Start by locating the drive shaft assembly – it connects the engine to the cutting head and is critical for power transmission. On the FS 91 R, this shaft is housed in a curved or straight rod, depending on the model variant, and is secured with snap rings at both ends. Check for wear on the splines if the trimmer loses power during operation. Replacement shafts are available under part numbers 4140-700-3005 (straight) and 4140-700-3006 (curved), matching OEM specifications.
Fuel system repairs require precise attention to the carburetor and its linkages. The FS 91 R uses a Zama C1Q-S63 carburetor, identified by its black plastic housing and three adjustment screws (H, L, and idle). Before disassembly, mark the positions of these screws relative to the body–factory settings are 1.25 turns out for both H and L screws. A common failure point is the fuel diaphragm; replace it with part 1132-120-2001 if cracks or deformation are visible under magnification.
The cutting head mechanism relies on a dual-line bump-feed system. To access the spool, remove the retaining nut (M14x1.5 thread) using a 22mm socket. The spool (4140-120-5306) should rotate freely when lifted–if it binds, inspect the drive dog for stripped teeth. For automatic line feeding, ensure the spring (part 4140-000-3505) is tensioned correctly; improper tension leads to uneven line dispensing or premature breakage.
Exhaust components are often overlooked but critical for performance. The FS 91 R’s muffler features an integrated spark arrestor screen (part 4140-701-3301). Remove carbon buildup with a brass wire brush–never use steel wool, as metallic particles can contaminate the cylinder. If the screen is torn, replace it immediately to prevent debris from entering the combustion chamber, which can score the piston (part 1131-030-1300).
For electrical diagnostics, focus on the ignition module (part 4140-400-3011). Test resistance between the coil leads–values should read for primary windings and 5–15 kohms for secondary. If the trimmer fails to start but cranks, check the spark plug (NGK CMR6H) gap (0.6–0.7mm) and electrode condition. Replace plugs every 20 operating hours or if electrode erosion exceeds 0.5mm depth.
FS 91 R Maintenance Blueprint: Exact Component Locations and Replacement Tips
Begin disassembly by removing the spark plug (BOSCH WSR 6 F or equivalent) using a 19mm socket to prevent accidental starts. The air filter housing sits above the carburetor–release two 10mm bolts and lift the cover; replace the foam element (OEM #4229 120 0610) if debris exceeds 0.5mm thickness. Next, detach the fuel tank by unscrewing three 8mm bolts–label fuel lines A (supply) and B (return) with masking tape to avoid misconnection.
| Component | OEM Code | Tool Required | Torque (Nm) |
|---|---|---|---|
| Flywheel nut | 4220 120 0600 | 22mm socket + flywheel holder | 40-45 |
| Muffler screws | 4228 180 0305 (x2) | 4mm hex key | 8-10 |
| Clutch drum | 4137 190 0600 | 17mm spanner | 25-30 |
For the cutting head, lock the shaft by inserting a 6mm rod through the housing hole while rotating the assembly to align the clutch drum’s cutout. Remove the M14x1.5 star nut using a drift punch–replace the automatic line feed spring (OEM #4148 710 6700) if it fails to retract the cutting cord fully. Clean the centrifugal clutch pads (NGK BP6ES or DENSO W20FP) with brake cleaner; grooves deeper than 0.8mm require replacement (OEM #4137 190 1210).
Critical Wear Indicators and Procurement
Inspect the crankshaft seals (P/N 1128 100 3800) for leaks at the PTO side–dark oil traces indicate failure. Replace the recoil starter pawls (OEM #4130 190 3502) if engagement takes over 3 pulls; lubricate the spring with molybdenum grease (NLGI #2). For carburetor adjustments, use a tachometer: set idle to 2,800 RPM (±200) and high-speed mixture to 9,500 RPM (±500) with the air screw 1.5 turns out from lightly seated. Order consumables from dealers listing real-time stock–OEM filters and gaskets are batch-coded (e.g., “23/08”) for warranty claims.
Finding Fuel System Elements on Your Trimmer’s Schematic
Begin by identifying the fuel tank–typically marked with a rectangular or oval shape near the rear of the technical drawing, often labeled with a fuel symbol (⛽) or text like “reservoir” or “fuel chamber.” The tank’s position relative to the carburetor (a smaller, box-like component connected via hoses) helps confirm its location, as the two must align for proper flow.
Trace the fuel line from the tank’s outlet port (usually a small nipple or fitting) toward the carburetor inlet. The schematic will show a thin, curved line–sometimes color-coded red or black–indicating the path. Look for arrowheads or dashed segments to denote direction; these reveal whether the line runs above, below, or through other assemblies like the engine housing.
Key Fuel System Markings to Decode
Locate the primer bulb, a circular or oval feature on the schematic, often positioned between the tank and carburetor. It should be labeled with terms like “choke” or “pump” and connected by two distinct lines: one returning to the tank (vent line) and one advancing fuel to the carburetor (supply line). Misidentifying these can lead to incorrect reassembly.
Examine the carburetor’s internal symbols–small rectangles or channels labeled with numbers (e.g., “1,” “2,” or “A,” “B”) represent jets, diaphragms, or adjustment screws. A “main jet” usually sits near the fuel inlet, while an “idle jet” or “mixture screw” appears closer to the engine side. These are critical for tuning but rarely stand out without close inspection.
The fuel filter–often a tiny basket or mesh symbol–attaches to the tank’s interior or the end of the supply line. Schematics show it as a trapezoidal or domed shape with a dot pattern, labeled “filter” or “strainer.” If absent, check the tank’s exploded view, where it’s listed as a separate numbered item. Overlooking this component risks clogging the carburetor during operation.
Cross-reference any ambiguous symbols with the legend or component list adjacent to the schematic. Fuel-related parts are typically grouped under sections like “Fuel Supply” or “Carburetion,” with corresponding numbers linking to the image. Colors, dashed borders, or shading on the drawing indicate assembly order–for example, a red outline may signal a critical seal or gasket location.
Locating the Trimmer Head and Line Renewal Components in FS 91 R Schematics
First, pinpoint the trimmer head assembly at the base of the shaft in the exploded view. This unit typically appears as a cylindrical housing with two protruding eyelets–these secure the cutting line during operation. Reference the item labeled near the bottom right of the blueprint, often marked with a number like “4228 700 4001” for rapid cross-checking.
Verify the line feed mechanism by tracing the path from the head upward. The spool (commonly tagged “4128 709 5400”) sits inside the housing, wound with pre-cut monofilament. Ensure compatibility: the FS 91 R accepts 2.4 mm or 2.7 mm line diameters–check the original spool edge for precise sizing before sourcing replacements.
Critical Wear Elements
The bump knob (item “4128 702 7200”) protrudes below the spool; this plastic component endures repeated impact against hard surfaces. Inspect for cracks or rounded edges–either signal imminent failure. Replace if thread grooves appear stripped, as binding prevents automatic line advancement.
Examine the drive shaft adapter (referenced as “4137 710 7000”), connecting the engine to the head. Splines wear predictably after 150–200 operating hours; a stripped adapter prevents torque transfer, halting rotation. Compare spline integrity against a new unit using calipers–tolerance exceeds 0.3 mm indicates replacement.
Line Renewal Protocol
Remove the cutting head by depressing the two tabs on opposite sides while rotating counterclockwise. Disassemble the spool by pulling the inner hub upward–note the clockwise arrow molded into the casing dictates rewind direction. Load fresh line starting from the center hub outward, ensuring equal lengths (approximately 20 cm) extend through each eyelet.
Secure the spool by aligning its internal spring with the bump knob shaft, then press down until the hub locks audibly. Reassemble the housing rotating clockwise until seated firmly–tighten by hand only, excessive force risks thread cross-threading. Trim protruding line flush with the eyelets using shears for clean cuts; uneven edges accelerate fraying.
Test automatic feed by operating at full throttle against a soft target–line should advance incrementally under light resistance. Manual tap feed requires moderate impact to the head against grass; sluggish response suggests spool binding or line tangling–disassemble and recheck winding symmetry.
Locating the Engine Housing and Air Filter Assembly in the FS 91 R Breakdown
On the exploded schematic, the engine housing sits at position 21, identified as the *crankcase cover*–a die-cast magnesium component directly bolted to the powerhead’s main body. Adjacent to it, the air filter assembly occupies positions 38 (filter base), 39 (filter element), and 40 (filter cover), forming a compact unit mounted above the carburetor intake. For precise removal, note the three M6 screws securing the cover and the single locating pin that aligns the base; mismatching these during reassembly will disrupt airflow calibration.
Key Reference Points in the Schematic
To avoid confusion, cross-reference the numeric labels with the color-coded sections: the housing is part of the powertrain group (shaded blue), while the filter assembly falls under the air intake group (green). If replacing the filter element, also inspect the sealing gasket (position 41)–deformation here causes unmetered air leaks, leading to erratic idle or fuel mixture issues. Always torque the cover screws to 9 Nm in a star pattern to prevent warping.