This workshop manual is mainly intended for Husqvarna 50, 50 Special, 51, and 55 models, but certain chapters can also be used for other models. The manual gives information about how to repair the saw and how to use special tools. In order to make sure that the saw will function properly, always use Husqvarna original spare parts and accessories.
Page Technical Specifications Service Data Service Tools Safety Equipment Rewind Starter Ignition System Centrifugal Clutch Lubrication System, Oil Pump Anti-Vibration System Fuel System Exhaust System Cylinder & Piston Crankcase & Crankshaft...
Engine Displacement - cc (cu. in.) 49 (3.0) 51 (3.1) 51 (3.1) 53 (3.2) Bore - mm (in.) 44(1 .73) 45(1 .77) 45(1 .77) 46(1 .81 ) Stroke - mm (in.) 32(1 .26) 32(1 .26) 32(1 .26) 32(1 .26) Max. power of speed - rpm 8400 8400 9000...
Allen key, 4 mm 5025018-01 5025019-01 Allen key, 3 mm 5025022-01 Socket spanner, 8 mm Puller, complete 5025026-01 5025030-09 Crankshaft installation device, Model 50 serial # 9250857 and up 5025030-12 Crankshaft installation device Piston Stop 5025033-01 Pressure gauge 5025038-01 5025046-01 Sleeves, 2 pieces...
The chain brake system has a strong spring under tension. This spring activates the chain brake band when the chain brake is engaged. The spring is located under a cover plate that must be removed during servicing. Anytime the cover is removed and the spring is still under tension it may dislodge without warning.
Turn cover over. Using a suitable punch, drive out remaining half of pivot sleeve. Separate hand guard from cover. Remove the five screws and lift off the cover plate. CAUTION ! The spring is under tension. Always wear safety goggles for eye protection. Using the tip of a flat bladed screwdriver, carefully lift the end of spring out of the cover.
Turn the cover over and press out the brake bank retaining pin with a punch. Check the brake band for wear. Measure the thickness of the band. The original thickness of the brake band is 0.80 mm (0.031 in.). The brake band must be exchanged if the band is worn anywhere along its length to 0.65 mm (0.026 in.) or less.
Assemble the brake band and knee link. Insert the roll pin into the brake band so the open side of the pin is up as shown in photo. Insert the linkage and band assembly into the cover. Secure the knee link to the cover with the retaining pin. Slide the brake spring over the linkage spring post.
Press the front hand guard into position. Install the pivot sleeves and screw. Pull the hand guard back to disengage the brake. Mount the bar, chain and chain brake to the saw. Before running the saw, check the brake function by moving the hand guard forward until the brake engages.
Throttle Trigger Lockout Disassembly Press out the pin. On the 50 and 50 Special models, the throttle lockout lever has a small tab midway on the left side. Insert a small screwdriver between the lever and lever opening on the left side so that the tab will clear the handle then...
Drive out the pin for the throttle trigger. Raise the throttle trigger up into the handle and unhook the spring. Assembly Insert the spring down through the recess in the handle. Hook the spring onto throttle trigger. Align the pin hole in the trigger with the corresponding hole in the handle and install the pin.
Align the pin holes and drive in the pin. After assembly check throttle lockout function. It should not be possible to move the throttle trigger (accelerate the saw) without first depressing the lockout lever. NOTE ! On 51 and 55 models, the fast idle system is integrated into the choke system.
30 cm (12 inches) and hold starter pulley. Pull a loop of starter rope up between pulley and housing. On 50 and 50 Special models, engage rope in pulley notch. On 51 and 55 models, pull rope up between any notched section in pulley.
Assembly New starter rope length is 88 cm (34.5 inches). Tie a knot in one end of the rope. Insert the other end through the rope pulley first, then through the rope eyelet in the housing and the starter grip. Tie the rope at the starter grip end with a double loop knot.
Pull the starter cord up into the pulley notch and rotate the pulley clockwise two turns. NOTE ! Check that the starter pulley can be turned at least one half of a turn further when the starter cord is pulled to the end of rope travel. To install the starter assembly on crankcase, first pull the starter grip out approximately 15 cm (6 in.) then position the housing against the crankcase.
Ignition Ignition System These models are equipped with an electronic ignition system. One of the characteristics of this system is that the spark across the spark plug electrode gap has a higher energy and shorter burning time (arc duration time). When viewing the spark on a properly functioning ignition system, the spark will appear to be weaker compared to breaker point ignition systems due to the shorter arc duration time.
Attach the high tension lead to the spark plug. Ground the spark plug against the cylinder. With the-chain saw secure from movement, pull the starter quickly while watching for a spark across the spark plug electrode. If no spark is evident, retest with a new spark plug or test plug P/N 5027113-01.
Check the air gap between the ignition module and flywheel magnets. The air gap should be 0.3 mm (0.012 in.). The ignition air gap can be checked using air gap tool 5025134-02. If the ignition air gap requires adjustment, first loosen the ignition module retaining screws.
If the ignition module is new, it will be necessary to install a new spark plug connector spring on the high tension lead. Before installing the connector spring, first correctly route the high tension lead from the module to the spark plug.
Carefully pull the spark plug boot over the spring. Assemble the starter to the saw and check for spark as previously instructed in the Checking for Spark section. Flywheel Removal Remove the spark plug and thread the piston stop P/N 5025033-01 into the spark plug opening in the cylinder.
Hold onto the flywheel and lift the saw slightly above the work surface. Strike the tool squarely on its end with a hammer to free the flywheel from the crank- shaft. Remove the tool and flywheel. The starter pawls and springs are serviceable. To remove the starter pawls, position the flywheel with the starter pawls down on two pieces of wood so that the pawls are exposed.
On flywheels equipped with two crankshaft key-ways, use the key-way marked “5” for correct ignition timing. NOTE ! From serial number 9250857 the flywheel key is cast into the flywheel. Install the flywheel and tighten the flywheel retaining nut to 25-30 Nm (18.4-22.1 ft.lbs.) torque. Check the air gap between the flywheel magnets and the ignition module as instructed in the Ignition Module section.
Centrifugal Clutch Disassembly Remove the spark plug and thread the piston stop P/N 5025033-01 into the spark plug opening in the cylinder. NOTE ! Make sure that the piston stop is threaded completely into the cylinder. Rotate the clutch clockwise by hand until the piston contacts the piston stop.
To separate the clutch shoes and spring from the clutch hub first secure the clutch hub in a suitable vise. Pry one shoe out far enough to remove the spring. Check the clutch shoes for wear by measuring the material thickness at the shoes thinnest point. If the shoe is worn to less than 1 mm (0.040 in.) of material at the most worn part, the shoes must be exchanged.
On models equipped with a floating rim sprocket, carefully examine the rim sprocket and the sprocket splines on the clutch drum. On models equipped with a spur sprocket, check for saw chain drive link wear grooves around the spur sprocket on the clutch drum. An excessively worn rim or spur sprocket will cause premature wear of a new chain.
On models so equipped, assemble the rim sprocket to the clutch drum. Lightly lubricate the clutch drum needle bearing then slide the washer, needle bearing and clutch drum onto the crankshaft. Install the clutch hub and shoes. Tighten the clutch hub to 30-40 Nm (22.1 -29.5 ft.
Pump Disassembly Pour out the oil and clean the oil tank. Check the oil tank ventilation by slightly pressurizing the tank through the filler opening with compressed air. The oil tank vent is located at the top and to the front of the bar pad. Air should readily pass through the vent.
Using oil pump drive gear puller P/N 5025026-01 and sleeve P/N 5025099-01, remove the gear from the crankshaft. Separate and clean the oil pump components. Exchange worn or damaged parts. Assembly Thread the oil pump drive gear into the sleeve P/N 502 5099-01 until the end of the gear is flush with the end of the sleeve.
After removing the tool, make sure the drive gear is flush against the shoulder on crankshaft. install the oil suction tube and filter in the oil tank. Slip the pump piston into the pump housing. Fill onto pump cylinder with grease and slide the cylinder the piston.
Fill the space around the oil pump drive gear with grease. Install the pump assembly into the crankcase ensuring that the pump piston and the pump cylinder engage the corresponding pin and notch in the crank- case. Fit the rubber grommet around the oil discharge port in the pump cylinder.
Anti-Vibration System In order to prevent injuries in the operator’s hands and fingers caused by vibrations from cutting equipment and the oscillating internal parts of the engine, rubber elements isolate the cutting equipment and engine from the front and rear handles. To maintain the greatest possible protection from vibration during use, the rubber elements should be exchanged at least once a year, regardless of the amount of hours used.
Use a screwdriver to remove the rubber elements.
Disconnect the fuel hose at the carburetor and attach the pressure gauge P/N 5025038-01 to the hose. Pressurize the fuel tank to 50 kPa (7.2 psi) as indicated by the pressure gauge. The pressure should reduce to 20 kPa within 60 seconds (3 psi/60 sec.). The fuel tank ventilation should be serviced if the pressure reduction to 20 kPa takes longer than 60 seconds.
Assembly Install the fuel tank vent using a drift the same diameter as the vent tube. The remainder of assembly is the reverse of disassembly. NOTE ! The exposed length of the fuel tank hose measured between the fuel tank and the carburetor should be 115 mm (4 1/2 in.) Carburetor Removal...
Carburetor Operation The carburetor is a multi-position, integral diaphragm type which can be divided into three main sections, the metering section, the mixing section and the pumping section. The metering section consists of the metering diaphragm. the fuel nozzles and the fuel flow control functions. The mixing section consists of the venturi, where the fuel and air are mixed and where the throttle valve and (on the model 51 and 55) the choke valve are located.
Connect the pressure gauge P/N 5025038-01 to fuel inlet fitting on the carburetor. Immerse the carburetor in a suitable container of fuel. Pressurize the carburetor to 50 kPa (7.2 psi). No air leakage is permitted. If air leakage is observed, remove the screw retaining inlet lever pivot shaft and carefully remove the lever shaft, spring and needle valve.
Remove the one screw retaining the fuel pump cover and remove the cover. Carefully remove the fuel pump diaphragm gasket and fuel pump diaphragm. Using a pick, carefully remove the fuel inlet screen.
Remove the low and high speed mixture screws. Exchange the mixture screws if the screw tips are bent or damaged. If the carburetor is very dirty it will be necessary to clean the fuel passages located underneath the fuel circuit plate ( Walbro WA) or the welch plug(s) ( Walbro WT).
Clean the carburetor body and blow out all passages and channels with low pressure compressed air. Check the throttle and (if equipped) choke valves and shafts for wear and exchange if necessary. Assembly New gaskets, diaphragms, fuel inlet screen and welch plugs must be used to ensure correct carburetor function.
Begin assembly with the fuel metering side. Install the fuel mixture adjustment screws. Depending on the carburetor type, either install the fuel circuit plate and gasket or the welch plugs. Use a suitable drift to install welch plugs. Install the needle valve, spring, lever and shaft.
After assembly, pressure test the carburetor for air leaks. Pressure test at 50 kPa (7.2 psi). No air leaks are permitted. On carburetors equipped with choke valves, check that the levers of the fast idle system are not worn and that they have positive engagement.
Carburetor Installation Check the fuel pump pulse passage in the intake manifold to ensure that it is open and clean. Check the pulse passage seal and rubber intake tube on the cylinder. Exchange the seal and tube if the material has hardened or is damaged.
Mount the carburetor to the air filter plate and insert the retaining screws. Attach the throttle and (if equipped) the choke linkage. Install the carburetor. Make sure the gasket between the carburetor and manifold is installed correctly. Tighten the retaining screws evenly. Install the air filter plate to crankcase retaining screw.
Final Adjustment NOTE ! A suitable two-stroke single cylinder tachometer such as the one supplied by Husqvarna P/N 5027114-01 must be used to ensure correct engine operating rpm’s preventing possible unnecessary and costly engine damage. Start the saw and allow the engine to reach normal operating temperature.
Begin with the low speed mixture screw. Using carburetor screwdriver P/N 5016002-02, turn the L screw in until the engine rpm’s just start to decrease and note the screw- driver position. This is referred to as the lean drop setting. Now turn the L screw out until the engine rpm’s just start to decrease again and note the screwdriver position.
Trouble-shooting Chart Start 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Idle Acceleration, Deceleration Start Acceleration, deceleration Hard starting K. Will not accelerate Fuel dripping from carburetor L.
Exhaust System The law in some countries requires that the exhaust system be equipped with a spark arrester screen. Check the screen and clean or exchange it if necessary. A clogged or even partially clogged spark arrester screen will cause poor engine performance and overheating resulting in potential piston an cylinder damage.
Cylinder& Piston Disassembly Remove the cylinder cover and air filter. Clean around the cylinder base to prevent dirt and debris from entering the crankcase when the cylinder is removed. Remove the carburetor and intake manifold as described in section 10. Fuel System. Remove the spark plug and the exhaust system.
Remove the wrist pin needle bearing from the connecting rod. Fault Remedy Broken cooling fins, damaged In serious cases - change the threads or broken screw at cylinder. Repair the thread the exhaust port. HeIi-Coil-inserts. Seizure marks in the cylinder Grind the damaged spot with a fine wall (especially at the exhaust emery cloth so that the adhered port).
Position the piston over the connecting rod with arrow on the piston crown pointing towards the exhaust side. Align the piston pin boss holes with the connecting rod pin hole and install the pin. Carefully install the wrist pin clips ensuring that the clips are fully seated in the retaining grooves.
Crankcase &Crankshaft Changing a Bar Stud Empty and rinse out the oil tank. Using a drift, tap the bar stud into the oil tank. Extract the stud through the oil filler opening. Insert a smaller diameter wire through the bar stud hole in the crankcase and out the oil filler opening.
Mount tool P/N 5025161-01 on the clutch side of the crankshaft and crankcase. Separate the crankcase halves by pressing the crankshaft out of the right crank- case half. Mount the tool on the left crankcase half and press out the crankshaft. Remove the crankshaft bearings from the crankcase halves by heating up the crankcase around the crank- shaft bearing then pressing the bearing out of the case.
Checking the Crankshaft The crankshaft is not serviceable and must be exchanged if defective. Some crankshafts may have a slight blue discoloration around the crank pin. This is a normal condition caused by the heat treatment around the crank pin hole during assembly. Very carefully inspect the connecting rod big-end (crank pin) bearing.
Check the small-end of the connecting rod. If seizure marks or discolored spots are evident in the bearing seat, the crankshaft must be replaced. Assembly Thoroughly clean all parts before assembly. Make sure the crankcase half mating surfaces are clean and free of all gasket material.
Using assembly tool P/N 5025030-10 pull the crankshaft into the ignition side crankcase half. Make sure the crankshaft is flush against the bearing. Position the crankcase gasket onto the ignition side crankcase half using a light coat of grease to hold it in place.
Always use new crankshaft seals. Begin with the ignition side. Apply grease to the crankshaft seal lip and carefully slide the seal over the end of the crankshaft. The open side of the seal must face inward. Drive the seal into the crankcase using tool P/N 5025079-01 until the seal is flush with the crankcase.