TWO-CYCLE ENGINE TRAINING
TROUBLE SHOOTING TWO-CYCLE ENGINES
SAW FAILS TO START DUE TO A LACK OF FUEL SUPPLY:
« CHECK FUEL TANK FOR FUEL
« IS FUEL FREE OF WATER AND ALCOHOL?
« CHECK FUEL FILTER FOR FREE FLOW OF FUEL
« IS FUEL LINE CRACKED OR DAMAGED?
« IS THE FUEL TANK VENT OPERATING PROPERLY
« AIR FILTER CLOGGED WITH SAW DUST
« ALL GASKETS INSTALLED PROPERLY
« IS CARBURETOR ADJUSTED ACCORDING TO MANUFACTURE SPECIFICATIONS
« IS MUFFLER SCREEN CLOGGED NOT ALLOWING DISCHARGE OF DIRTY AIR
SAW ACCELERATES POORLY OR SLOWLY
« LOW AND HIGH SPEED NEEDLES TWO LEAN
« INLET CONTROL LEVER SETTING SHOULD BE LEVEL WITH METERING CHAMBER
« INLET NEEDLE SHOULD BE FREE FROM STICKING
« CONNECTING HOLE TO ATMOSPHERE ON DIAPHRAGM COVER COULD BE BLOCKED
« METERING DIAPHRAGM SHOULD NOT BE STIFF OR DAMAGED
SAW FLOODS AND ENGINE STALLS
« CARBURETOR LEAKS DURING PRESSURE TEST
« CHECK INLET NEEDLE FOR PROPER SEAL
« IS CONTROL LEVER FREE ON AXLE?
« CHECK SPRING FOR PROPER PLACEMENT BETWEEN CONTROL LEVER AND CARBURETOR BODY
« IMPROPER CONNECTION BETWEEN CONTROL DIAPHRAGM AND CONTROL LEVER
« CONTROL LEVER SET IMPROPERLY
ENGINE SPEED DROPS QUICKLY UNDER LOAD
« CHECK AIR FILTER FOR EXCESSIVE SAW DUST
« IS FUEL TANK VENT OPERATING PROPERLY
« CHECK FUEL LINE FOR CRACKS OR HOLES
« FUEL FILTER DIRTY OR HARDENED
« FUEL PUMP DIAPHRAGM FAULTY OR IMPROPERLY INSTALLED
« HIGH SPEED NEEDLE BORE PORT BLOCKED
« IS CARBURETOR FUEL SCREEN CLOGGED
ENGINE FAILS TO IDLE OR IDLES TO HIGH
« LOW SPEED NEEDLE TO RICH
« CHECK LOW SPEED NEEDLE BORE PORT FOR BLOCKAGE
« ADJUST THROTTLE VALVE OPENING WITH THE IDLE SPEED SCREW
LACK OF SPARK
« IS SPARK PLUG BOOT FIRMLY SEATED ON SPARK PLUG
« CHECK CONDITION OF SPARK PLUG
« CHECK HIGH TENSION LEAD FROM IGNITION COIL TO SPARK PLUG
« CHECK FOR CORRECT AIR GAP FOR CORRESPONDING COIL
« REPLACE IGNITION COIL
LACK OF COMPRESSION
« TEST CYLINDER FOR SUFFICIENT COMPRESSION (100 PSI MINIMUM)
« PRESSURIZE CRANKCASE TO CHECK FOR ANY LEAKS AT SEALS OR GASKETS
HOODS AND COVERS
The covers for the cylinders ensure that the cool air generated by the flywheel fins is passed to the cylinder area.
All the chain saws are equipped with an insert for summer winter operation. The insert generally will always be set for summer operation. This will decrease the temperature of the carburetor during running.
Summer operation (constant temperature above 32ç Fahrenheit): To prevent overheating of the carburetor and starting problems, the passage must be closed.
Winter operation (constant temperatures below freezing): An open passage will cure problems of icing of the filter and carburetor.
The small homeowner saws have and orange insert in the filter hood. The insert is removed from the underside and then should be rotated 180ç to the correct setting. The top of the hood is embossed with symbols ( ) is for summer operation and (*) is for winter operation.
The mid-range saws have a white plastic plug in the air box bottom wall between the cylinder and carburetor. Summer operation is obtained by having the plug in place, removal of the plug will set the saw for winter operation.
In the lower end range of Professional chain saws there is a shutter in the cylinder hood. The shutter is opened and closed from the carburetor side of the hood. Closed will be the summer operational setting and open will be for winter.
The larger end of the Professional chain saws uses a cylindrical valve in the air filter hood. On the top of the hood are the symbols (0) for summer use and (*) for winter use. The top of the valve has an arrow that is molded into the plastic valve and can be turned with the assistance of a screwdriver to the correct setting for the current temperature.
All hoods and covers for all products (Chain Saws, Power Cuts, String Trimmers, Brush Cutters, Blowers and Pressure Washers) should be checked periodically for cracks or damage. If such a crack exists, dirt ingestion may occur, causing damage to the cylinder and piston.
Air filters are provided to remove sand, dust, sawdust and other foreign materials from the aspirated air, thus reducing wear of the cylinder and piston.
Depending on the ambient and cutting conditions, air filters must be cleaned at least once a day. Chain Saw filter contamination depends upon whether dry or fresh timber is cut, and whether the chain saw chain is dull or sharpened. Power Cut filters will become contaminated sooner when the tool is being used to cut stone or concrete as opposed to steel. Filters for the
line trimmers and brush cutters will require cleaning at least once a month.
A spark plug having a sooted up or oily insulator nose shows that its heat rating is too high.
A spark plug whose heat rating is too low will show pronounced electrode erosion after only a short operating time.
The operating temperature of the spark plug should not exceed 500çC. At even longer operating periods, at full throttle, the operation temperature should not rise above 900çC. 500çC is the self-cleaning temperature of the spark plug at which the oil and soot particles deposited on the insulator nose will burn up. If the temperature rises above 900çC the fuel-air mixture will not be ignited by the spark, but will have been ignited prematurely by the red-hot parts of the spark plug (know as auto or pre ignition).
NOTE: Auto ignition can cause serious damage to the engine. Incorrect spark plug usage will void the warranty.
The purpose of the ignition system is to ignite the compressed fuel-air mixture in the cylinder at the correct time. Ignition occurs when the ignition coil generates a high voltage and discharges a strong spark at the electrode of the spark plug. In view of the extreme high speed of the engine, great demands are made on the function of the ignition coil. Efficient combustion of the fuel, and consequently the output of the engine, depends to a high degree upon the intensity of the ignition spark at the precise moment of ignition.
Removal of the flywheel is done with the assistance of the following special tools:
Models Tool Required
DCS-330s 944 500 880
DCS-6800i 957 408 001
DCS-9000 957 427 010
The purpose of the chain brake is to completely immobilize the chain in the event of a kickback and to protect the operator from serious injuries that may result from a running chain.
Activation of the chain brake:
1. Automatically (in the event of a kickback during operation);
2. Manually (by pushing the hand guard in the direction of the end of the guide bar).
Sprocket guard cannot be fitted.
Brake system released.
Models Method of release
DCS-330s The band can be released by lifting the gold portion of the brake band up until the - plastic catch springs up and holds the brake band in the released position. DCS-401
DCS-430 With the bar removed from the saw place the chain brake over the bar studs. The tab on the disengaging lever must be inserted into the two slots at the top of the chain brake. With the tab in both slots push the hand guard forward until the brake band is in the released position.
DCS-6800i Push the hand guard forward to release the brake band.
The chain brake fails to stop the chain
Brake band broken.
Replace the brake band.
NOTE: For models DCS-430 / DCS-6800i, tool (number 950 237 000) will be need to remove and install the tension spring. For the DCS-9000, tool (number 944 603 080) will be needed to install the tension spring.
An ignitable fuel air mixture is prepared in the carburetor. This fuel air mixture must always be
the same ratio (1:14) to ensure optimum combustion.
The diaphragm carburetor is comprised of three essential assemblies: 1. The venturi tube (1); SEE SECTION 16 2. The pump system (2);
3. The control system (3);
During the suction and compression stroke, the piston moves from bottom dead center to top dead center. In the course of this movement, vacuum is formed in the crankcase, causing air to be aspirated through the bore in the carburetor. The narrowest part of this bore is called the venturi tube. When air passes through this orifice, its velocity of flow is increased. Immediately behind this point, the tube widens, resulting in a drop of pressure about the outlet opening of the venturi nozzle. The resulting vacuum causes the fuel to be aspirated from the venturi nozzle. A fuel-air mixture in the form of an atomized emulsion is then supplied to the engine.
PUMP SYSTEM SEE SECTION 16
The rhythmically alternating pressure conditions (aspiration = underpressure; compression = overpressure) brought about by the movement of the piston in the crankcase are used for driving the pump diaphragm (4). The pulse passage (5) connects the internal crankcase with the external pump diaphragm. During underpressure in the crankcase (the piston moves toward top dead center), the following occurs: The pump diaphragm is pulled upwards, causing the fuel space in the pump chamber to extend; the suction valve (6) is opened; the exhaust valve (7) is closed; and fuel from the tank flows into the fuel space.
During overpressure in the crankcase (piston moves toward bottom dead center), the following occurs: The pump diaphragm is pulled downwards; the suction valve is closed; and exhaust valve is open.
The fuel, via the fuel filter, is supplied to the inlet needle.
CONTROL SYSTEM SEE SECTION 16
The inlet needle (9) opens and closes under the movement of the control diaphragm (10). The dry side of the control diaphragm is exposed to atmospheric pressure; the wet side is affected by the vacuum generated through the fuel extraction during the rotation of the engine. The inlet needle is opened by the control lever (11). The fuel from the delivery side of the pump can now reach the dosaging chamber and through nozzles L and H (L= Low speed or idling (12); H= High speed or full throttle (13)), can also reach the venturi tube, where the mixing of air and
fuel takes place. Control of the inlet needle by the control diaphragm ensures a continuos and uniform proportion of fuel in the dosaging chamber. The diaphragm is fitted with an aluminum stabilizing plate to ensure that the control diaphragm will not flutter under the rapidly alternating pressure differences, but will smoothly open and close the inlet needle.
STARTING SEE SECTION 17
For cold starting, a rich mixture is required, i.e. the choke (1) is closed and the throttle plate (2) is slightly or half-open. For higher engine speeds, the choke must be opened to permit a larger amount of air to enter.
IDLING SEE SECTION 17
For idling speed, the throttle plate is slightly open. The suction action of the engine now occurs exclusively via the primary fuel bore (3).
SPEED TRANSITION SEESECTION 18
In order to obtain a satisfactory run-up from idling speed to full throttle, an increase in the fuelŒair ratio is required. Engine output is increased by a single secondary idling bore (4) located behind the primary bore (3). The secondary bore is uncovered when the throttle plate is opened.
FULL THROTTLE SEESECTION 18
The fully opened throttle plate causes an increase in the air velocity through the venturi tube (5). Fuel aspirates through the main nozzle (6), reaches the airstream and enters the engine in the form of an atomized emulsion.
INJECTION CARBURETORS SEE SECTION 15
The injection carburetor does not have a choke shutter. When the saw is started an additional valve injects the fuel. An exact quantity of fuel is injected into the intake channel if you open the injection valve when starting the saw. The engine obtains a fuel-air mixture that is flammable under all running conditions.
The basic construction of the carburetor did not change, you can test and maintain the pump and control diaphragms in the same manor as you do on the traditional diaphragm carburetor.
Testing the injection valve should be done when the engine is cold. Start the engine in the semi-throttle lock position with valve opened. Let the engine run and observe the rpm behavior. If the engine speed reduces considerably after seconds and then dies, the valve is operating properly.
ROTARY VALVE CARBURETORS SEESECTION 19
One of the most unique features of the rotary or barrel valve carburetors is the throttle assembly. It represents a significant change from our standard throttle valve design. The valve, as used in this application, was developed to deliver the best mixture of fuel and air during transition form idle to wide-open throttle and, at the same time, maintain maximum engine performance. The main advantage of the barrel design is that it allows smoother idle transition, even though it maintains the venturi effect of a standard diaphragm carburetor. This is important because line trimmers and brush cutters are not always used at wide-open throttle. These machines are operated at various part-throttle settings.
These carburetors are preset with no adjustment needed to the high or low running needed. 'The or\y adjustment necessary is to the idle speed screw. Should the need arise to make adjustments to the high-speed rpm it will be necessary to replace the jet (14). Different sizes are available. The size of the jet will depend on the location altitude where the unit is being operated.
List of items for Rotary Valve Carburetors: 1. Throttle Valve
3. Low Speed Needle 4. Inlet Screen
5. Needle Valve 6. Fuel Inlet
7. Metering Lever Spring 8. Metering Lever 9. Metering Diaphragm 10. Metering Chamber
11. Fuel Pump Diaphragm 12. Fuel Outlet
13. Engine Pulse 14. Main Jet
16. Idle Adjusting Screw (This screw is set and nonadjustable.)
17. Primer Pump
18. Combination Valve
FIXED JET CARBURETORS
Fixed jet carburetors are diaphragm carburetors without high-speed adjustment screws. These carburetors are installed on models DPC-7001 and DPC-9501. The absence of the high-speed needle eliminates the chance of improper adjustment at the high-speed setting. Improper adjustment on the high speed for a power cut can cause damage to the cylinder and piston due to a lean running condition and with the unit running above the maximum allowable rpm could cause the abrasive blade to come apart, causing injury to the user.
Connect the pressure tester (956 004 000) hose to the fuel inlet nipple. Close the vent screw on the rubber bulb and pump air into the carburetor until the pressure gauge shows a reading of approximately 0.8 bar (12psi). If the pressure remains constant the carburetor is airtight. However, if it drops, there are two possible causes:
1. The inlet needle is not sealing (foreign matter in valve seat or sealing cone of inlet needle is damaged or inlet control lever sticking).
2. The metering diaphragm is damaged.
In either of these cases the carburetor must be removed and serviced.
it is advisable to check the serviceability of the fuel pump whenever the carburetor is removed for repair.
1. Remove the carburetor
2. Unscrew and remove the fuel pump end cover.
3. Remove the gasket and pump diaphragm.
NOTE: The diaphragm and gasket often stick to the cover or carburetor body. If this is the case, take particular care when separating them.
If the fuel strainer in the pump side of the carburetor body is dirty, use a scriber to pry it out and then clean or replace it.
NOTE: If the screen is damaged always replace it.
In such a case the fuel pickup filter should also be inspected and cleaned or replaced if necessary.
To disassemble the carburetor, unscrew the metering chamber end cover and lift it away. 1. Remove the metering diaphragm and gasket from the carburetor body and cover. 2. Carefully separate the diaphragm and gasket.
NOTE: The diaphragms are the most delicate parts of the carburetor. They are subjected to continuous alternating stresses and the material eventually show signs of fatigue, i.e. the diaphragms distort and swell. They have to be replaced when this stage is reached.
The inlet needle valve is located in a recess in the metering diaphragm chamber. Take out the retaining screw. Remove the inlet control lever with axle, spring and inlet needle. If there is an annular indention on the sealing cone of the inlet needle, it will be necessary to replace the inlet needle because it will no longer seal properly. Constant flooding of the carburetor even though the needle is clean indicates this. Remove the carburetor adjusting screws. Pry the welch plug out of the metering chamber.
NOTE: The welch plug is destroyed during removal. It should therefore, only be removed if a replacement is available.
Wash the carburetor body and all serviceable parts in fresh white spirit and blow clean with compressed air, paying special attention to the bores and ports.
Fit the needle and the spring in their respective bores. Insert axle in the inlet control lever, engage clevis in annular grove on the head of the inlet needle and tighten down the retaining screw. Make sure that the spring locates on the control lever's nipple. Check for easy action of the inlet control lever.
NOTE: The top edge of the inlet control lever must be exactly level with the metering diaphragm seating face. If necessary, use suitable pliers to carefully bend the control lever into position.
Fit the gasket, metering diaphragm and end cover. The metering diaphragm and gasket are held in position by the integrally cast pegs on the body. Insert the fuel screen at the pump side. Fit the pump diaphragm, gasket and end cover and tighten down securely. The pump diaphragm and gasket are held in position by the integrally cast pegs on the end cover. Refit the carburetor adjusting screws.
When the engine is tested at the factory the carburetor is set to obtain a slightly richer mixture to provide the cylinder bore and bearings with additional lubricant during the break-in period. This setting should be left as it is for the first three tank fillings. The high speed adjusting screw may then be turned up to 1/8 turn clockwise to obtain a leaner mixture.
If the unit is used at high altitudes it may be necessary to alter the carburetor setting slightly. This correction is made at the two adjusting screws (H and L) as follows: Turn clockwise for a leaner mixture.
NOTE: Even very slight variations at the adjusting screws produce a noticeable change in the engine running behavior. Always make sure that the engine is warm and the air filter clean before carrying out carburetor adjustments. The setting of the high-speed adjusting screw not only affects the engine's performance but also its maximum no-load speed. If the setting is too lean (adjusting screw turned too far clockwise), there is a risk of damaging the engine as a result of insufficient lubrication and overheating. Corrections to the high speed adjusting screw may only be carried out if an accurate tachometer is available. Using the tachometer, run the engine at full throttle and turn the high speed adjusting screw clockwise to obtain the engine's maximum speed. Then turn the high-speed adjusted screw counterclockwise from that position until engine speed falls about 150 rpm.
If the carburetor has to be adjusted from scratch, carefully screw both adjusting screws both adjusting screws clockwise down onto their seats to obtain a starting point for fine-tuning. Then make the following adjustments:
High speed adjusting screw H:
back off 1 full turn
Low speed adjusting screw L: back off 1 full turn
If an accurate tachometer is not available, do not turn the high speed adjusting screw beyond this basic setting to obtain a leaner mixture.