Fuel not reaching the carburettor: how to diagnose

If a brush cutter cranks, has spark, but simply won’t fire — or fires briefly on primer fuel and then dies — the fault is very often upstream of the carburettor rather than inside it. Four parts do all the work of getting fuel from tank to carb, and checking them in the right order usually finds the culprit in under twenty minutes.

Why fuel stops getting through

The fuel path on a typical 2-stroke brush cutter is short: tank, pickup body with a filter, a line to the carburettor, a pump diaphragm inside the carb that draws fuel using crankcase pulses, and a vented cap that lets air back into the tank as fuel is drawn out. A fault at any one of these stops fuel dead, and each has a distinct symptom that narrows things down quickly.

Cause	Typical symptom	How common
Blocked fuel filter or pickup	Starts on primer fuel, dies once that’s used up; weak or no draw on the bulb	Very common
Kinked or perished fuel line	Visible pinch point, or line collapses under suction; cracking when flexed	Common
Blocked tank vent	Runs briefly then dies, restarts after loosening the cap, gets worse as tank empties	Common, often missed
Faulty carb pump diaphragm	No fuel draw at all despite good filter, line and vent; primer bulb doesn’t hold prime	Less common, but the one people skip to too early

The test order

Work through these in sequence rather than jumping straight to the carburettor — most delivery faults sit upstream of it, and pulling the carb apart first just adds a reassembly step to a job that didn’t need it.

1. Check the primer bulb and visible line for obvious damage, cracking, or air bubbles that don’t clear after several primes.
2. Pull the line off the carb inlet and check for actual flow with the tank above the fitting.
3. Trace and inspect the fuel filter and pickup body inside the tank.
4. Check the tank vent by running with the cap loosened.
5. Only then test the carburettor’s pump side — the diaphragm, gasket, and pulse (impulse) line from the crankcase.

Always work on a cool engine with the machine in the open air, away from any ignition source. Fuel system checks involve open fuel — no smoking, no sparks, and keep a rag or catch tray under any fitting you disconnect.

Step 1: blocked fuel filter or pickup

This is the single most common cause, especially after a machine has sat over winter with fuel left in the tank. The filter is a fine mesh or sintered-bronze element on the end of the pickup line, usually 80-120 micron mesh, sitting loose in the bottom of the tank so it can reach fuel at low levels. Ethanol-blended fuel that’s been sitting for more than a month or two breaks down and leaves a gummy varnish that clogs the mesh from the outside in.

How to check:

1. Drain the tank into a suitable container (worth doing anyway — old fuel is often part of the problem).
2. Fish the pickup body out through the filler neck using a bent wire hook or needle-nose pliers.
3. Look at the filter itself: a healthy one is a uniform pale colour and lets light through the mesh; a clogged one looks dark, sticky, or has visible debris caked on.
4. Try blowing gently through the pickup line from the carb end — a clear pickup passes air easily; a blocked one resists or won’t pass air at all.

The fix: the filter is a cheap, direct-fit part — pull it off the pickup body’s barbed end and press the new one on until it seats fully. While the tank’s open, wipe out any sediment or varnish residue from the bottom with a lint-free rag before refilling with fresh, correctly mixed fuel.

Step 2: kinked or perished fuel line

A line can look fine from the outside and still be the problem — a kink inside a tight bend, or a line that’s gone soft and collapses under the suction the pump diaphragm generates, doesn’t always show as an obvious external fault.

How to check:

1. Trace the full length of the line from tank to carb, checking for any tight bend, especially where it’s routed past a bracket, clip, or through a grommet.
2. Flex the line gently between finger and thumb — pliable and slightly glossy is healthy; stiff, brittle, or showing fine surface cracks means it’s due for replacement.
3. With the line disconnected at the carb and the tank above it, check for a steady gravity flow — a trickle or none at all with the filter confirmed clear points to a line restriction.
4. Check both ends push fully onto their fittings — a line that’s pulled slightly loose can draw air instead of fuel, which starves the carb just as effectively as a blockage.

The fix: replace with fuel-rated line (not generic vinyl tubing) cut to the same length, routed exactly as the old one to avoid reintroducing a kink at the same point.

Step 3: blocked tank vent

Easy to miss because the symptom looks intermittent: the engine runs for anywhere from a few seconds to a couple of minutes, then dies as if starved, and often restarts once you loosen the fuel cap. What’s happening is a vacuum building up inside a sealed tank as fuel is drawn out — with nowhere for replacement air to get in, the tank eventually pulls hard enough on the diaphragm’s supply side that it can’t draw fuel through.

How to check:

1. Run the engine until it dies or bogs down in the way you’ve been seeing.
2. Loosen the fuel cap a quarter turn (don’t remove it) and try restarting immediately.
3. If it fires straight back up and runs normally, the vent is confirmed as the fault.
4. Inspect the cap’s vent valve for dirt, a stuck reed, or a perished seal — most vented caps have a small one-way valve that lets air in but not fuel out.

The fix: clean the vent passage with compressed air or a fine wire if it’s just clogged with dirt; replace the cap outright if the valve itself is stuck or damaged, since the valve isn’t usually a separate serviceable part.

Step 4: faulty carb pump diaphragm

If the filter, line, and vent are all confirmed good and there’s still no fuel draw, the fault is inside the carburettor itself — specifically the pump diaphragm, a thin rubber or synthetic membrane roughly 0.2-0.3mm thick that flexes with crankcase pressure pulses (delivered through a small impulse line, usually 2-3mm bore) to draw fuel from the tank and push it toward the metering chamber.

Symptoms specific to this fault:

• Primer bulb won’t hold a prime, or fuel drains straight back out of it.
• No fuel movement at all when cranking, even with a confirmed-clear filter and line.
• The impulse line itself is cracked, split, or has come off its fitting — check this before condemning the diaphragm, since it’s a much cheaper and quicker fix.
• Diaphragm has gone stiff, torn, or is stuck to its gasket — visible once the pump cover is off.

The fix: a carburettor gasket and diaphragm kit (pump diaphragm, metering diaphragm, and gaskets together) is the standard repair — remove the pump cover, lift out the old diaphragm and gasket, fit the new ones the correct way round (check orientation against the old parts before discarding them), and reassemble evenly to avoid pinching the new diaphragm. This is a fiddly job with small parts on an uneven surface, so work over a tray and don’t rush the reassembly.

Don’t force the pump cover screws down unevenly — a diaphragm pinched or torn on refitting causes the exact same no-fuel symptom you just fixed, and you’ll be doing the job twice.

When to call a dealer

Hand it to a dealer if you’ve confirmed filter, line, and vent are all good and the diaphragm still isn’t drawing fuel after a fresh kit — that points to a worn pump chamber or a cracked carb body, which needs bench diagnosis. Also take it in if the tank itself shows internal contamination (rust, heavy varnish, or debris you can’t fully clean out) or if you’re not confident stripping a carburettor pump cover without a parts diagram to hand.

Quick checklist before you call it fixed:

• Fresh fuel filter fitted and old fuel drained, not just topped up.
• Full length of the line checked for kinks and flexed for perishing — replaced if in doubt.
• Cap-loosening test done to rule out (or confirm) a vent blockage.
• Impulse line checked before assuming the diaphragm itself has failed.
• Pump cover reassembled evenly, diaphragm and gaskets in the correct orientation.
• Fresh, correctly mixed fuel used for the retest — don’t put old fuel back through a system you’ve just cleaned.