MTD 25cc Professional Shop Manual
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MTD 25cc Professional Shop Manual

25cc 2-cycle engines
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For Parts Call 606-678-9623 or 606-561-4983
Professional Shop Manual
25cc 2-Cycle P25 Series Engines
NOTE: These materials are for use by trained technicians who are experienced in the service and repair of outdoor power
equipment of the kind described in this publication, and are not intended for use by untrained or inexperienced individuals.
These materials are intended to provide supplemental information to assist the trained technician. Untrained or inexperi-
enced individuals should seek the assistance of an experienced and trained professional. Read, understand, and follow all
instructions and use common sense when working on power equipment. This includes the contents of the product's Oper-
ators Manual, supplied with the equipment. No liability can be accepted for any inaccuracies or omission in this publication,
although care has been taken to make it as complete and accurate as possible at the time of publication. However, due to
the variety of outdoor power equipment and continuing product changes that occur over time, updates will be made to these
instructions from time to time. Therefore, it may be necessary to obtain the latest materials before servicing or repairing a
product. The company reserves the right to make changes at any time to this publication without prior notice and without
incurring an obligation to make such changes to previously published versions. Instructions, photographs and illustrations
used in this publication are for reference use only and may not depict actual model and component parts.
© Copyright 2010 MTD Products Inc. All Rights Reserved
www.mymowerparts.com

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Summary of Contents for MTD 25cc

  • Page 1 Instructions, photographs and illustrations used in this publication are for reference use only and may not depict actual model and component parts. © Copyright 2010 MTD Products Inc. All Rights Reserved www.mymowerparts.com...
  • Page 2 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com...

  • Page 3: Table Of Contents

    For Parts Call 606-678-9623 or 606-561-4983 Table of Contents Chapter 1: Introduction 1 Professional Shop Manual intent ........1 Fasteners .
  • Page 4 For Parts Call 606-678-9623 or 606-561-4983 Chapter 5: Fuel system and carburetor 25 Fuel system troubleshooting ........25 Inspecting the fuel .

  • Page 5: Chapter 1: Introduction

    CHAPTER 1: INTRODUCTION Professional Shop Manual intent This Manual is intended to provide service dealers with an introduction to the mechanical aspects of the 25cc P25 series of 2-cycle engines. Disclaimer: The information contained in this manual is correct at the time of writing. Both the product and the infor- mation about the product are subject to change without notice.

  • Page 6: Assembly

    Some instructions may refer to other parts of the manual for subsidiary procedures. This avoids repeating the same procedure two or three times in the manual. Description The P25 series of engines are used on a variety of string trimmers. These engines have: • 25cc’s of displacement. • Piston ported/loop scavenged design. • Cantilever crankshaft. Figure 1.1...

  • Page 7: Identifying Engines

    For Parts Call 606-678-9623 or 606-561-4983 Introduction Identifying engines MTD currently has two cantilever crank, 2-cycle engines: the A31 and the P25. NOTE: The starter is mounted between the engine and the drive shaft on all cantilever engines. Fuel tank •...

  • Page 8: Model And Serial Numbers

    ...H................... Month of production (H = August) ..13 ..................Day of the month ..0 ..................Last digit of the year ...D................... Plant it was built in (MTD Southwest) ....I ................. Assembly line number ....1521 ............... Number of unit built www.mymowerparts.com...

  • Page 9: Chapter 2: Maintenance

    The information in this manual applies to the P25 series of engines. Some basic principles may apply to other engines produced by MTD and other manufacturers. As the saying goes “an ounce of prevention is worth a pound of cure”. The same can be said about preventive maintenance on outdoor power equipment.

  • Page 10: Spark Plugs

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of engines Spark plugs The spark plug used in the 25cc trimmer is a Cham- pion RDJ7J (794-00055) gapped to 0.025” (0.655 mm). See Figure 2.3. Center electrode Wear rate will vary with severity of use. If the edges...

  • Page 11: Spark Arrestor

    For Parts Call 606-678-9623 or 606-561-4983 Maintenance Spark arrestor A spark arrestor is a screen that is in the outlet of the muffler. It’s job is to collect any sparks or hot embers that are exiting the muffler. The spark arrestor will develop a carbon build up over time.

  • Page 12: Fuel Filer

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of engines Fuel filer A dirty fuel filter can result in a lean run condition. The fuel filter should be inspected every 25 hours of use. NOTE: The weighted fuel filter (clunk) keeps the filter submerged in the fuel at any angle of operation. The fil- ter removes dirt and air bubbles from the fuel.

  • Page 13: Storage

    For Parts Call 606-678-9623 or 606-561-4983 Maintenance Storage When storing a piece of gas powered equipment for longer than 4 weeks, the following procedures will prolong the engine life and minimize issues such as hard starting after storage. Run the engine until it is out of gas. Open the air filter cover.
  • Page 14 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of engines www.mymowerparts.com...

  • Page 15: Chapter 3: Troubleshooting

    For Parts Call 606-678-9623 or 606-561-4983 Troubleshooting CHAPTER 3: TROUBLESHOOTING Definitions Troubleshooting - The act of gathering information by performing tests and direct observations. Diagnosis - A theory of what the problem is, based on the information gathered by troubleshooting. Introduction Diagnosing an engine is an art form that is built on several factors.
  • Page 16 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines There are tools that the technician can use in order to define the problem, such as: Interview the customer. • Get a good description of their complaint. • If it is an intermittent problem, verify what conditions aggravates the problem as best as possible. •...

  • Page 17: Identify Factors That Could Cause The Problem

    For Parts Call 606-678-9623 or 606-561-4983 Troubleshooting Identify factors that could cause the problem This is the second step in the troubleshooting process. Crankshaft will not turn. • Hard to pull rope, steady pressure - This usually indicates a mechanical bind of some sort. the likely sus- pects are: A parasitic load from a jammed attachment or drive shaft.
  • Page 18 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Starts, runs poorly • Starts, then dies Run the engine with a spark tester in-line between the spark plug wire and the spark plug or use an oscilloscope and see if the spark goes away at the same time the engine dies. Check choke operation.
  • Page 19 For Parts Call 606-678-9623 or 606-561-4983 Troubleshooting • Too much fuel: needle valve stuck or metering / emulsion issues with the carburetor. b. Blue smoke, usually light. • Too much oil in the fuel mix. IX. Piston rings a. Confirm with compression test. b.

  • Page 20: Repairing The Problem

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Repairing the problem The third step in the diagnostic process is to repair the problem. This step consists of: Form a diagnosis by using all of the information gathered from the troubleshooting that was performed. Physically perform the repair.

  • Page 21: Prime Test

    For Parts Call 606-678-9623 or 606-561-4983 Troubleshooting Prime test To perform a prime test: 1. Prime the engine through the carburetor throat using a squirt bottle, filled with clean fresh gasoline/oil mix. NOTE: Inspect the air filter while priming the engine. Look for a dirty or plugged filter that could prevent air flow or a missing filter that would indicate dirt ingestion.

  • Page 22: Compression Testing

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Compression testing To perform a compression test: Gauge is zeroed NOTE: If the engine will run, start the engine and let it warm up first for a better reading. Disconnect the high-tension lead from the spark plug and ground it well away from the spark plug hole.

  • Page 23: Chapter 4: Ignition

    NOTE: The spark should be a minimum of 10 Kv (10,000 volts) at pull over speed. Make sure the engine stop switch is in the “RUN” position. NOTE: Since 2005, most stop switches used on MTD handheld products are spring loaded to the “RUN” position when they are released. This prevents no-...

  • Page 24: Testing The Module

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Testing the module To test the module: Remove the starter housing by following the proce- dures described in Chapter 6: Starter. wire Check the air gap for the module. Set it to 0.010” by following the steps described in the module section Red wire of this chapter.

  • Page 25: Test The Engine Stop Switch

    For Parts Call 606-678-9623 or 606-561-4983 Ignition Test the engine stop switch To test the engine stop switch: Switch in the engine Open the grip by removing the five screws with a T-20 run position torx driver. Disconnect the wires from the engine stop switch. Connect an ohm meter or continuity light to the switch.

  • Page 26: Module

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Module Remove the ignition module: Black wire on mounting screw Remove the starter by following the procedures described in Chapter 6: starter. Disconnect the red wire from the module. See Figure 4.6. Red wire NOTE: The black wire will come off with the module mounting screws.

  • Page 27: Flywheel

    For Parts Call 606-678-9623 or 606-561-4983 Ignition Flywheel The flywheel of an engine performs four functions: • It stores energy. • Provides the momentum that keeps the crankshaft Gently pry while striking rotating between power strokes. • It times the firing of the ignition module. •...
  • Page 28 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines www.mymowerparts.com...

  • Page 29: Chapter 5: Fuel System And Carburetor

    For Parts Call 606-678-9623 or 606-561-4983 Fuel System And Carburetor CHAPTER 5: FUEL SYSTEM AND CARBURETOR The function of the fuel system is to store fuel/oil mixture, mix the fuel/oil with air and deliver it to the combustion chamber. The fuel system consists of the following components: •...

  • Page 30: Check The Fuel/Oil Mixture

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Check the fuel/oil mixture To check the fuel in the carburetor: Get a plain piece of paper. Divide it into four sections and label them: • Plain gas • Two cycle mix •...

  • Page 31: Test Fuel For Alcohol

    A 10% ethanol (E10) mix is acceptable for MTD engines. Anything higher than that will result in perfor- mance issues. NOTE: E20 and E85 fuels are not to be used in any MTD engines. There are several alcohol test kit available commer- cially.

  • Page 32: Fuel Filer

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Fuel filer A dirty fuel filter can result in a lean run condition. The fuel filter should be inspected every 25 hours of use. NOTE: The weighted fuel filter (clunk) keeps the filter submerged in the fuel at any angle of operation. The fil- ter removes dirt and air bubbles from the fuel.

  • Page 33: Fuel Lines

    Black fuel lines = low permeation fuel line NOTE: MTD started using low permeation fuel lines on some of the P25 series of engines in 2009. In 2012, all MTD hand held engines will have the low permeation fuel line to comply with EPA guidelines. The low permeation fuel lines can be identified by their black color.

  • Page 34: Fuel Tank

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Fuel tank To remove/replace the fuel tank: Drain the fuel tank. Disconnect the fuel lines from the carburetor. See Figure 5.9. Disconnect the fuel lines Figure 5.9 Remove the two screws that hold the fuel tank to the starter housing using a T-20 torx driver.

  • Page 35: Carburetor

    For Parts Call 606-678-9623 or 606-561-4983 Fuel System And Carburetor Carburetor The P25 series of engines uses the Walbro WT series carburetors, or a carburetor of similar design from another manufacturer. The main distinguishing features of this carburetor design are: •...
  • Page 36 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines If the fuel/air mixture is lean: • Inspect the carburetor spacer for cracks. • Inspect the spacer O-ring. NOTE: A cracked spacer or a leaking O-ring between the spacer and the cylinder or the spacer and the carburetor could result in a Spacer lean run condition or prevent the impulses...

  • Page 37: Disassembly Of The Carburetor

    For Parts Call 606-678-9623 or 606-561-4983 Fuel System And Carburetor Disassembly of the carburetor Remove the two screws that hold the primer pump Screws body and the primer bulb to the carburetor using a #2 phillips screwdriver. See Figure 5.15. Figure 5.15 Separate the primer pump and the bulb assembly, Diaphragm...
  • Page 38 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Inspect for dirt or varnish build up inside the carbu- retor. If there is a lot of dirt/varnish in the carburetor, Metering chamber replace it. See Figure 5.18. Cover Check that the needle valve is set to the right height.
  • Page 39 For Parts Call 606-678-9623 or 606-561-4983 Fuel System And Carburetor 11. Remove the throttle plate using a #2 phillips screw- driver. See Figure 5.21. Butterfly valve Figure 5.21 12. Remove the throttle shaft E-clip. See Figure 5.22. 13. Slide the throttle shaft out of the carburetor. E-clip Figure 5.22 14.
  • Page 40 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Fuel flow Impulse port NOTE: The fuel pump in the carburetor is driven by Pump chamber pressure pulses in the crank case. The impulse port from the spacer lets the pulses pass through the carburetor body into the pump camber.

  • Page 41: Re-Assembly Of The Carburetor

    For Parts Call 606-678-9623 or 606-561-4983 Fuel System And Carburetor Re-assembly of the carburetor NOTE: The carburetor used in this manual is a Walbro. Needle valve Depending on the application, the engine may seat have a different carburetor. All carburetors have a manufacturer name cast on them.
  • Page 42 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Inspect the primer pump body for dirt and/or var- nish. See Figure 5.28. Install the diaphragm and gasket. Install the primer pump body and the primer bulb. Tighten the screws using a #2 phillips screwdriver. Perform a needle valve pop off test by following the carburetor manufacturer’s recommendations.

  • Page 43: Carburetor Insulator

    For Parts Call 606-678-9623 or 606-561-4983 Fuel System And Carburetor Carburetor insulator NOTE: A cracked spacer or a leaking gasket between the spacer and the cylinder could result in a lean run or prevent the impulses from the engine from driving the fuel pump.
  • Page 44 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Inspect the O-ring between the insulator block and the cylinder. See Figure 5.32. Install the insulator by following the previous steps in reverse order. O-ring NOTE: Apply a small amount of releasable thread locking compound such as Loctite®...

  • Page 45: Chapter 6: Starters

    For Parts Call 606-678-9623 or 606-561-4983 Starters CHAPTER 6: STARTERS Recoil Starter Removal To remove the starter assembly: Remove the clutch following the steps described in the section on clutch removal. NOTE: The drive shaft will come off with the clutch hous- ing.
  • Page 46 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Remove the three screws in the front of the starter housing. See Figure 6.2. Slide the starter housing off of the engine. Install the starter assembly by following the previous Screws steps in reverse order.

  • Page 47: The Starter Rope, Pulley And Springs

    The starter rope, pulley and springs The 25cc P25 series of engines are equipped with the Assist Spring Technology (AST) starters.The AST system allows the customer to pull the recoil rope at any pace they please, yet still generate enough force to start the engine.
  • Page 48 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines NOTE: The recoil pulley assembly consists of three parts, but it is serviced as one assembly. The three parts are: the inner pulley, the starter assist (accumulator) spring, and the outer pulley.
  • Page 49 For Parts Call 606-678-9623 or 606-561-4983 Starters To assembly the starter pulley assembly: Install a new starter rope in the recoil pulley assembly. 1a. Remove the old rope from the pulley. 1b. Cut a piece of #3 1/2 (7/64”) starter rope 43” (109 cm) long.
  • Page 50 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Secure the recoil handle to the rope using two half hitch knots. See Figure 6.11. Release the slip knot. 2 half hitch knots Figure 6.11 Install the pulley retainer using a deep socket to Deep well socket apply even pressure to the retainer while driving it into place.

  • Page 51: Chapter 7: Clutch And Upper Drive Shaft

    For Parts Call 606-678-9623 or 606-561-4983 Clutch and Upper Drive Shaft CHAPTER 7: CLUTCH AND UPPER DRIVE SHAFT Drive shaft assembly There are two types of drive shaft assemblies available with the P25 series of engines: • Straight shaft • Curve shaft NOTE: The procedure to remove the drive shaft assembly from the engine is the same for both styles.
  • Page 52 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Remove the anti-rotation screw with a T-20 driver. Loosen the clamping bolt with a T-20 driver. See Figure 7.3. Anti-rotation screw Slide the drive shaft housing out of the clutch hous- ing.

  • Page 53: Clutch Removal/Replacement

    For Parts Call 606-678-9623 or 606-561-4983 Clutch and Upper Drive Shaft Clutch removal/replacement NOTE: The upper drive shaft may be left connected to the clutch housing. Removing the three screws in the Clutch housing clutch housing will allow the clutch housing and the screws upper drive shaft to be removed as one.
  • Page 54 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Remove the clutch rotor with a clutch removal tool. Part number NOTE: The P25 engines use a rubber clutch. Clutch 797-00070 tool number 797-00070 can be used to remove it. See Figure 7.8. NOTE: The clutch is used to disconnect the drive shaft for starting.

  • Page 55: Chapter 8: Engine Assembly

    For Parts Call 606-678-9623 or 606-561-4983 Engine Assembly CHAPTER 8: ENGINE ASSEMBLY Theory of operation The P25 series of engines are piston ported engines. As the piston travels upward: • The exhaust port is covered. • The fuel/air mixture above the piston is com- pressed.

  • Page 56: Engine Disassembly

    For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Engine disassembly Economics and parts availability will determine how much engine disassembly is feasible. There are two main levels of engine disassembly. Short block installation. Short block installation consists of removing the engine, transferring the external parts to the replacement short block, and installing the engine.
  • Page 57 For Parts Call 606-678-9623 or 606-561-4983 Engine Assembly Rotate the flywheel so that the magnets are away from the module. Remove the two screws that secure the ignition mod- ule to the block using a T-20 torx driver. See Figure 8.4. Screws Remove the module.
  • Page 58 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Remove the two cylinder bolts using a T-30 torx driver. See Figure 8.7. Slide the cylinder off of the piston and connecting rod. Remove the O-ring from the cylinder base. Cylinder bolts Figure 8.7 NOTE: The following steps are shown only for the...

  • Page 59: Engine Reassembly

    For Parts Call 606-678-9623 or 606-561-4983 Engine Assembly Engine Reassembly Attach the piston to the connecting rod, if removed, by inserting the piston pin. Lettering NOTE: The side of the connecting rod with the lettering on the bearing faces the side of the piston the cap is on.
  • Page 60 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines Exhaust port NOTE: Apply a coating of Slick 50 or similar prod- ® uct to the connecting rod bearing as a assembly lube. Place the cylinder O-ring on the crankcase. Slide the connecting rod onto the crankshaft pin.
  • Page 61 For Parts Call 606-678-9623 or 606-561-4983 Engine Assembly Install the flywheel and module by following the procedures described in Chapter 4: Ignition. Install the muffler. NOTE: Apply a thread locking compound such as Loctite 266 to the muffler screws. Tighten the screws to a torque of 80 - 90 in.
  • Page 62 For Parts Call 606-678-9623 or 606-561-4983 P25 Series of Engines www.mymowerparts.com...
  • Page 63 For Parts Call 606-678-9623 or 606-561-4983 www.mymowerparts.com...
  • Page 64 For Parts Call 606-678-9623 or 606-561-4983 MTD Products Inc - Product Training and Education Department FORM NUMBER - 769-06465 10/2010 www.mymowerparts.com...

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P25 series

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