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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. See Figure 8.1.

• Fuel/air mix is drawn into the crankcase.

• The spark plug will fire just before the piston

reaches top dead center, igniting the fuel/air mix-
ture.

As the piston travels downward:

• The exhaust post is uncovered, allowing the burn

gases to escape.

• The piston compresses the fuel/air mix in the

crankcase, forcing the mixture up through trans-
fer ports in the side of the cylinder.

• The fuel/air mixture enters the combustion camber,

flowing towards the intake side of the cylinder.

NOTE:

This flow pattern helps to push the exhaust gases

out of the cylinder.

Figure 8.1

Exhaust port

Intake port

Figure 8.2

Exhaust

CHAPTER 8: ENGINE ASSEMBLY

For Parts Call 606-678-9623 or 606-561-4983

www.mymowerparts.com

Содержание 25cc

Страница 1: ...ility 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 va...

Страница 2: ...For Parts Call 606 678 9623 or 606 561 4983 www mymowerparts com...

Страница 3: ...rage 9 Chapter 3 Troubleshooting 11 Definitions 11 Introduction 11 Steps to troubleshooting 11 Identify factors that could cause the problem 13 Repairing the problem 16 Diagnostic tests 16 Prime test...

Страница 4: ...roubleshooting the carburetor 31 Disassembly of the carburetor 33 Re assembly of the carburetor 37 Carburetor insulator 39 Chapter 6 Starters 41 Recoil Starter Removal 41 The starter rope pulley and s...

Страница 5: ...asteners used on the engine are metric Some are sized in fractional inches For this reason wrench sizes are frequently identified in the text and measurements are given in U S and metric scales If a f...

Страница 6: ...hunt for information The level of assembly instructions provided will be determined by the complexity and of reassembly and by the potential for unsafe conditions to arise from mistakes made in assem...

Страница 7: ...ines A31 31 cc s of displacement Muffler on back side of engine Fuel cap on top of engine Air filter on rear of engine Reed valve loop scavenged design P25 25cc s of displacement Muffler on side of en...

Страница 8: ...umber mean is as follows 41 Hand held product B Sales level D Product type 160 Unique Identifier G Packaging code 965 Customer number The serial number is 1H130DI1521 The serial number reads as follow...

Страница 9: ...difficult The air filter should be cleaned every 10 hours of use depending on area of use dusty areas require more frequent cleanings To clean replace the air filter 1 Remove the air filter cover by p...

Страница 10: ...e insulator and electrodes indicate an over rich fuel air mixture too much fuel or not enough air Wet black deposits on the insulator and electrodes indicate the presence of oil in the combustion cham...

Страница 11: ...cessible for cleaning It should be inspected every 25 hours of use When it develops a carbon build up replace the muffler NOTE The spark arrestor also serves to keep blockages out of the exhaust syste...

Страница 12: ...fuel lines do not stand up to the removal installation methods used in the field Because of this the fuel filter and fuel lines are serviced with the fuel tank To inspect the fuel filter 1 Bend a piec...

Страница 13: ...ge 1 Run the engine until it is out of gas 2 Open the air filter cover 3 Spray some oil down the throat of the carburetor 4 Close the air filter cover 5 Pull the starter rope to turn the engine over a...

Страница 14: ...P25 Series of engines 10 For Parts Call 606 678 9623 or 606 561 4983 www mymowerparts com...

Страница 15: ...s and the order of the steps that follow are a suggested approach to troubleshooting the trim mer engine The technician does not necessarily have to follow them as described in this chapter Define the...

Страница 16: ...ndicate the level of care the equipment has received II Make sure the engine and attachments are securely fastened III The tune up factors NOTE Most hard starting and poor running conditions can be so...

Страница 17: ...ne engine diagnosis involves isolating problems in the four critical factors an engine needs to run properly I Ignition sufficient spark to start combustion in the cylinder occurring at the proper tim...

Страница 18: ...ton or similar damage IV Low compression case leak a Check compression Engine hard to start May start and run if prime test is used May run well at high RPMs but will not idle Sluggish pick up from lo...

Страница 19: ...er run unburned fuel igniting past exhaust port Mixture not burning completely in combustion chamber It may be too rich or it may be a spark plug or an ignition problem Skip Usually ignition related N...

Страница 20: ...d to the first repair 2 Delivery to customer We are not just repairing equipment we are repairing customers Inoculate against recurring problem with education e g if the problem was caused by stale fu...

Страница 21: ...event air flow or a missing filter that would indicate dirt ingestion 2 Make sure the ignition switch is in the run position 3 Attempt to start the engine 4 If the engine starts and runs long enough t...

Страница 22: ...e combustion chamber will result in an artificially high compression reading 4 Install a compression gauge in the spark plug hole 5 Confirm that the gauge is zeroed then pull the starter rope repeated...

Страница 23: ...peed 6 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 preven...

Страница 24: ...ection of this chapter 5 Install the clutch as described in the clutch section of this chapter 6 Pull the starter rope with the spark tester still hooked up to the spark plug wire If there is spark no...

Страница 25: ...scribed the switch is bad and should be replaced If the switch is working properly there is a short in the wires NOTE If there is reason to suspect that the ignition timing is off Remove the starter b...

Страница 26: ...ack wire is on the proper mounting screw See Figure 4 6 6 Place a non magnetic 0 010 feeler gauge on the flywheel magnets and rotate the flywheel until the magnets line up with the module Let the magn...

Страница 27: ...ab the crankshaft like a pawl nut when compressed To remove the washer use the flywheel as a slide hammer and drive the washer off of the crankshaft Discard the damaged washer NOTE Inspect the flywhee...

Страница 28: ...P25 Series of Engines 24 For Parts Call 606 678 9623 or 606 561 4983 www mymowerparts com...

Страница 29: ...than 90 psi 6 2 bar 1 Drain the fuel 2 Inspect the fuel 3 Check the fuel oil mixture 4 Test for alcohol 5 Inspect the fuel filter 6 Inspect the fuel lines 7 Check the fuel tank vent 8 Inspect the pri...

Страница 30: ...aper labeled sample from car buretor See Figure 5 1 4 Place a sample of two cycle mix where it says two cycle mix straight gas with straight gas and straight oil with straight oil See Figure 5 2 5 Mat...

Страница 31: ...o stoichiometric ratio for an engine burning alcohol is much richer than the stoichi ometric ratio an engine running on gasoline An engine tuned to run on gasoline will not run well on alcohol The mor...

Страница 32: ...fuel lines do not stand up to the removal installation methods used in the field Because of this the fuel filter and fuel lines are serviced with the fuel tank To inspect the fuel filter 1 Bend a pie...

Страница 33: ...ome 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...

Страница 34: ...re 5 9 3 Remove the two screws that hold the fuel tank to the starter housing using a T 20 torx driver See Figure 5 10 4 Slide the tank out to the rear 5 Install the fuel tank by following the previou...

Страница 35: ...ore subtle While specific problems with a carburetor can be identified on tear down identification of the carburetor as the location of the problem is usually done by process of elimination Treat the...

Страница 36: ...e engine from driving the fuel pump See Figure 5 13 7 Inspect the butterfly valve assemble for debris and freedom of movement See Figure 5 14 8 Perform a needle valve pop off test by following the car...

Страница 37: ...diaphragm and gasket If torn damaged or brittle install a diaphragm kit 4 Inspect the primer bulb 5 Inspect the inside of the pump chamber for dirt or varnish build up NOTE If there is a lot of dirt v...

Страница 38: ...NOTE If there is a minor amount of dirt varnish in the carburetor it would be worth while to clean and rebuild the carburetor by following the procedures recommended by the carbu retor manufacturer N...

Страница 39: ...e shaft E clip See Figure 5 22 13 Slide the throttle shaft out of the carburetor 14 The orifice check valve can also be seen from the pump side of the throttle housing See Figure 5 23 Figure 5 21 Butt...

Страница 40: ...The impulse port from the spacer lets the pulses pass through the carburetor body into the pump camber See Figure 5 24 15 Make sure these ports are clean and free of debris Figure 5 24 Impulse port Pu...

Страница 41: ...ottle valve assembly 5 Clean the metering orifice check valve with carbure tor cleaner 6 Install the needle valve valve lever and spring as one assembly 7 Tighten the hold down screw using a 2 phillip...

Страница 42: ...d the primer bulb 14 Tighten the screws using a 2 phillips screwdriver 15 Perform a needle valve pop off test by following the carburetor manufacturer s recommendations 16 Install the carburetor on th...

Страница 43: ...etor section of this chapter 2 Remove the throttle cable by pressing in the tabs located in the slots while lifting the cable out See Figure 5 29 3 Remove the three screws that secure the engine cover...

Страница 44: ...us steps in reverse order NOTE Apply a small amount of releasable thread locking compound such as Loctite 242 blue to the carburetor insulator block to cyl inder screws Tighten them to a torque of 45...

Страница 45: ...f with the clutch hous ing 2 Remove the screw that hold the engine cover to the starter housing using a T 20 torx driver See Figure 6 1 3 Remove the two fuel tank mounting screws using a T 20 torx dri...

Страница 46: ...sing See Figure 6 2 6 Slide the starter housing off of the engine 7 Install the starter assembly by following the previous steps in reverse order 8 Test run the engine before returning it to service F...

Страница 47: ...vice the rope pulley and springs 1 Remove the starter housing assembly by following the procedures described in the previous section of this chapter 2 Remove the two screws securing the pressure plate...

Страница 48: ...ssion of the engine it releases its energy spinning the inner pulley and the flywheel rapidly This allows the operator to pull the recoil rope slowly yet still generate enough cranking speed to start...

Страница 49: ...e recoil pulley assem bly fits over with a good quality lithium base grease 3 Install the pulley assembly into the starter housing assembly See Figure 6 10 4 Install a new retaining ring 5 Pass the fr...

Страница 50: ...has a shoulder on it that acts as a stop for the pawl nut Drive the pawl nut down till it stops See Figure 6 13 11 Test the operation of the starter recoil 12 Install the starter housing assembly be f...

Страница 51: ...le inner drive shaft The hous ing and the drive shaft are serviced separately To remove the drive shaft assembly 1 Remove the six screws in the grip with a T 20 torx driver See Figure 7 1 2 Let the gr...

Страница 52: ...acing the drive shaft housing remove the han dle bar shield and the trimmer head 7 Slide the drive shaft out of the housing See Figure 7 4 8 Follow these steps in reverse order to install the drive sh...

Страница 53: ...iver See Figure 7 5 2 Disconnect and remove the spark plug 3 Insert at least 18 0 5 M of starter rope in the spark plug hole to keep the crank shaft from rotating Keep some of the rope out so it can b...

Страница 54: ...housing See Figure 7 9 NOTE The small thrust washer goes on top of the rotor NOTE The side of the rotor with the flange faces the clutch drum See Figure 7 10 7 Torque the rotor to 210 230 in lbs 24 2...

Страница 55: ...ix ture As the piston travels downward The exhaust post is uncovered allowing the burn gases to escape The piston compresses the fuel air mix in the crankcase forcing the mixture up through trans fer...

Страница 56: ...in warrantable situations must be made using the 2 3 replacement cost rule Outside of warranty the decision is left to the dealer and the customer NOTE There are a few different paths that can be foll...

Страница 57: ...8 5 NOTE If replacing a short block exchange the crankcase and cylinder with the short block and follow the previous steps in reverse order 9 Remove the muffler using a T 27 torx driver or a 5 16 wre...

Страница 58: ...See Figure 8 9 16 Withdraw the wrist pin and remove the piston NOTE If there has been metal transfer from the pis ton skirt to the cylinder wall replace the short block 17 Immediately reinstall the wr...

Страница 59: ...n ring groove to prevent the end of a ring from getting caught on a port See Figure 8 11 NOTE When installing the piston rings the ring gaps must line up with the pins NOTE The piston rings are symmet...

Страница 60: ...Figure 8 13 7 Apply a thread locking compound such as Loctite 266 to the cylinder bolts Install the cylinder bolts using a T 30 torx driver and tighten them to a torque of 110 120 in lbs 12 14 Nm See...

Страница 61: ...and drive shaft assembly by following the procedures described in Chapter 7 Clutch and Drive Shaft 15 Install the spark plug 16 Test run the engine before returning it to service Apply a small amount...

Страница 62: ...P25 Series of Engines 58 For Parts Call 606 678 9623 or 606 561 4983 www mymowerparts com...

Страница 63: ...For Parts Call 606 678 9623 or 606 561 4983 www mymowerparts com...

Страница 64: ...MTD Products Inc Product Training and Education Department FORM NUMBER 769 06465 10 2010 For Parts Call 606 678 9623 or 606 561 4983 www mymowerparts com...

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