If the cutting head creeps round while the engine is just ticking over, or the engine stalls every time you let go of the trigger, the fault is almost always idle speed rather than anything more serious. The T screw sets it, and getting it right takes about five minutes once the engine is warm.
What the T screw actually does
The T screw (sometimes marked LA, or just “idle”) is a mechanical stop, not a fuel adjustment. It sits against the throttle linkage and controls how far the throttle plate is allowed to close when the trigger is released - turn it in and the plate rests further open, raising idle rpm; turn it out and the plate closes more, lowering idle rpm. It has no effect on the fuel-to-air ratio at all. That job belongs to the L and H mixture screws, which are a separate adjustment covered in our carburettor screws and carburettor adjustment guides - this article stays strictly on idle speed.
Because T only moves a mechanical stop, it can’t fix a mixture problem. If idle is rough, surges, or dies the moment you touch the throttle, winding T up as a workaround just masks a lean or rich L circuit and usually makes the underlying problem worse, not better.
Why the target matters
A correctly set idle does two jobs at once: it keeps the engine running smoothly without you touching the trigger, and it stays low enough that the centrifugal clutch stays fully disengaged. On most handheld two-stroke brush cutters, that clutch typically starts to bite somewhere in the 3,600-4,200 rpm range, so there needs to be real margin between idle and engagement.
| Idle rpm | What happens |
|---|---|
| Below ~2,600 rpm | Engine hesitates, hunts, or stalls at idle, especially when cold or with any load from a dirty filter |
| ~2,800-3,000 rpm | Target range for most handheld units - stable idle, head and blade fully stationary |
| Above ~3,400 rpm | Idle creeps toward clutch engagement; head or blade may start to rotate with the trigger released |
Figures vary a little by make and engine size, so check the shroud sticker or manual if your machine states a different target - but 2,800-3,000 rpm is a reliable starting point across the vast majority of handheld two-stroke units.
Before you touch the screw
- Warm the engine for 2-3 minutes at a fast idle or light throttle. A cold engine idles differently to a warm one, and any adjustment made cold will drift once it heats up.
- Confirm the air filter is clean. A clogged filter richens the mixture and can make idle hunt regardless of where T is set, leading you to chase a fault that isn’t there.
- Check the throttle cable has a touch of freeplay - roughly 1-2 mm of slack at the trigger - rather than being pulled tight at rest. A cable with no slack holds the throttle open slightly and no amount of T adjustment will bring idle down properly.
- Make sure the spark plug is clean and gapped correctly (typically around 0.5 mm); a fouled plug makes the engine idle unevenly and muddies your read on whether T is actually the problem.
Step-by-step: setting idle speed
- With the engine warm and running, note how it currently idles - stable, hunting, too fast, or too slow.
- Locate the T screw, usually a small slotted screw near the carburettor body, close to where the throttle cable or linkage attaches.
- Turn the screw in small increments - an eighth of a turn at a time - clockwise to raise idle, anticlockwise to lower it.
- After each adjustment, let the engine settle for a few seconds before judging the result; idle speed lags slightly behind the screw movement.
- Aim for a steady, even idle in the 2,800-3,000 rpm range. If you have a tachometer, use it; if not, listen for a smooth, consistent note with no surging.
Safety note. After every adjustment, physically look at the cutting head or blade for several seconds with the engine idling. If it rotates at all - even a slow creep - the idle is set too high or the clutch needs inspection. Never assume it’s fine because the engine “sounds” normal; check by eye every time, with your body clear of the cutting path.
- Once idle sounds and looks correct, blip the throttle open and let it snap back to idle two or three times. It should return to the same steady speed each time, with the head stopping cleanly and promptly.
If idle won’t behave, it usually isn’t T
Some symptoms look like an idle-speed problem but actually point elsewhere, and winding T further in either direction won’t fix them:
| Symptom | Likely real cause |
|---|---|
| Idle hunts or surges even after T is reset | L mixture screw too lean or too rich - a mixture issue, not a speed issue |
| Head keeps spinning even with idle rpm confirmed low | Worn or glazed clutch shoes/drum, not needing more speed adjustment |
| Idle fine when cold, creeps up once warm | Cable freeplay tightening as components expand, or a mixture drifting lean with heat |
| Engine won’t hold any idle at all | Air leak at the intake boot or crankcase seal, well beyond what T can compensate for |
If you’ve set T correctly to the 2,800-3,000 rpm range and any of these symptoms persist, the fix lies with the mixture screws, the clutch, or an air/fuel leak - not with further turns of the idle stop.
When to call a dealer
- The head still rotates at idle after T is turned all the way to its lowest safe setting - this points to a worn or contaminated clutch, which needs inspection rather than a speed adjustment.
- Idle rpm won’t settle anywhere, hunting constantly regardless of T position, air filter, or plug condition - likely an intake air leak or a diaphragm fault inside the carburettor.
- You don’t have a way to check rpm and the head’s movement at idle is genuinely borderline - a dealer can confirm the correct speed with a tachometer in a few minutes, and it’s cheap peace of mind on a machine that spins a cutting attachment.
- The idle screw feels loose, won’t hold its position, or the linkage behind it looks worn or bent - this is a mechanical repair, not a tuning job.
Quick checklist
- Engine warmed up for 2-3 minutes before adjusting
- Air filter clean, plug gapped correctly (~0.5 mm)
- Throttle cable has 1-2 mm of freeplay at the trigger
- T adjusted in small increments, checking result after each
- Idle settles smoothly around 2,800-3,000 rpm
- Head or blade confirmed stationary by eye, every time
- Throttle snaps back to a clean, consistent idle after blipping