Tecumseh 2 Cycle Small Engine Repair Manual

This manual covers the following basic type or model numbers dependent on age of product: AH520, AH600, AV520, AV600, HSK600, TVS600. This manual covered many engines under an Old form of Identification which will need to be reviewed as well. TYPE / SPECIFICATION NUMBER 638-670 1398-1642 and Craftsman 200 Series Models. 2-CYCLE ENGINES T E C H N I C I A N ’ S H A N D B O O K TECUMSEH

i Contents PRIMER BULB (DIAPHRAGM CARBURETOR) ................................................ 11 CARBURETOR CHECK VALVE ......................... 11 CARBURETOR SERVICE PROCEDURE ......... 12 EMISSIONIZED DIAPHRAGM CARBURETION 13 OUTBOARD CARBURETORS .............................. 13 OUTBOARD CONTROL PANEL ........................ 14 CARBURETOR ADJUSTMENTS .......................... 15 IDLE SPEED ADJUSTMENT ............................. 15 FLOAT TYPE-FIXED MAIN, IDLE ADJUST ...... 15 DIAPHRAGM - SINGLE AND DUAL ADJUSTMENT .................................................. 15 TROUBLESHOOTING CARBURETION ............... 16 FLOAT CARBURETOR CHECKS ...................... 16 CHECKS FOR DIAPHRAGM ............................. 16 STANDARD SERVICE CARBURETORS .............. 17 FUEL FITTING .................................................... 17 INLET FUEL FITTING ......................................... 17 CHOKE SHAFT ................................................... 17 THROTTLE LEVER ................................................ 17 IDLE SPEED ADJUSTMENT SCREW .................. 17 GOVERNORS AND LINKAGE .............................. 18 AIR VANE GOVERNORS ................................... 18 OPERATION ........................................................ 18 LINKAGE INSTALLATION ..................................... 18 REWIND STARTERS, ELECTRIC STARTERS AND ALTERNATORS ............................................. 20 REWIND STARTERS .......................................... 20 COMPONENTS ................................................... 20 ROPE SERVICE ................................................. 20 STYLIZED REWIND STARTER ......................... 21 VERTICAL PULL STARTER HORIZONTAL ENGAGEMENT TYPE ........................................ 22 STYLIZED REWIND STARTER WITH PLASTIC RETAINER ........................................................ 23 VERTICAL PULL STARTER, VERTICAL ENGAGEMENT TYPE ...................................... 24 REWIND STARTER MOUNTED IN HOUSING . 25 ELECTRIC STARTERS .......................................... 26 DRIVE ASSEMBLY SERVICE ............................ 26 STARTER CHECKING AND SERVICE ............. 27 CHECK FIELD ..................................................... 27 ARMATURE CHECK ........................................... 27 TROUBLESHOOTING STARTERS ....................... 27 ALTERNATORS ...................................................... 28 CHECKING THE SYSTEM ................................. 28 FLYWHEEL (INSIDE-EDGE) BRAKE SYSTEM ... 29 INSIDE EDGE SYSTEM ..................................... 29 COMPONENTS ................................................... 29 SERVICE ............................................................. 29 CONTROL CABLE .............................................. 30 Page Page GENERAL INFORMATION ...................................... 1 ENGINE IDENTIFICATION ................................... 1 INTERPRETATION OF MODEL NUMBER .......... 1 ENGINE CARE ......................................................... 2 SHORT BLOCKS .................................................. 2 STORAGE: ............................................................ 2 TUNE-UP PROCEDURE ...................................... 3 EXHAUST PORT CLEANING .............................. 3 2-CYCLE THEORY OF OPERATION ...................... 4 OPERATION OF PISTON PORT STYLE ............ 4 OPERATION OF REED PORTED STYLE WITH LOOP SCAVENGING .............................. 4 TERMS .................................................................. 4 EXHAUST OR SCAVENGE PHASE .................... 4 PORTS .................................................................. 4 EXHAUST PORTS ................................................ 4 THIRD PORT ........................................................ 4 REED VALVE ........................................................ 4 LUBRICATION ...................................................... 4 AIR CLEANERS, CARBURETORS, GOVERNORS AND LINKAGE ......................................................... 5 AIR CLEANERS .................................................... 5 POLYURETHANE-TYPE AIR CLEANER ............ 5 KLEEN-AIREﬁ SYSTEM ...................................... 5 PAPER-TYPE AIR CLEANER SERVICE ............. 5 GENERAL CARBURETOR INFORMATION ........... 6 CARBURETOR IDENTIFICATION ....................... 6 FLOAT-TYPE CARBURETORS ........................... 6 DIAPHRAGM (PRESSURE DIFFERENTIAL) CARBURETORS ................................................... 6 OPERATION .......................................................... 7 CARBURETOR SERVICE .................................... 7 THROTTLE ............................................................ 7 CHOKE .................................................................. 8 ADJUSTING SCREWS ......................................... 8 FUEL BOWL RETAINING NUT ............................ 8 CARBURETOR VENTING .................................... 8 HIGH TILT FLOAT STYLE .................................... 8 FUEL BOWL .......................................................... 8 FLOAT ................................................................... 8 INLET NEEDLE AND SEAT (FLOAT CARBURETORS) .............................................. 9 NEEDLE AND SEAT OPO-OFF TEST ................ 9 FLOAT HEIGHT SETTINGS ................................. 9 PRIMER BULB .................................................... 10 CLEANING CARBURETOR BODY .................... 10 DIAPHRAGMS .................................................... 10 INLET NEEDLE AND SEAT ............................... 11 FUEL INLET FITTING ......................................... 11 Copyright © 1994 by Tecumseh Products Company All rights reserved. No part of this book may be reproduced or transmitted, in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission in writing from Tecumseh Products Company Training Department Manager.

ii IGNITION ................................................................. 31 IGNITION OPERATION (Pre- 1985 Production) ........................................................ 31 MAGNETO IGNITION ......................................... 31 CONTACT POINTS ............................................. 31 CONDENSER ..................................................... 31 OPERATION OF THE TECUMSEH MAGNETO IGNITION ....................................... 31 SOLID STATE IGNITION (1985 to Present) ...... 32 SOLID STATE IGNITION OPERATION ............. 32 IGNITION SERVICE ............................................... 33 SPARK PLUG SERVICE .................................... 33 FLYWHEEL REMOVAL AND SERVICE ............. 33 FLYWHEEL MAGNETS ...................................... 33 FLYWHEEL KEYS ............................................... 34 ADAPTER KEY TO FLYWHEEL ASSEMBLY .... 34 FLYWHEEL SLEEVE .......................................... 34 FLYWHEEL TORQUE ......................................... 34 REPLACING MAGNETO BREAKER POINTS .. 34 CONDENSER CHECK ....................................... 34 IGNITION COIL ................................................... 34 IGNITION TIMING .............................................. 34 FIXED TIME SPEED (EXTERNAL COIL) .......... 36 SOLID STATE (CDI) ........................................... 36 OTHER IGNITION SYSTEMS ............................ 36 OUTBOARD TIMING .............................................. 38 OUTBOARD TIMING (STANDARD IGNITION) . 38 OUTBOARD TIMING (SOLID STATE) ............... 39 CYLINDERS & INTERNAL COMPONENTS ......... 41 PISTON & RING SERVICE ................................ 41 CONNECTING ROD SERVICE .............................. 42 NEEDLE BEARINGS .......................................... 42 CRANKSHAFT, BEARING AND OIL SEAL SERVICE ................................................................. 43 NEEDLE BEARINGS .......................................... 43 Page This manual contains information on Tecumseh Engines built to comply with emission regulations. As a technician it is unlawful to modify or cause a change in the original calibration of these engines. All speed adjustments must remain within the limits that are specified for each engine, and are not to exceed the maximum. Any deviation must be specifically approved by Tecumseh Products Company. OIL SEALS .......................................................... 43 ONE PIECE SEALS ............................................ 43 THREE PIECE SEALS ....................................... 44 OUTBOARD SEAL SERVICE ............................ 44 CYLINDERS, REEDS & COMPRESSION RELEASE ................................................................ 45 CYLINDER & HEAD SERVICE .......................... 45 GASKETS ............................................................ 45 REED VALVES .................................................... 45 AUTOMATIC COMPRESSION RELEASE (DOUBLE REED TYPE) ................................. 45 AUTOMATIC COMPRESSION RELEASE (SINGLE REED TYPE) ................................... 45 CYLINDER EXHAUST PORTS .......................... 46 TROUBLESHOOTING ............................................ 47 ENGINE FAILS TO START OR STARTS WITH DIFFICULTY ............................................. 47 ENGINE KNOCKS .............................................. 47 ENGINE MISSES UNDER LOAD ...................... 48 ENGINE LACKS POWER ................................... 48 ENGINE OVERHEATS ....................................... 48 ENGINE SURGES OR RUNS UNEVENLY ....... 49 ENGINE VIBRATES EXCESSIVELY ................. 49 SPECIFICATIONS .................................................. 50 ENGINE TYPE NUMBER AND LETTER REFERENCE ................................................... 50 SEARS CRAFTSMAN CROSS REFERENCE ...... 51 TABLE OF SPECIFICATIONS ....................... 52 - 61 TABLE OF TORQUE LIMITS ................................. 62 EDUCATIONAL MATERIALS AND TOOLS ......... 63 AVAILABLE TECHNICIAN’S HANDBOOKS ...... 63 AVAILABLE FOREIGN TECHNICIAN’S HANDBOOKS .................................................. 63 TOOL KIT 670195D ................................................ 64 TOOLS ..................................................................... 65 Page

1 GENERAL INFORMATION ENGINE IDENTIFICATION Tecumseh has used two different methods of identifying 2 cycle engines. The first method was used until the mid 1980’s using the base model number with a type number. The type number was used to identify variations for the OEM’s specific needs (Fig. 1) INTERPRETATION OF MODEL NUMBER The letter designations in a model number indicates the basic type of engine. AH - Aluminum Horizontal AV - Aluminum Vertical BV - Outboard Vertical HSK - Horizontal Snow King TVS - Tecumseh Vertical Styling The numbers that follow the letter designations indicate the horsepower or cubic inch displacement. Tecumseh engine model, specification, and D.O.M. number are now located on a decal adhered to the blower housing. TVS600-661030M 7352 1975 YEAR 30th DAY 5th. MONTH (MAY) T-670 TECUMSEH 6874603 TYPE SERIAL NUMBER FRONT SIDE REVERSE SIDE (BUILD DATE) TYPE SERIAL NUMBER YEAR 219th DAY 1976 or Aug. 6th 1401J TECUMSEH 6219 TYPE SERIAL NUMBER 670-100 TECUMSEH 62290322 YEAR 229th DAY 1976 or Aug. 17th 5 30 75 FUEL REGULAR UNLEADED FUEL/OIL MIX 32:1 HSK600-1688S 8023B TVS600- 661030M 7352 The second method was put in place to standardize 2 cycle and 4 cycle engine identification. The type number has now become the specification number, which is preceded by the engine model as illustrated in Figure 2 and 3. Following the engine size are the engine specification numbers which are used for identification when ordering parts. The final set of numbers are the date of manufacture (D.O.M.). Example: HSK600 1688S HSK - Horizontal Snow King 600 - 6 Cubic Inch displacement 1688S - Specification number 8023B - Serial number D.O.M. (date of manufacturing previously serial number) 8 - First digit is the year of manufacturing (1998) 023 - Indicates the calendar day of the year (23th day or Jan. 23, 1998) B - Represents the line and shift on which the engine was built at the factory 4 2 1 FUEL REGULAR UNLEADED FUEL/OIL MIX 32:1 HSK600-1688S 8023B 3

2 ENGINE CARE SHORT BLOCKS New short blocks are identified by a tag marked S.B.H. (Short Block Horizontal) or S.B.V. (Short Block Vertical). Original model identification numbers of an engine should always be transferred to a new short block for correct parts identification. THIS SYMBOL POINTS OUT IMPORTANT SAFETY INSTRUCTIONS WHICH IF NOT FOLLOWED COULD ENDANGER THE PERSONAL SAFETY OF YOURSELF AND OTHERS. FOLLOW ALL INSTRUCTIONS. ENGINE FUEL MIX U.S. U.S. METRIC METRIC Amount of Oil Amount of Oil Gasoline To Be Added Petrol To Be Added 24:1 1 Gallon 5.3 oz. 4 Liters 167 ml 2 Gallons 10.7 oz. 8 Liters 333 ml 32:1 1 Gallon 4 oz. 4 Liters 125 ml 2 Gallons 8 oz. 8 Liters 250 ml 50:1 1 Gallon 2.5 oz. 4 Liters 80 ml 2 Gallons 5.0 oz. 8 Liters 160 ml FUELS. Tecumseh Products Company strongly recommends the use of fresh, clean, UNLEADED regular gasoline in all Tecumseh Engines. Unleaded gasoline burns cleaner, extends engine life and promotes good starting by reducing the build up of carbon deposits. Premium gas or gasohol containing no more than 10% ethanol can be used if unleaded fuel is not available. NOTE: NEVER USE GASOLINE CONTAINING METHANOL, GASOHOL CONTAINING MORE THAN 10% ETHANOL, UNLEADED REGULAR GASOLINE CONTAINING MORE THAN 15% M.T.B.E. OR ETBE, GASOLINE ADDITIVES, OR WHITE GAS BECAUSE ENGINE/FUEL SYSTEM DAMAGE COULD RESULT. NOTE: In countries where unleaded gasoline is not available, regular gas can be used. Make sure that gasoline and oil are stored in clean, covered, rust free containers. Dirt in fuel can clog small ports and passages in the carburetor. Use fresh gasoline only. Gasoline standing for long periods of time develop a gum that will result in fouled spark plugs, clogged fuel lines, carburetors and fuel screens. SHORT BLOCK IDENTIFICATION TAG SBV-2316 SER 4291 SERIAL NUMBER SBV OR SBH IDENTIFICATION NUMBER Using fuel that is not fresh will cause engines to be hard starting, especially in cold temperatures. Clean gas cap, tank and fuel container spout when filling fuel tank, to assure that dirt will not get into fuel system. If gasohol is used, special care is required when the engine is to be stored for extended periods. ENGINE OIL. Use Clean, high quality 2 CYCLE OIL. NMMA TC-WII, or TC-W3. FUEL/OIL MIX. Follow fuel and engine oil requirements listed in the Owners Manual. Disregard conflicting instructions found on oil containers. NOTE: Sears Craftsman models use a 40:1 mix which is acceptable. NOTE: ALWAYS DOUBLE THE AMOUNT OF OIL FOR THE FIRST GALLON OF MIX FOR ENGINE BREAK IN. To assure thorough mixing of oil and gasoline, fill container with gasoline to one quarter full, add oil as recommended, shake container vigorously and then add remainder of gasoline. Do not mix directly in engine or equipment fuel tank. TWO-CYCLE FUEL/OIL MIX RATIOS 24:1 AV520 All Basic Specifications 670 & 653 AV600 All Basic Specifications 600-10 & Up 32:1 TVS600 All Specifications AH600 50:1 HSK600 STORAGE: NEVER STORE ENGINE WITH FUEL IN TANK INDOORS OR IN ENCLOSED, POORLY VENTILATED ENCLOSURES WHERE FUEL FUMES MAY REACH AN OPEN FLAME, SPARK OR PILOT LIGHT AS ON A FURNACE, WATER HEATER, CLOTHES DRYER, ETC. If engine is to be stored over 30 days, prepare for storage as follows: 1. Remove all gasoline from fuel tank to prevent gum deposits from forming in tank and other fuel system components, causing possible malfunction of the engine. DRAIN FUEL INTO APPROVED CONTAINER OUTDOORS, AWAY FROM OPEN FLAMES. Run engine until fuel tank is empty and engine stops due to lack of fuel. Fuel stabilizer (such as Tecumseh’s Part No. 730245) is an acceptable alternative in minimizing the formation of fuel gum deposits during storage.

3 Add stabilizer to fuel in fuel tank or storage container. Always follow mix ratio found on stabilizer container. Run engine at least 10 minutes after adding stabilizer to allow it to reach carburetor. If gasohol has been used, complete preceding instructions and then add a small amount, one pint or less, of unleaded regular automotive gasoline properly mixed with the specified lubricating oil and run the engine until the fuel tank is empty and the engine stops due to lack of fuel. 2. Pull starter handle slowly until resistance is felt due to compression pressure, then stop. Release starter tension slowly to prevent engine from reversing due to compression pressure. This position will close both the intake and exhaust ports to prevent corrosion of the piston and the cylinder bore. 3. Clean unit by removing any dirt from exterior of the engine and equipment. TUNE-UP PROCEDURE. The following is a minor tune- up procedure. Repair procedures for this engine and its components are listed in this manual if the engine does not perform to satisfaction after tune-up is performed. CAUTION: Remove spark plug wire before doing any service work on engine. 1. Service or replace air cleaner as necessary. NOTE: Snow King ﬁ engines do not use an air filter due to the clean operating environment and to prevent filter freeze-up. 2. Remove blower housing, clean all dirt, grass or debris from air intake screen, cylinder cooling fins, and carburetor governor levers and linkage. 3. Remove carburetor, clean and install a carburetor kit, make adjustment presets where needed. Presets are found in the carburetor section of this manual. Make sure fuel tank, fuel filters and fuel lines are clean. Reinstall carburetor, replacing any worn or damaged governor springs or linkage. Make proper governor adjustment. Adjustments are found in the manual under governor linkage and adjustment and engine reassembly. 4. Replace the spark plug with a correct equivalent. 5. Make sure all ignition wires are properly routed, so they will not rub on flywheel. Inspect all ignition wires for abrasion or damage. Remove flywheel and check flywheel key, reinstall flywheel, torque flywheel nut to specifications. Set air gap between solid state module and flywheel at .0125’. Air gap gauge, part no. 670297 may be used. 6. The engine must be firmly mounted to the associated equipment. On rotary lawnmowers, make sure blade is properly balanced and correctly torqued. See disassembly section for flywheel removal and air gap setting procedure. 7. Make sure all remote linkage is properly adjusted for operation. 8. Fill tank with the proper fuel/oil mix. 9. Start the engine, allow it to warm up 5 minutes, then adjust carburetor and engine R.P.M. according to Tecumsehs recommendations. Snow King ﬁ engines should be run in outdoor temperatures for engine adjustments. NOTE: Emissionized engines have non-adjustable carburetors in compliance with CARB (California Air Resource Board) and U.S. EPA (United States Environmental Protection Agency). These engines can not be modified without specific authorization. EXHAUST PORT CLEANING. The exhaust ports should be cleaned after each seventy-five (75) to one hundred (100) hours of use. Before cleaning the ports place the piston in the Bottom Dead Center position. Then clean the ports using a pointed 3/8’ diameter wooden dowel or similar tool. Be sure not to scratch the port area. Also remember to remove all the loose carbon particles from the engine. NOTE: When cleaning the exhaust ports, check and clean the compression release passage. COMPRESSION RELEASE PASSAGE

4 2-CYCLE THEORY OF OPERATION As the mixture flows into the combustion chamber through the transfer ports, it collides and is directed to the top of the combustion chamber looping when it strikes the cylinder head, thus forcing all spent gases out through the open exhaust ports. TERMS. Listed are common terms often referred to on 2-cycle engines. EXHAUST or SCAVENGE PHASE. The burned gases must be cleared out of the combustion chamber and replaced by a fresh charge of fuel-air mixture through the intake ports. The exhaust passes out through the exhaust ports into the outside air. PORTS. Openings in the cylinder allow gases to pass into and out of the combustion chamber. The ports are opened or closed by the upward and downward movement of the piston. EXHAUST PORTS. Allow the burned gases to pass out of the combustion chamber. THIRD PORT (Sometimes called piston port). A third port is for entry of the fuel-air and oil mixture to the crankcase. From the crankcase the fuel-air mixture enters the combustion chamber through the intake ports. The third port is controlled by the piston skirt. REED VALVE. A reed valve is activated by crankcase pressure or vacuum. A decrease in crankcase pressure opens the reed allowing the fuel air and oil mixture to enter the crankcase. Increased crankcase pressure closes the valve, preventing escape of the fuel-air and oil mixture back through the carburetor. LUBRICATION. Tecumseh 2-cycle engines are lubricated by a gas oil mix. The correct mix ratio of oil and gas combines with air in the venturi and enters the crankcase. During engine operation the oil clings to all the internal moving parts for lubrication. 3. 1. 4. INTAKE AND IGNITION POWER 2. COMPRESSION OPERATION OF REED PORTED STYLE WITH LOOP SCAVENGING The following illustrates the loop scavenge design which uses a vacuum-pressure activated reed valve. Here the ports are located on three sides of the cylinder; the intake ports are on two sides opposite each other, and the exhaust ports are illustrated by the three holes just above the head of the piston. OPERATION OF PISTON PORT STYLE. A low pressure area is created in the crankcase as the piston moves upward to compress the air/fuel mixture in the cylinder. When the piston moves far enough to uncover the intake port, the air/fuel mixture from the carburetor flows into the engine crankcase due to higher pressure atmospheric air. Just before the piston reaches top dead center (TDC), the spark plug ignites the air / fuel mixture in the cylinder. The expanding combustion gases force the piston down. The downward piston travel causes a pressure buildup in the crankcase. The piston uncovers the exhaust port first, followed by the transfer ports. The exhaust flows out the exhaust port while the pressurized air/ fuel mixture enters the cylinder from the crankcase through the transfer ports. As the piston travels upward the sequence is repeated. Air cooled 2-cycle engines differ from 4-cycles by having one power stroke for every revolution of the engine versus every other stroke on 4-cycles.

5 CUPPED SCREEN MUST BE POSITIONED WITH EDGES AWAY FROM ELEMENT COVER POLYURETHANE ELEMENT FLOCKED SCREEN POLYURETHANE ELEMENT PLUG MUST BE IN PLACE FLOCKED SCREEN AIR CLEANERS, CARBURETORS, GOVERNORS AND LINKAGE AIR CLEANERS Service the air cleaner frequently to prevent clogging of the cleaner and to prevent dust and dirt from entering the engine. Dust bypassing an improper or damaged air filter can quickly damage an engine. Always make certain covers and air cleaner connections are tightly sealed to prevent entry of dirt. NOTE: Snow King ﬁ models do not use air cleaners due to the clean environment that they operate in and also to prevent air cleaner freeze-up. When excessive carburetor adjustment or loss of power results, inspect the air filter for clogging. NOTE: Use factory recommended parts only. POLYURETHANE-TYPE AIR CLEANER. These serviceable air cleaners utilize a polyurethane element which will clog up with use. The element should be cleaned and serviced in the following manner. Wash element in a detergent and water solution and squeeze (dont twist) until all dirt is removed. Rinse thoroughly. Wrap in clean cloth and squeeze (dont twist) until completely dry. Clean air cleaner housing and cover. Dry thoroughly. Re-oil element by applying generous quantity of oil to all sides. Squeeze vigorously to distribute oil and to remove excess oil. (S.A.E. 30) When reassembling polyurethane oval type air cleaners, place cupped screen into housing with edge against carburetor end of housing. Screen should be installed to hold element away from housing to allow full utilization of air cleaner element. NOTE: Polyurethane type filters will lose effectiveness if stored for extended periods of time, due to oil migration (settling Down) through the filter. Re-oil filter as necessary. KLEEN-AIRE ﬁ SYSTEM. This system uses a polyurethane type element. Service as described under polyurethane-type air cleaner. When removing air cleaner body from carburetor, remove plug in the body to gain access to the mounting screw. Make certain plug is put back in place. If it shows damage, replace. PAPER-TYPE AIR CLEANER SERVICE. Replace air filter once a year or more often in extremely dusty or dirty conditions. DO NOT ATTEMPT TO CLEAN OR OIL PAPER-TYPE FILTER. Be sure to clean base and cover thoroughly before installing new paper filter. NEVER RUN THE ENGINE WITHOUT THE COMPLETE AIR CLEANER INSTALLED ON THE ENGINE. NOTE: Serious damage to the engine may result in using any other but the specified part number filter. Use factory recommended parts only.

6 GENERAL CARBURETOR INFORMATION When servicing carburetors, you may use either the engine model specification number or the manufacturing number on the carburetor to properly identify. Information regarding replacement parts or kits are available in the master parts manual, microfiche catalog or electronic parts look-up systems. Tecumseh uses two basic types of carburetors for their 2 cycle engines, float-type and diaphragm. FLOAT-TYPE CARBURETORS. Float-type carburetors use a hollow metal float to maintain the operating level of fuel in the carburetor. As the fuel is used, the fuel level in the carburetor bowl drops and the float moves downward. This actuates the inlet needle valve, to allow fuel to flow into the fuel bowl. As the fuel level in the bowl again rises, so will the float. This float action adjusts the fuel flow and keeps the fuel at the proper mixture level. Some carburetors are of the fixed main type. On these models, the main adjusting screw and nut are replaced by a fixed main jet bowl nut. DIAPHRAGM (PRESSURE DIFFERENTIAL) CARBURETORS This type of carburetor uses a rubber-like diaphragm which is exposed to intake manifold pressure on one side and to atmospheric pressure on the other. Tecumseh diaphragm carburetors use the diaphragm as a metering device. As the intake manifold pressure decreases due to downward piston travel, the atmospheric pressure on the vented side of the diaphragm moves the diaphragm against the inlet needle. The diaphragm movement overcomes the spring tension on the inlet needle and moves the inlet needle off the seat. This permits the fuel to flow through the inlet valve to maintain the correct fuel volume in the fuel chamber. The inlet needle return spring closes the inlet valve when the pressure on the diaphragm equalizes or a pressure higher than atmospheric exists on the intake side (upward piston travel). The diaphragm meters a correct fuel volume in the fuel chamber to be delivered to the mixing passages and discharge ports. A main or idle adjustment needle may be replaced by an internally fixed jet on some models. The main nozzle contains a ball check valve. The main purpose of this ball check is to eliminate air being drawn down the main nozzle during idle speeds and leaning the idle mixture. An advantage of the diaphragm carburetor over the float system is that the diaphragm carburetor increases the angle that the engine may be operated at. ALTERNATE LOCATION FOR MANUFACTURING NUMBER CARBURETOR MANUFACTURING NUMBER CARBURETOR CODE DATE 89 4 F 5 CHOKE SHUTTER FUEL INLET CHECK BALL DIAPHRAGM MAIN ADJUST IDLE ADJUST AIR BLEED IDLE AND INTERMEDIATE PORTS THROTTLE SHUTTER INLET NEEDLE AND SEAT INLET SEAT *MAIN ADJUST FLOAT INLET NEEDLE AIR BLEED CHOKE SHUTTER IDLE AIR BLEED IDLE FUEL CHAMBER THROTTLE SHUTTER SECONDARY IDLE DISCHARGE PRIMARY IDLE DISCHARGE *IDLE ADJUSTMENT SCREW IDLE FUEL TRANSFER PASSAGE IDLE FUEL TRANSFER PASSAGE REDUCTION ROD CARBURETOR IDENTIFICATION. Tecumseh carburetors are identified by a manufacturing number and code date stamped on the carburetor as illustrated. *Both jets may be fixed - non adjustable 89 4 F5

7 OPERATION In the CHOKE or START position, the choke shutter is closed, and the only air entering the engine enters through openings around the shutter. As the engine starts to rotate the downward piston travel will create a low air pressure area in the engine cylinder above the piston. Higher pressure (atmospheric air) rushes into the engine to fill the created low pressure area. Since the majority of the air passage is blocked by the choke shutter, a relatively small quantity of air enters the carburetor at increased speed. The main nozzle and both idle fuel discharge ports are supplying fuel due to the low air pressure in the intake of the engine. A maximum fuel flow through the carburetor orifices combined with the reduced quantity of air that passes through the carburetor, make a very rich fuel mixture which is needed to start a cold engine. At engine IDLE speed, a relatively small amount of fuel is required to operate the engine. The throttle is almost completely closed. A fuel / air mixture is supplied through the primary idle-fuel discharge orifice during idle. During INTERMEDIATE engine operation, a second orifice is uncovered as the throttle shutter opens, and more fuel mixture is allowed to atomize with the air flowing into the engine. During HIGH SPEED engine operation, the throttle shutter is opened. Air flows through the carburetor at high speed. The venturi, which decreases the size of the air passage through the carburetor, further accelerates the air flow. This high speed movement of the air decreases the air pressure at the main nozzle opening. Fuel is forced out the main nozzle opening due the difference in the atmospheric air pressure on the fuel in the carburetor bowl and the reduced air pressure at the main nozzle opening. CARBURETOR SERVICE. Carefully disassemble carburetor, removing all non-metallic parts, i.e., gaskets, viton seats and needles, O rings, fuel pump valves, etc. Nylon check balls used in some diaphragm carburetors are not serviceable. Nylon can be damaged if subjected to harsh cleaners for prolonged periods. Remove the primer bulb (if equipped) by grasping with a pliers and pulling and twisting out of the body. Remove the retainer by prying and lifting out with a screwdriver. Do not re-use old bulb or retainer. Remove all welch plugs if cleaning the carburetor. Secure the carburetor in a vise equipped with protective jaws. Use a small chisel sharpened to a 1/8" wide wedge point. Drive the chisel into the plug to pierce the metal and push down on the chisel to pry the plug out of the hole. Clean all metallic parts with solvent. ABOUT 1/8" WIDE WELCH PLUG TO BE REMOVED PRY OUT PLUG PIERCE PLUG WITH TIP SMALL CHISEL DO NOT ALLOW CHISEL POINT TO STRIKE CARBURETOR BODY OR CHANNEL REDUCER SMALL CHISEL NEW WELCH PLUG SAME OR LARGER DIAMETER OF PLUG FLAT-END PUNCH THROTTLE PLATE THROTTLE LEVER To install a new welch plug after cleaning, place welch plug into receptacle with raised portion up. With a punch equal to the size of the plug, merely flatten the plug. Do not dent or drive the center plug below the top surface of the carburetor. After installing the welch plug, seal the outer diameter with finger nail polish. (Do not use clear polish). THROTTLE. Examine the throttle lever and plate prior to disassembly. Replace any worn and/or damaged parts. When reassembling, it is important that the lines on the throttle plate are facing out when in the closed position. Position throttle plates with the two lines at 12 and 3 oclock. If throttle plate has only one line, the line should be positioned in the 12 oclock position. If binding occurs, correct by loosening the screws and repositioning the throttle plate.

Tecumseh 2 Cycle Small Engine Repair Manual

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