Stihl HT 75 Repair Manual

The Stihl HT 75 is a professional-grade pole pruner designed for tree maintenance and pruning at heights. This manual provides comprehensive repair, maintenance, and troubleshooting information for the HT 75 powerhead, including its engine, fuel system, ignition, cutting attachment, and pole assembly. Below are key sections covering safety, specifications, disassembly, component repair, assembly, and troubleshooting procedures.

Safety Precautions

Before performing any repair, ensure the engine is off and cool. Disconnect the spark plug boot to prevent accidental starting. Work in a clean, well-ventilated area. Use proper personal protective equipment (PPE): safety glasses, gloves, and hearing protection.

Hazard	Precaution
Fuel Fire/Explosion	Drain fuel tank before repair; no smoking or open flames
Sharp Components	Handle cutting attachment with care; wear cut-resistant gloves
Spring Tension	Be aware of tension in starter assembly and throttle linkage
Chemical Exposure	Avoid skin contact with fuel, oil, and cleaning solvents
Electrical Shock	Disconnect spark plug before working on ignition system

Specifications & Components

Key components: 2-stroke engine, diaphragm carburetor, electronic ignition (CDI), centrifugal clutch, gearhead, drive shaft, and cutting attachment (saw chain or brush cutter).

• Engine: 27.2 cc, air-cooled, single-cylinder 2-stroke
• Fuel: Gasoline/oil mix (50:1 with STIHL HP Ultra oil)
• Powerhead Weight: Approx. 6.8 lbs (3.1 kg)
• Ignition: Electronic, non-adjustable
• Carburetor: STIHL diaphragm carburetor with H and L adjustment screws
• Clutch: Centrifugal, engages at ~2,800 RPM
• Drive: Flexible drive shaft inside pole
• Cutting Attachment: Interchangeable (guide bar/chain or brush cutter)

Tools Required

1. STIHL special tool set (spark plug wrench, flywheel holder, clutch tool)
2. Torx screwdrivers (T15, T20, T27)
3. Socket set (8mm, 10mm, 13mm)
4. Flathead and Phillips screwdrivers
5. Compression tester
6. Feeler gauges
7. Carburetor adjustment tool
8. Clean rags and parts cleaning solvent

NOTE: Use genuine STIHL tools for best results and to avoid damage.

Disassembly Procedure

Follow steps in order to avoid damage.

1. Remove cutting attachment and pole assembly.
2. Drain fuel and oil from tanks.
3. Remove starter assembly (pull off slowly, noting spring orientation).
4. Remove air filter cover and air filter.
5. Remove carburetor cover and disconnect throttle linkage.
6. Remove fuel tank by unscrewing mounting screws.
7. Remove cylinder cover and muffler.
8. Remove clutch assembly using clutch tool.
9. Remove flywheel using flywheel puller.

WARNING! Label or photograph parts and connections for reassembly reference.

Engine Repair

Common engine issues: low compression, piston scoring, crankshaft seal leaks.

Component	Inspection	Repair
Cylinder & Piston	Check for scoring, carbon buildup	Clean with solvent; replace if scored
Piston Rings	Check ring gap and freedom in groove	Replace if worn or stuck
Crankshaft Seals	Inspect for leaks (air/fuel)	Replace seals using seal driver
Crankcase	Check for cracks or air leaks	Seal with gasket maker if needed
Bearings	Check for smooth rotation, no play	Replace if rough or loose

Compression Test: Should be above 120 psi for proper operation.

Fuel System Repair

Components: fuel tank, fuel lines, filter, carburetor, and primer bulb.

1. Inspect fuel lines for cracks or hardening; replace if necessary.
2. Replace fuel filter inside tank.
3. Clean carburetor: disassemble, soak in cleaner, blow out passages with compressed air.
4. Replace diaphragm and gaskets in carburetor kit.
5. Check primer bulb for cracks; replace if leaking.
6. Reassemble carburetor and set initial screw settings: H and L screws 1 turn open from lightly seated.

Tip: Use only STIHL OEM carburetor kits for proper fit and performance.

Ignition System Repair

Components: spark plug, ignition coil (CDI), stop switch, and wiring.

• Spark Plug: Check gap (0.020 in / 0.5 mm); replace if fouled or cracked.
• Ignition Coil: Check air gap (0.3 mm / 0.012 in) between coil legs and flywheel.
• Stop Switch: Test continuity; replace if circuit is always open or closed.
• Wiring: Inspect for breaks or shorts; repair with heat-shrink connectors.

Testing: Use spark tester to verify strong spark. If no spark, replace coil.

Cutting Attachment Repair

For guide bar and chain attachment:

1. Remove side cover and bar nuts.
2. Inspect guide bar for straightness and groove wear; dress or replace if worn.
3. Check chain for sharpness and damaged cutters; sharpen or replace.
4. Inspect sprocket at gearhead for wear; replace if teeth are hooked.
5. Lubricate drive shaft splines with high-quality grease.
6. Adjust chain tension: snug but pullable by hand along bar.

WARNING! Always wear gloves when handling chain; ensure engine is off.

Pole & Drive Shaft Repair

Components: pole sections, flexible drive shaft, gearhead, and bearing points.

• Drive Shaft: Inspect for kinks or wear; replace if damaged.
• Pole Sections: Check for cracks or bends; replace if compromised.
• Gearhead: Check gears for wear; replace gear set if teeth are chipped.
• Bearings: Lubricate upper and lower bearings with grease.
• Connections: Ensure locking collars and pins are secure.

Assembly: Apply light grease to drive shaft before inserting into pole.

Reassembly & Adjustment

Reverse disassembly steps, noting the following:

1. Install new gaskets and seals where removed.
2. Torque clutch to 40 Nm (30 ft-lb) using clutch tool.
3. Set flywheel key properly; torque flywheel nut to 25 Nm (18 ft-lb).
4. Adjust carburetor: Start engine, warm up, adjust L screw for smooth idle, H screw for max RPM (13,500 RPM no-load).
5. Check throttle linkage for smooth operation and full travel.
6. Test stop switch function.
7. Reinstall cutting attachment and adjust chain tension.

Final Check: Run engine and check for leaks, unusual noises, and proper cutting attachment operation.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, fuel issue, compression	Check spark, fuel delivery, compression; clean carburetor
Engine starts then dies	Fuel restriction, air leak	Replace fuel filter, check fuel lines, inspect crankcase seals
Loss of power	Clogged air filter, exhaust screen, carb adjustment	Clean/replace air filter, clean spark arrestor, adjust carburetor
Cutting attachment doesn't turn	Clutch worn, drive shaft broken	Inspect clutch shoes; replace drive shaft if damaged
Excessive vibration	Unbalanced cutting attachment, bent drive shaft	Balance attachment; inspect and replace drive shaft

Service Support: For genuine STIHL parts and technical assistance, contact authorized STIHL dealer or visit www.stihlusa.com.