String Trimmer Won't Start | Troubleshooting
String trimmers (also called weed eaters or line trimmers) mostly use two-stroke engines requiring oil/fuel mixture. When your trimmer won’t start, the causes usually involve fuel mix issues, carburetor problems or ignition failures. This guide covers systematic diagnosis for gas and battery string trimmers from Stihl, Echo, Husqvarna and other major brands.
Table of Contents
- Quick Diagnosis
- Two-Stroke Fuel Requirements
- Fuel System Problems
- Ignition System
- Compression Issues
- Battery Trimmer Issues
- Brand-Specific Problems
- Starting Procedure
Quick Diagnosis
Before tearing into the trimmer, run through these checks:
| Symptom | Likely Cause | Solution |
|---|---|---|
| No pull resistance | Engine seized | Major repair needed |
| Pulls but nothing | Fuel or spark | Check both systems |
| Fires then dies | Fuel delivery | Carburetor or fuel lines |
| Only runs on choke | Lean condition | Carburetor cleaning |
| Starts but bogs | Air/fuel restriction | Filters and carb |
[NEED REAL IMAGE: String trimmer diagnostic flowchart]
Two-Stroke Fuel Requirements
Two-stroke engines require oil mixed with gasoline. This is non-negotiable - straight gas destroys these engines within minutes.
Correct Mix Ratios
| Brand | Ratio | Oil per Gallon |
|---|---|---|
| Stihl | 50:1 | 2.6 oz |
| Echo | 50:1 | 2.6 oz |
| Husqvarna | 50:1 | 2.6 oz |
| Older models | 40:1 | 3.2 oz |
| Very old | 32:1 | 4 oz |
Always check your manual - wrong ratio causes problems either way:
- Too much oil: Fouled plug, carbon buildup, hard starting
- Too little oil: Engine seizure, scoring, destruction
Fuel Quality
Fresh fuel is critical. Two-stroke mix degrades faster than straight gasoline.
- Use fuel under 30 days old
- Ethanol-free fuel lasts longer
- Add stabilizer if storing over 30 days
- Never use E15 or higher ethanol content
For more on fuel problems, see our small engine fuel problems guide.
Fuel System Problems
The fuel system is responsible for 80%+ of no-start problems on string trimmers.
Carburetor Issues
Small two-stroke carburetors clog easily from old fuel varnish. Ethanol in modern gasoline makes this worse.
Symptoms:
- Cranks but won’t fire
- Starts then dies immediately
- Only runs on choke
- Surges or hunts
Quick test: Spray starting fluid into the carburetor intake.
- Fires briefly then dies = Carburetor problem
- Nothing at all = Spark or compression issue
Solutions:
- Remove and clean carburetor thoroughly
- Replace diaphragms and gaskets (they harden over time)
- Adjust mixture screws (L, H and idle)
- Replace carburetor if worn internally
See our how to clean a carburetor guide for step-by-step instructions.
For Stihl trimmers, check our Stihl trimmer problems guide.
Fuel Lines
Two-stroke trimmers have fuel pickup lines inside the tank that deteriorate from ethanol exposure.
Symptoms:
- Won’t prime (bulb stays soft)
- Loses prime quickly
- Fuel leaks from tank area
- Hard starting after sitting
Solution: Replace all fuel lines and the in-tank filter together. This is common maintenance on trimmers over 2-3 years old. See our fuel line replacement guide.
Fuel Filter
The weighted fuel filter sits inside the tank on the pickup line.
Symptoms of clogged filter:
- Hard starting
- Dies under load
- Fuel starvation at high RPM
Solution: Replace with the fuel lines - they typically fail together.
Primer Bulb
The primer bulb draws fuel from the tank into the carburetor.
Testing:
- Should pump firmly and return
- Fuel should be visible after 5-8 pumps
- Cracks or holes prevent proper priming
Solution: Replace primer bulb assembly.
[NEED REAL IMAGE: Trimmer fuel system components]
Ignition System
If fuel delivery is confirmed good, check the ignition system.
Spark Plug Testing
Procedure:
- Remove spark plug
- Reconnect plug wire
- Ground plug body against engine block
- Pull starter cord
- Watch for spark at electrode
Good spark: Bright blue, snaps audibly Weak spark: Orange/yellow, inconsistent No spark: Nothing visible
Spark Plug Specs
| Brand | Plug Type | Gap |
|---|---|---|
| Stihl | NGK BPMR7A | 0.020” |
| Echo | NGK BPMR8Y | 0.024” |
| Husqvarna | Champion RCJ7Y | 0.020” |
Two-stroke plugs foul more easily than four-stroke due to oil in the fuel. Replace annually or every 50 hours.
Ignition Coil
No spark with a good plug indicates coil failure.
Testing:
- Disconnect kill switch wire from coil
- Retest for spark
- Spark now = kill switch or wiring problem
- Still no spark = coil failure
Air gap: Set coil to flywheel gap at 0.010-0.014” (business card thickness works for rough setting).
Kill Switch
A shorted kill switch grounds the ignition, preventing spark.
Test: Disconnect kill switch wire from coil and test for spark. If spark returns, the switch or wiring is grounding.
Compression Issues
Low compression prevents starting regardless of fuel and spark condition.
Compression Test
Procedure:
- Remove spark plug
- Install compression gauge
- Pull starter cord 4-5 times
- Note highest reading
Specs:
- Normal: 90-120 PSI
- Marginal: 70-90 PSI (may start but run weak)
- Low: Under 70 PSI (won’t start reliably)
Low Compression Causes
- Worn piston rings
- Scored cylinder
- Damaged crankshaft seals
- Head gasket failure
Low compression on a two-stroke usually means engine replacement is more economical than repair.
Battery Trimmer Issues
Battery-powered trimmers have different failure modes.
Won’t Turn On
Check:
- Battery fully charged?
- Battery seated correctly?
- Safety trigger engaged?
- Error lights flashing?
Common fixes:
- Remove and reseat battery
- Clean battery contacts with alcohol
- Try different battery if available
Motor Doesn’t Spin
Causes:
- Debris jamming head
- Motor failure
- Controller failure
- Trigger switch failure
Test: Remove trimmer head and try running motor. If it spins freely, debris was the problem.
Short Runtime
Causes:
- Battery age (capacity decreases)
- Cold temperatures
- Constant maximum power use
- Thick vegetation
Solutions:
- Use variable speed when possible
- Keep battery warm before use
- Replace aging batteries (typically 3-5 year lifespan)
[NEED REAL IMAGE: Battery trimmer troubleshooting]
Brand-Specific Problems
Stihl Trimmers
Common issues on FS series:
- Fuel line deterioration (3-year service item)
- Zama carburetor needs diaphragm replacement
- Spark arrestor clogging reduces power
See our detailed Stihl trimmer problems guide.
Echo Trimmers
Common issues on SRM series:
- Fuel system maintenance needed every 2-3 years
- Speed-Feed head requires correct line loading
- Carburetor responds well to cleaning
See our Echo trimmer problems guide.
Husqvarna Trimmers
Common issues:
- Smart Start system requires correct procedure
- Air Injection system needs maintenance
- Fuel lines crack from ethanol exposure
See our Husqvarna trimmer problems guide.
Starting Procedure
Correct starting procedure prevents many problems and avoids flooding.
Cold Start
- Set choke to FULL (closed)
- Set throttle to START or fast idle
- Prime bulb 5-8 times (fuel should be visible)
- Pull starter until engine fires (may not run)
- Move choke to HALF
- Pull until engine runs
- Move choke to RUN when warm
Warm Start
- Choke OFF
- Throttle at idle
- Pull - should start in 1-2 pulls
Flooded Engine Recovery
Strong fuel smell indicates flooding.
- Remove spark plug
- Pull cord 10+ times to clear cylinder
- Let sit 5-10 minutes
- Reinstall plug (or use new one)
- Start with choke OFF, throttle FULL
- Pull until it fires
- Return to normal throttle
When to Replace vs Repair
Worth Repairing
- Carburetor cleaning/rebuild ($15-30)
- Fuel line replacement ($10-20)
- Spark plug ($5-8)
- Air filter ($8-15)
- Ignition coil ($25-50)
Consider Replacement
- Scored cylinder or worn piston
- Cracked crankcase
- Damaged crankshaft
- Multiple systems failing
- Trimmer over 8-10 years old
Related Guides
- How to Clean a Carburetor
- Stihl Trimmer Problems
- Echo Trimmer Problems
- Husqvarna Trimmer Problems
- Walbro Carburetor Repair
- Zama Carburetor Repair
- Fuel Line Replacement
- Small Engine Fuel Problems
Summary
String trimmer starting problems almost always come down to the fuel system. Old fuel mix gums up carburetors quickly. Replace fuel lines every 2-3 years and always use fresh 50:1 mix. If fuel and spark are both confirmed good, check compression - low compression means the engine is worn out. For battery trimmers, verify the battery is charged and seated properly. Most starting problems can be fixed for under $30 in parts.