of the procedure than because of the machine. Confirm all of it:

• Cold start: ignition on, choke fully closed, throttle lock engaged if the saw has one, primer bulb pressed until it fills with fuel if fitted. Pull briskly.
• The saw should fire — not run — within about three or four pulls on full choke. As soon as it coughs, release the choke and continue. Leaving it on full choke after it has fired is how you flood it.
• Warm start: no choke, often with the throttle lock engaged.
• A decompression valve (fitted to larger saws) makes pulling easier. If it is stuck open, compression is lost and the saw will not start — check that it closes and returns.
• Pull the cord like you mean it. A slow, half-hearted pull often will not generate a reliable spark at low cranking speed. Let the cord retract fully before each pull, and use short, sharp strokes.

3. Is it flooded?

The symptom is unmistakable: the pull gets harder and the saw smells strongly of petrol. Pull the plug — it will be wet with fuel.

To clear it: switch the ignition off, hold the throttle wide open, and pull the starter several times. Leave the plug out while you do it, dry the plug electrodes, then refit and try again with the choke off. If the plug is soaked, replace it — a fuel-wet plug frequently will not fire again even after being wiped.

4. Is there spark?

With the engine off and the plug removed:

• Refit the plug into its boot, and hold the plug body firmly against bare metal on the cylinder head — the plug's metal base against the metal, not your hand.
• Pull the starter and watch the gap. You want a strong, bright blue-white spark that jumps the gap crisply.
• A weak, orange or intermittent spark points to a worn plug, a failing coil, a dirty coil air gap, or a fault in the lead.
• While you are there, check the electrode: wet and black means flooding or a rich condition; dry and greyish-tan means the plug is fine and the problem is fuel delivery; a plug that is fouled with hard black carbon gets replaced, not cleaned a fifth time.
• Check the gap and reset it if needed — commonly around 0.020 in / 0.5 mm on small two-strokes. Check your manual.

If there is no spark at all, eliminate the cheap causes in this order: a new plug (correctly gapped), the plug boot and lead, then the stop switch and its wiring — a shorted kill-switch circuit is a classic "no spark" fault that looks exactly like a failed coil. Disconnecting the stop switch wire at the coil is the standard way to rule it out.

5. Is fuel actually reaching the cylinder?

The test: with the plug out, pull the starter a few times and look into the plug hole. You should see or smell a fine mist of fuel, and the plug should come out damp with petrol after a few pulls on choke.

If the plug stays bone dry, the fault is in fuel delivery. Work through it in this order:

1. Fuel in the tank? Obvious, and it happens to everyone once.
2. The in-tank fuel filter — a clogged filter starves the engine. It is a cheap consumable, replaced at least annually, and every three months if you run ethanol-blended fuel.
3. The fuel line — ethanol makes fuel lines go hard, then crack. A cracked line lets air in instead of fuel, and the engine runs lean or not at all. Squeeze the line: if it has lost its flexibility, replace it.
4. The fuel tank vent — a blocked vent creates a vacuum in the tank and stops the flow. The giveaway symptom: the saw runs perfectly for a few minutes, then dies, and restarts immediately after you loosen the fuel cap. Clear the vent rather than working around it.
5. The primer bulb and its lines (if fitted) — a cracked bulb or line is an air leak straight into the fuel supply.
6. The carburettor — a stiff or perished metering diaphragm, a stuck needle valve, or resinous deposits left behind by evaporated stale fuel. This is where a spring no-start turns up most often, and it is the reason stale fuel left in the tank over winter is the most common cause of a saw that will not start in spring.

6. Is there compression?

Pull the plug and pull the starter slowly by hand. If the resistance is mushy and the saw spins over with almost no effort, or you can hear air hissing past the piston, the top end is worn or scored.

A compression gauge gives a number: a healthy small two-stroke typically reads somewhere around 130 to 150 psi; below roughly 110 psi the top end is tired and the saw will be hard to start and down on power. Hold the throttle wide open while you crank it, and note that gauges vary between brands — the most useful comparison is against a saw you know to be healthy.

Hard starting with low compression is a mechanical fault, not a tuning fault. No carburettor adjustment will fix a worn ring or a scored piston.

7. Air and exhaust

The two remaining restrictions:

• Air filter — packed with fine dust, it starves the engine. Clean it properly or replace it (see the section on power loss below). Never run the saw without one fitted, even "just for one cut".
• Spark arrestor screen — a carbon-blocked screen in the muffler chokes the engine. A plugged exhaust port does the same thing more slowly. Both are covered in Fault 2.

Hot start failure is a different fault entirely

A saw that starts from cold on the third pull but refuses to restart when hot is not suffering from a starting problem. It is telling you one of two things, and the spark plug tells you which:

Reading the plug after a hot no-start

Plug condition after a hot no-start	Diagnosis	What to do
Wet with fuel	Flooding. The engine is getting more fuel than it can burn.	Starting procedure, rich carburettor setting, or a worn needle valve. Clear it as described above.
Bone dry	No fuel reaching the cylinder. The engine is running lean, or the fuel is boiling in the line and carburettor before it arrives.	This is the dangerous one. Suspect overheating, vapour lock, a restricted fuel supply, or an air leak. Stop and diagnose rather than pulling until it fires.

A saw that is only hard to restart when hot is a saw running too hot. Go straight to Fault 4.

Fault 2: It starts but has no power

First, decide what "no power" actually means

Owners use the same words for three different faults, and they have different causes:

What you observe, and where the fault actually is

What you actually observe	Where the fault is
The engine will not rev out, even with no load. It idles, but bogs or stumbles the moment you squeeze the trigger.	Airflow, exhaust, or fuel delivery.
The engine revs hard and sounds right, but the chain will not pull itself into the wood.	The cutting attachment: chain, sprocket, bar, clutch.
It cuts well for a few minutes, then fades and loses its note.	Heat-related: a lean condition, a failing coil, or a fuel supply that cannot keep up.

Get this classification right before you touch anything, and you will skip about half of the possible causes.

Engine-side causes, in the order you should check them

1. The air filter

The single most common cause of a lazy, underpowered saw. Fine dust packed into the pleats starves the engine of air and makes the mixture rich, which shows up as a sooty plug, poor fuel economy and dull throttle response.

• Pleated paper elements: tap them out from the inside, or blow compressed air from the inside out at low pressure. Never blow from the outside in — that drives the dirt into the pleats — and never wash a paper element.
• Felt, fabric or oiled-foam elements: wash in warm water with a little dish soap, squeeze gently, then squeeze dry. Never wring or twist — that tears the material. Air-dry completely, then re-oil lightly if it is an oiled type. An over-oiled foam filter is nearly as bad as a clogged one, because the excess oil pulls dust into the element.

An element that stays dark grey inside after cleaning is finished. Replace it.

2. The spark arrestor screen and the exhaust port

This is the classic cause of a saw that starts fine but will not pull revs — it idles happily and bogs the instant you load it.

The spark arrestor is a fine mesh screen in the muffler, and it cokes up with carbon. Remove the muffler cover, lift the screen out, and clean it with a wire brush. A screen that is solid with carbon gets replaced, not scraped. While the cover is off, check the exhaust port in the cylinder for hard carbon buildup, and check the muffler gasket and fasteners — oily black streaks around the exhaust port mean a blown gasket or a loose muffler.

3. Fuel delivery

If the saw runs lean at full throttle — losing power and surging rather than bogging — the engine is not getting enough fuel:

• The in-tank fuel filter, partially clogged. Cheap, replace it.
• The fuel line, gone hard and possibly collapsing under suction, or cracked and drawing air. Ethanol-blended petrol is the usual culprit.
• The tank vent, partially blocked. The saw cuts fine until the vacuum builds, then fades.
• The carburettor metering diaphragm, gone stiff with age, so it cannot deliver enough fuel at high demand. It is a service part, and it is the reason a saw that was fine in autumn feels flat in spring.

4. The carburettor setting — and its limits

Most modern saws ship with limiter caps on the adjustment screws, which deliberately restrict how far you can go. That is an emissions and engine-protection decision, not an obstacle to be defeated with a pair of pliers.

• Set the idle so the saw ticks over steadily without the chain turning. A saw whose chain turns at idle is not safe to use — the chain should be stationary whenever you are off the trigger.
• Adjust the carburettor last, after the air filter, the plug and the exhaust are known good. A clogged filter or a tired plug mimics a carburettor fault, and you will spend an hour turning the wrong screw.

5. An air leak — the fault that destroys engines

An air leak leans the mixture regardless of how the carburettor is set. The symptoms: the saw will not hold a steady idle, it races or "hangs" at idle, the idle screw needs an unusually long turn to keep it running, and it is hard to restart when hot.

Suspect the intake boot, the impulse line, the manifold gaskets and the crankshaft seals. Diagnosis needs a pressure or vacuum test, which is a workshop job. No carburettor adjustment can fix an air leak — and continuing to use a saw with one is how a "no power" complaint becomes a top-end rebuild.

6. Compression

Low compression robs power across the whole rev range. Symptoms: easy pulling, a fast and hollow-sounding starter, poor idling, and difficulty starting when hot. Test as described above. Below roughly 110 psi, the top end needs work.

7. The ignition coil

A coil that breaks down under load or with heat is easy to miss, because it sparks perfectly when you test it cold. The pattern is a saw that runs well from cold, fades after ten or fifteen minutes, then recovers after it cools. The plug may look perfect. If you have eliminated airflow, exhaust, fuel and compression and the fault is strictly temperature-related, this is the remaining suspect.

Cutting-attachment causes

If the engine sounds healthy but the chain will not cut, work down this list:

1. A dull chain. The single most common cause. A sharp chain produces chips. A dull chain produces fine powder. If you are pushing the saw to make it cut, or it pulls to one side in the kerf, stop and file. A dull chain does not "get through the last bit" — it loads the engine, overheats the bar, and turns a safe low-kickback chain into one that grabs.

2. Depth gauges set too high. This is the most commonly missed cause of a chain that "looks sharp and cuts nothing". As cutters are filed back, the rakers must come down with them. If the cutters have been filed but the rakers have not, the chain rides on the rakers and takes nothing but dust. Most chains want 0.025 in (0.65 mm); most low-kickback garden chains want 0.018 in (0.45 mm). A depth-gauge file guide turns this into a two-minute job. Match the round file to the chain's pitch:

File and depth-gauge sizing by chain pitch

Chain pitch	Round file diameter	Depth-gauge setting
1/4 in and 3/8 in Low Profile	5/32 in (4.0 mm)	0.018 in (0.45 mm) on low-kickback chains
.325 in	3/16 in (4.8 mm)	0.025 in (0.65 mm)
3/8 in (full size) and .404 in	7/32 in (5.5 mm)	0.025 in (0.65 mm)

3. Chain tension. A chain that is too tight drags on the bar and the sprocket, loads the crankshaft bearing, and steals a noticeable amount of power before it does any cutting. Set it cold: the chain should hang just below the bar's underside at mid-bar and lift about 1/8 to 1/4 in (3 to 6 mm) off the top rail with light finger pressure, with the drive links still seated in the groove.

4. The drive sprocket. A worn sprocket wears into hooked shapes — the industry nickname is "shark fins" — and a hooked sprocket will not drive a chain properly. It also ruins a new chain within a couple of tanks. The rule of thumb is one sprocket per two or three chains.

5. The bar. A pinched or worn groove, burred rails, or a seized nose sprocket all drag on the chain and steal power. If the nose sprocket is stiff, gritty or rocks side to side, the bar is finished.

6. The chain brake. A brake that is partially applied — a sticky mechanism, a glazed band, or a hand guard that does not fully return — drags the chain constantly and heats everything up. Test it: brake released, the chain should pull around the bar freely by hand.

7. The clutch. A centrifugal clutch with weak or broken springs slips under load. The signature: the engine revs freely but the chain stalls or creeps, and the clutch drum gets very hot and smells burnt. A chain that turns at idle means the idle is too high or the clutch is dragging — either way it is a safety fault, not a tuning preference.

Fault 3: It leaks oil

First, establish which oil is leaking

A petrol chainsaw has no crankcase full of oil — the lubricant for the engine arrives dissolved in the fuel. So when a saw leaks, it can only be one of two things, and they have completely different urgency:

Telling bar oil from fuel mix

What is leaking	How to tell	How urgent
Bar and chain oil	Amber to dark, slippery, no petrol smell. It gathers under the clutch cover, along the bar and in the saw's storage spot.	Almost always harmless. Sometimes a cheap seal to replace.
Fuel mix	Smells strongly of petrol, feels thin, evaporates rather than sitting. You will also usually notice the level dropping in the fuel tank.	A fire risk. Never normal. Fix it before you store or use the saw again.

If you cannot tell by sight, leave a clean white sheet of card under the saw overnight, then smell the stain. Petrol smell settles it immediately.

Bar oil: what is actually normal

Most "my chainsaw leaks oil" complaints are not faults at all, and it is worth understanding why before you start replacing parts.

A chainsaw's bar-oil system is deliberately not sealed. The oil reservoir is vented so that the pump can draw oil without pulling a vacuum, and there is no seal at the point where the oil is fed onto the bar pad — the oil is pumped into an open channel and carried away by the moving chain. That means a certain amount of oil is always sitting in an unsealed path, and gravity eventually wins.

Three everyday reasons a perfectly healthy saw leaves a spot:

1. It was stored full. A tank filled to the brim has oil sitting above the feed point, so it weeps out overnight.
2. It was used outdoors in the cold and then brought somewhere warm. The oil and the air above it expand with the temperature change, and the expanding volume pushes oil out through the vent and the feed channel. This is the classic "it leaked the day I brought it into the garage" case.
3. The oiler is set high. Saws with an adjustable oiler, set to maximum for a long bar, deliver more oil than the chain can carry away, and the surplus collects under the cover and drips.

Bar oil leaks: normal versus fault

Bar-oil leaks: normal versus fault

What you see	Verdict
A small puddle under the clutch cover after a full day of cutting	Normal. Surplus oil thrown off the chain collects there.
A few spots on the shelf, especially in the first day after the saw comes indoors	Normal. Thermal expansion and a full tank.
Oil slowly darkening a patch on the shelf over weeks with the saw unused	Normal weeping through an unsealed feed path. Empty the tank for long storage and it stops.
A steady drip while the saw sits level, with the oil tank draining over a day or two	A seal or gasket has failed. See below.
Oil appearing at the fuel cap area, or in the air filter box	Not bar oil — you have a fuel or oiler-line problem. Investigate immediately.

The real bar-oil faults

When a saw genuinely leaks bar oil, it is one of a short list of inexpensive parts:

• The oil tank cap and its gasket or O-ring. The most common genuine cause, and the cheapest fix. A hardened, flattened or split gasket will not seal, and the saw will weep from the cap every time it is warm. Caps are also easily cross-threaded, which produces the same result.
• A cracked or porous oil tank. Less common, usually after a drop or long exposure. Clean the tank area thoroughly, dry it, and look for the trail starting at a seam.
• The oil pump's O-ring and seals. The pump is driven off the crankshaft, and the seals between the pump and the tank and between the pump and the bar pad harden with age. A failed pump seal leaks continuously while the engine runs, which is why the puddle grows during use rather than sitting still.
• The oil line or pickup hose in the tank. Chafing, hardening or a loose connection lets oil bypass the pump and drain out of the tank.
• A worn oiler body or pump. When it wears, oil bypasses internally and drips under the clutch cover.
• A missing or damaged gasket at the bar pad, or deep scores in the bar pad face, which let oil escape around the bar rather than travelling down the groove.

How to find any leak in ten minutes

Reading the saw will not work — by the time oil has spread, it has spread everywhere. This procedure will:

1. Empty the tank and clean the saw thoroughly with a degreaser or warm soapy water. Get the area around the cap, the tank seam, the pump area and the bar pad genuinely clean, then dry it completely.
2. Refill the oil tank — about half full is plenty.
3. Dust the suspect areas with talcum powder or plain flour. A light dusting, not a layer. This is the trick that makes the leak visible.
4. Let it stand for an hour, then check. Look for the first dark trail. That is your leak, and where it starts is where the fault is — ignore where it ends up.
5. If nothing shows when it is standing, run the saw for a minute or two (bar off is fine) and check again. A leak that only appears while running is a pump-seal leak, not a cap leak.

Fuel leaks are a different category

The usual sources:

• The fuel cap gasket — same failure mode as the oil cap, higher stakes.
• A cracked or hardened fuel line — ethanol-blended petrol is the usual accelerator. Squeeze the line: if it has lost its flexibility, replace it.
• A cracked primer bulb or its lines, if fitted.
• The tank vent, which can leak if it is damaged by over-enthusiastic clearing.
• Carburettor gaskets and the fuel inlet needle, which leak when the needle does not seat and the carburettor floods — often first noticed as a strong petrol smell and a saw that is hard to start.

One more caution: do not try to seal a leaking gasket with silicone or a general-purpose sealer. Petrol dissolves most sealants, and the debris ends up inside the carburettor. Gaskets, O-rings and fuel hose cost very little and are the correct repair.

Fault 4: It runs hot

There are two overheating problems, and they get confused constantly

Engine overheating versus bar overheating

What is overheating	The evidence	The usual cause
The engine	Power fades, the exhaust note goes thin and shrieking, the idle races, the saw will not restart when hot, and the engine smells hot and dry.	Restricted cooling airflow, or a lean mixture.
The bar and chain	The bar rails turn blue or purple, the paint burns off the bar, the bar oil smokes, the nose sprocket seizes, and the chain stretches fast.	No bar oil reaching the chain, a chain that is too tight, or a worn bar.

They are related — a dragging bar loads the engine and makes it run hotter — but they are diagnosed separately. Check the bar first, because it is cheaper and it is often the real fault behind a machine that "feels gutless and smells hot".

Engine overheating: the causes, ranked by how often they are the answer

1. Blocked cooling fins and air intake

The cooling fins are the saw's radiator, and this is the single most common cause of an overheating chainsaw.

An air-cooled two-stroke dissipates its heat through fins on the cylinder and through the flywheel's fan, which draws air across them through the starter housing. A blanket of oil-soaked sawdust between the fins insulates the cylinder instead of cooling it. The engine runs hot, loses power, and eventually scores a piston — and the operator rarely notices anything wrong beyond "it's not cutting well today".

Clean the fins with a stiff brush and a wooden stick or a soft scraper — not a screwdriver, which nicks the fins and creates hot spots. Clean the air intake grille on the starter housing at the same time, and remember the sawdust that collects under the clutch cover and around the cylinder is oil-soaked and clings far harder than dry sawdust.

2. Running lean

A lean mixture — too much air, not enough fuel — burns hotter and destroys pistons. It is the most dangerous of the overheating causes because the saw can feel crisp and powerful right up until it seizes. Sources of an unintended lean condition:

• An air leak. Crank seals, the intake boot, the impulse line, manifold gaskets, or a loose spark plug. The signature is an idle that races or hangs, a saw that will not settle, and a hot restart failure. No carburettor adjustment can fix an air leak.
• A carburettor set too lean, usually in pursuit of more power.
• A restricted fuel supply, so the engine is starved at full demand: a clogged in-tank filter, a collapsing fuel line, or a blocked tank vent.
• Too little oil in the mix. Oil carries away a share of the piston's heat as well as lubricating it. A badly under-oiled mix both raises friction and reduces cooling.

3. Stale or separated fuel

Ethanol-blended petrol absorbs water, and when it takes on enough, the fuel undergoes phase separation: the ethanol-and-water mixture drops out of suspension and sinks, taking a share of the oil with it. A separated mix no longer delivers an even oil ratio no matter how hard you shake it, so the engine can run on a layer that is nearly straight petrol. It also burns poorly and can lean out.

4. A restricted exhaust

A carbon-blocked spark arrestor screen or a coked-up exhaust port traps hot gases in the cylinder. It feels like a power loss first, and it becomes a heat problem second.

5. A dull chain

This is the heat problem owners never connect to the engine. A dull chain does not cut; you push it through the wood instead, which holds the engine at full throttle under maximum load for minutes at a time with the chain barely moving — the worst possible combination for an air-cooled engine with no airflow over it while it is stationary against a log.

6. A chain that is too tight

A tight chain drags on the bar and the nose sprocket, which loads the crankshaft and the clutch drum, and the heat travels back into the powerhead. It also overheats the bar.

7. Long full-throttle work in high ambient temperature

An air-cooled two-stroke has a heat budget. Cut hard in the middle of a hot afternoon with a dull chain and a clogged air filter, and that budget is gone. Take a break, let the saw idle, and clear the fins before the next tank.

8. The wrong spark plug heat range

Rare, but real — an incorrect plug runs hotter than the engine was designed for. Fit the plug type your manual specifies.

Bar overheating: the causes

• No bar oil reaching the chain. Test it: run the saw at full throttle and point the bar tip a few inches from a light-coloured surface — a sawn log end, a piece of card, a concrete slab. You should see a fine line of oil thrown off within a second or two. If the bar stays dry, check in this order: the oil holes and groove in the bar (the most common blockage), the oiler port on the saw body, the pickup filter in the tank, and the oiler adjustment screw.
• The wrong oil. Bar oil is tackified so it stays on a chain travelling at speed. Motor oil flings off, so the bar runs dry exactly when it is working hardest — which is why a bar's paint burns off and the nose sprocket seizes. Used motor oil does the same while carrying abrasives into the groove.
• A tight chain, a pinched groove, or a seized nose sprocket. All three generate friction heat in the bar itself.
• A dull chain. Dull cutters rub rather than cut, and the friction goes straight into the bar.

What overheating feels like before it seizes

Worth memorising, because the window to save the engine is short:

1. Power fades at full throttle. The saw starts to feel tired rather than sharp.
2. The exhaust note goes thin and shrieking, and the engine may surge or hunt rather than holding a steady note.
3. The idle races or hangs, and the saw is reluctant to come back down. This is the lean signature.
4. It will not restart when hot, and the plug comes out dry. That is vapour forming in the fuel line and carburettor before the fuel can reach the cylinder.
5. It stops and will not turn over at all. By this point the piston has probably welded itself to the cylinder wall.

What to do when a saw gets hot

• Ease off the throttle and let it idle for half a minute before switching off. Never shut down straight from full throttle — let the airflow do its job and avoid the heat soak that cooks the top end after the engine stops.
• Let it cool completely before you refuel or remove the plug. Petrol on a hot engine, or a plug thread pulled out of a hot alloy head, are both avoidable accidents.

• Do not "clear it out" by holding full throttle when it is already lean. That is how a saving turn of the screw becomes a new piston.

The master symptom table

Where several faults share a symptom, this is the order to work in.

Which fault a shared symptom points to

Symptom	Fault 1: starting	Fault 2: power	Fault 3: leaks	Fault 4: heat
Fires on choke, dies when the choke comes off	Lean — fuel delivery or air leak	—	—	—
Runs a few minutes, dies, restarts when the cap is loosened	Blocked tank vent	Blocked tank vent	—	—
Starts fine, will not pull revs under load	—	Spark arrestor / exhaust, then air filter	—	Restricted exhaust
Revs freely, chain will not cut	—	Dull chain, depth gauges, sprocket, clutch	—	Dull chain
Pulls over too easily	Low compression	Low compression	—	—
Idle races or hangs, will not settle	Air leak	Air leak	—	Air leak — stop using the saw
Idles perfectly, chain turns at idle	Idle too high or clutch dragging	Clutch fault	—	—
Hard to restart when hot	Flooded (wet plug) or lean (dry plug)	—	—	Overheating — this is the classic heat symptom
Bar rails blue, paint burnt off	—	No bar oil, chain too tight	Oil not reaching the bar	Bar overheating
Power fades after ten or fifteen minutes and recovers when cold	—	Ignition coil breaking down with heat	—	Cooling airflow or a lean condition
Strong petrol smell, wet plug, hard starting	Flooding — carburettor needle not seating	—	Fuel leak — fire risk	—

The ten-minute roadside diagnosis

If you are standing in front of a saw that will not behave, this is the sequence that finds most faults with nothing but the saw's own toolkit.

1. Smell the fuel. Stale? Wrongly mixed? Straight petrol? This alone explains a large share of faults. Drain and refill if there is any doubt.
2. Pull the plug and read it. Wet and black, dry and grey, or blistered white? The colour tells you whether the engine is flooding, running correctly, or running lean.
3. Test the spark with the plug grounded against the cylinder. Strong blue-white or weak orange?
4. Look into the plug hole after a few pulls on choke. Damp mist means fuel is arriving; bone dry means the fault is in fuel delivery.
5. Clean the air filter and check the cooling fins. Five minutes, and it eliminates the two most common causes of both power loss and overheating.
6. Pull the bar and chain off and run the saw. Clean revs mean the fault is in the cutting attachment, not the engine.
7. Feel the compression by pulling the starter slowly. Mushy means the top end, and the session is over.
8. Only now consider the carburettor screws — and only after you have ruled out an air leak.

Seven things not to do

1. Do not use starting fluid or ether. It contains no lubricant. On an engine lubricated only by the oil dissolved in its fuel, it washes the oil film off the bore and can score a cylinder in seconds.

2. Do not turn the carburettor screws before checking the air filter, plug, exhaust and air leaks. A clogged filter and a tired plug both imitate a carburettor fault, and you will lean the saw out while chasing a problem that was never there. This is how a running saw becomes a scored one.

3. Do not remove the air filter to "get more power". It leans the mixture, which is the fastest route to overheating and piston damage.

4. Do not run a dull chain to finish the job. It loads the engine, overheats the bar, and makes you push — and pushing is what turns a routine cut into a kickback.

5. Do not keep pulling the starter on a saw that has stopped itself. If it stopped while hot without being switched off, treat it as a potential seizure and diagnose before you crank it again.

6. Do not pour water on a hot engine or bar. Thermal shock cracks castings and warps bars.

7. Do not seal a leak with silicone or general-purpose sealant. Petrol dissolves it and the debris ends up in the carburettor. Replace the gasket or the hose.

When to stop and take it to a workshop

Most of this guide is deliberately within reach of a home mechanic. These jobs are not:

• An air leak. The intake boot, impulse line, manifold gaskets and crank seals need a pressure or vacuum test to diagnose, and they are the leading cause of a destroyed top end.
• Low compression or a scored piston. A top-end rebuild is a genuine engine job, and continuing to run the saw makes it worse with every tank.
• A crankshaft or bearing rattle, or a clutch drum that will not stop dragging.
• An oiler that still does not deliver after the bar groove, oil holes, pickup filter and adjustment have been checked.
• Ignition faults beyond the plug and the stop switch — coils and their air gaps are set with a feeler gauge and are easy to get wrong.
• The chain brake does not latch, or engages slowly. Do not keep using the saw while you think about it.
• The saw is under warranty. Unauthorised disassembly can affect your coverage, and an engine failure showing the signature of a fuel or lubrication fault is a warranty question rather than a repair.

Frequently asked questions