Chainsaw Repairs 
Simple Chainsaw repairs for the determined Home-owner

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Chainsaws made for Home-owners to trim trees and cut firewood.
Some brands are available under 16" and 39cc. for less than $100.

Chainsaws make quick work of otherwise difficult chores.
But if they get cantankerous, they themselves can be difficult at best. AND they are about the most Dangerous Tool you can own. BE CAREFUL !!


Seasonal Storage: Bad news for Gasoline Engines! It is best to store them indoors where it is warm, and there is no sunshine. Plastic parts may become brittle, stretch, or shrink. Gas Tubing and Carburetor Diaphragms may develop pinholes or lose their elasticity.

ABOVE ALL: Don't leave Gasoline in a Chainsaw for extended periods of time!! The gas will evaporate and leave behind a residue of "varnish" that will coat and plug up everything it touches. That's VERY bad news for a Carburetor and its tiny Jets and passages.

Instead, drain the gas out, start it up, and work it or let it idle until it runs out of gas. Then put a teaspoon of 2-cycle Oil in the Spark Plug hole and slowly pull the Starter Rope one time. This will coat the moving parts with oil and help protect them from corrosion, oxidation and rust. You will get the best coating if the Spark Plug hole is straight up.


It's helpful to know that 4 common problems are inexpensive and within the realm of a determined do-it-yourselfer:

Spark Plugs: Inexpensive and easy to replace.

Spark Arrestor: Inexpensive and easy to clean or replace.

Gas lines: Inexpensive and not too difficult to replace

Carburetors: May only need to be adjusted. Kits are usually under $10.00 and not all that difficult. But be aware that Kits may not include instructions.
    
Remove the sharp Chain before attempting these Repairs !!
The Bar, Nuts, and cover can also be left off.


If it runs poorly:
(1) - Put Kickback Brake in "Run" position.
(2) - Make sure the Choke is turned "OFF".
(3) - Make sure the Gas is fresh.
(4) - Replace the Spark Plug with a new one.
(5) - Clean or replace the Spark Arrester

If your Chainsaw won't start:
(1) - Make sure the Switch is turned "ON".
(2) - Put Kickback Brake in "Run" position.
(3) - Check that it is filled with fresh Gas
(4) - Prime and Choke the Carburetor
       according to the Owner's Manual.
(5) - Replace the Spark Plug with a new one.


If the starter rope won't pull:
Under certain conditions fuel can build up in the Cylinder(flooding)
which prevents the piston from reaching top dead center.

If the switch does not work,
or if there is no spark,
or any condition which causes flooding,
may cause this impossible starting scenario.

In such cases, a removed spark plug is usually
found to have a drop of fuel between the electrodes,
which may be very hard to see.

Wipe the Electrodes with a Paper Towel,
it should show a wet spot.
The Rope should pull easily with the Plug removed.

Drain all the fuel out of the tank.
Allow the empty spark plug opening
to face downward for several hours
with dry newspaper under it.

Pull the Starter Rope occasionally.
Any fuel coming out of the Spark Plug hole
should be visible on the newspaper.

Then when you install the Spark Plug,
it should turn over normally again.

Put fresh fuel in it and try starting it
with the Switch "ON"
and NO CHOKING OR PRESSING OF ANY PRIMER BULB.

When it tries to start,
then follow the Starting Instructions in your Manual.


 

There are three Basic things you need, to have an Engine run: 
Proper Compression, Proper Spark, and Proper Fuel.

Compression can be tested with a gauge similar to the one that checks your car tires. It will have a tapered rubber tip that presses against the Spark Plug hole, or a threaded tip that screws into the Spark Plug hole. You pull the Starter rope to get a reading.

But you may know you have a compression problem by the effort required to pull the Starter Rope. If it suddenly pulls very easily, you probably have lost some compression. This can be caused by various problems, most of which require an experienced mechanic. He can also test  the compression for you.

Spark is delivered to the tip of a Spark Plug to ignite the gas inside the Cylinder. You can test for Spark by grounding the metal threads of the Plug against the Engine while you gently pull the rope. You should see a tiny Spark jump at the threaded end of the Plug.  

All this assumes that the Plug is good, you have the wire attached, and the Switch turned "ON". If you have No Spark, try a new Plug. 

Next check the wires that go to the "ON-OFF" Switch. They should be tight and not touching the engine metal anywhere in their length.

If you still have no Spark and the wire appears to be good; back to the "Experienced Mechanic".

Fuel: Here, more can go wrong than a little bit. 
Always make sure the Tank is full of FRESH Fuel first. 

Next, remove the Gas Line from the Carburetor that goes to the Gas Tank. Remove the Cap and put a Soda Straw into the Gas Tank with a plastic bag wrapped around the Straw to make an air-tight seal to the Tank. 

When you blow in the Straw, gas should squirt out of the disconnected Gas Line. If not; you have a plugged Fuel Filter inside the Tank. Replace it.

If the Gas Line is long enough to allow it, cut 1/2" off the end when you reconnect it to the Carburetor. Better yet; replace it. If you have gas at the Carburetor, and you have Spark and Compression, then chances are you have a very common Carburetor problem.

Carburetors may having sticking Needle Valves, leaking Gaskets, deformed Valves and bad fuel Pump Diaphragms or plugged screens. All these parts are included in a Carburetor Rebuild Kit. This page will help you install it.


Fig. 1- This simplified diagram shows how gasoline is sucked through a Carburetor and into the Cylinder, by the vacuum created when the Piston in made to move DOWN in the Cylinder, by pulling on the Starter Rope. Then the resulting upward travel of the Piston forces the exhaust gases out the Muffler. 

A Clutch and Sprocket assembly fastened to the Crankshaft, drives the saw Chain; but only when the engine speed is high enough to cause Weights within the Clutch to move outward by centrifugal force. 

Fig.2- The fuel pump is operated by vacuum created in the Sump below the Piston when it moves UP. The orange Vacuum Line sucks the blue rubber fuel-pump Diaphragm up, which lifts the red valve and sucks gasoline up the red Filter and tubing which hang in the gas Tank. An unshown valve isolates the green gas line to the Cylinder, preventing the pressure above the Piston from going into the Fuel Pump.

The red weighted flexible gas Filter drops to whichever side of the gas tank is down at any given moment, allowing Gas to enter the Carburetor, no matter what position the Chainsaw is working in. 



 
 


The top two photos are typical of Carburetors from tiny 2-cycle gasoline engines which have no oil sump. Instead the lubrication comes from 3 or 4 ounces of special 2-cycle oil which is mixed with 1 gallon of gasoline by the operator. These are different views of the popular Walbro Carburetor, showing the Valve Cover and Fuel Pump Cover.
 

Top Left photo shows the bottom of the Carburetor which gets bolted to the Engine-Intake with the Gasket shown, and allows the fuel-air mixture to enter the Cylinder. It's ignited by the Spark Plug which creates an internal explosion and makes power by causing rotation of the Crankshaft.

Inside the round throat, you can barely see the brass "Butterfly" which is a Valve that determines how much fuel/air mixture is allowed into the Cylinder. This Butterfly is about the size of a Penny. The big silver Screw allows access to the Valve Diaphragm.

At the bottom-left is a black Machine Screw with a Spring under the head. This is the Idle Adjustment which causes the half-moon cam to turn the Butterfly and change Idle Speed. The Spring behind the half-moon returns the Butterfly to idle speed. At top-left and bottom-right are Gas Inlets which carry gas from the Tank or the Primer Bulb.

Top Right photo shows the Choke Butterfly and the 4 screws that hold the cover on the Fuel Pump diaphragm chamber. The brass shaft at the top-left is moved by the Choke Lever to control the amount of additional gas used on cold-startups. The Screw in the center of this Butterfly allows its removal which releases the Choke Shaft for disassembly. 

The Accellerator Butterfly is similarly retained. These two Butterflies are connected to the manual controls that allow the operator to choke and accellerate the Engine.

Bottom-Left photo shows a K10-WAT Walbro Carburetor Kit which costs about $8.00 including the 2 pieces of Gas Line which are different diameters to fit the different diameter (why?) Gas Inlets! The bottom row shows the NeedleValve parts, Filter Screens, and Welch Plugs.

Bottom-Right photo shows a closeup of the Penny with the needle valve Rocker, a "C"-Ring Retainer, and a Screen so tiny it looks like the cutout for the hanger hole in the plastic bag. If you are not aware that this tiny part is in the kit, it can easily be lost.
 



Disassembly:
Lay out the Carburetor parts as you remove them so you can reassemble them in reverse order. A 7-compartment pill box is handy for this.

1 - Remove the large machine screw from the center of the Valve Cover.
2 - Remove the Valve Cover.
3 - Remove the rubber Valve / Gasket assembly

4- Remove the 4 small screws at the corners of the Fuel Pump Cover.
     (it has a raised dome with a hole at the edge)
5 - Remove the Fuel Pump Diaphragm / Gasket assembly.  

It has a round metal plate with a 1/8" long pin in the center.
This Pin operates the Needle Valve through the Rocker

 


The tiny Needle Valve Spring has no desire to "take its seat". In fact, when it's compressed into working position, it may very well take flight never to be seen again. A simple wooden toothpick can make you the boss. 

Put it through the Rocker into the Spring as shown above. Then press the whole assembly into place as you tighten the screw. It should be noted that first you should have started the screw a thread or two.

Be sure the "Y" Yoke of the Rocker is placed in the groove at the top of the Needle Valve, as shown above.

If your Rocker has no hole above the Spring, it may be best to take your Carburetor to a Chainsaw Shop which is experienced in dealing with flying springs. Of course, you have the option of working with your hands and the Carburetor inside a clear plastic bag which will limit the Springs' flight.



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