I see like usual every Spring there is a splurge of "WHY DON'T IT RUN?" threads popping up.
This thread is intended for newbies to read and help with initial diagnostics on what may be wrong with their saw. I wish I had a flow chart to show but will try to keep this simple. This is meant to help with diagnostics and not to be a complete repair manual. Keep in mind you need 5 basic things for a saw to run right: compression, fuel, spark, sealed fuel delivery system (pressure/vac), lastly air in/exhaust out.
Starting procedure: Make sure gas and oil are full and that your gas is mixed (this may seem obvious to some... but there are bone heads out there). Choke your saw and pull the starter cord till the saw pops, turn the choke off and the saw should start in the next few pulls. If you continue to choke the saw after it pops the first time you are probably flooding the cylinder preventing it from starting.
Commonly needed Tools:
compression tester
T27 screw driver (Stihl's mostly)
Slotted screw driver
Spark plug socket (or bar wrench)
Inline spark tester
Allen Wrenches (huskies and older stihls etc)
IF your saw does not start:
1. Check compression. Most late model saws will need a minimum of 135-140psi to run and operate; vintage and older mag saws can run at 100psi. Below this number you need new rings, new piston and rings, or new piston and cylinder. .
If you do have compression move to #2.
If you are below the above numbers above, I recommend removing the muffler and checking for vertical scoring on the piston. A small flashlight is useful so you can see to the intake side of the cylinder through the exhaust outlet. After determining the cylinder condition you can decide what parts to replace. If it's scored a new top end will be needed. If the cylinder is still in good shape I recommend a new piston and rings. I have not had good luck with aftermarket rings that come with cheap pistons... Save yourself the headache and get cabers if you buy a cheap piston, or do the best thing a buy an episan or meteor piston that comes with good rings. Keep in mind you will need to determine the cause of the scoring or low compression and fix the source of the problem before running the saw again. Putting a new top end on a saw that is broken somewhere else will just score the new cylinder. More direction on this later.
2: You have good compression but saw wont start.
A.) Check your muffler screen to make sure it's not clogged.
-if clogged throw it away or clean it. saw can't run right if exhaust can't get out
B.) Air filter clean? Saws do need an air/fuel mix to run. No air = no run.
C.) Check for spark: if none, make sure your flywheel are coil are gaped correctly (I just space them with a business card. If that doesn't work it may need new coil or if older the points cleaned. If you get to this point ask questions...
D.) Make sure you are getting gas to the cylinder, is your plug wet or dry after trying to start it?
-if plug is dry will it pop when you put a shot of fuel mix directly into carb? If the answer is yes you need to check fuel filter, fuel lines, impulse line and carb. If no, ask questions and include what you have done till this point
3.) Air leaks and vac test.
Air leaks in the fuel system cause saws to torch or score the cylinders and will cause any number of other problems so that a saw wont run right. On basic principle you need everything sealed up tight with no air leaks from the intake of the carb to the exhaust outlet. This includes the fuel line from the carb to the fuel filter. The most common air leaks are in the fuel/impulse lines (cracks and holes) or the oil seals. They can also come from a carb not seated right on the intake boot or the intake boot having a hole or tear. I could go at length about how to test for this but there are good threads on here about how to use a mityvac or similar system to do this testing.
4.) basic stuff
-Use synthetic 2stroke oil. it protects better against engine wear: don't care what kind
-Don't use ethanol fuel: Don't care what people say or read it's a saftey precaution. Also ethanol binds with water and carries that through a fuel system if for no better reason
-
This thread is intended for newbies to read and help with initial diagnostics on what may be wrong with their saw. I wish I had a flow chart to show but will try to keep this simple. This is meant to help with diagnostics and not to be a complete repair manual. Keep in mind you need 5 basic things for a saw to run right: compression, fuel, spark, sealed fuel delivery system (pressure/vac), lastly air in/exhaust out.
Starting procedure: Make sure gas and oil are full and that your gas is mixed (this may seem obvious to some... but there are bone heads out there). Choke your saw and pull the starter cord till the saw pops, turn the choke off and the saw should start in the next few pulls. If you continue to choke the saw after it pops the first time you are probably flooding the cylinder preventing it from starting.
Commonly needed Tools:
compression tester
T27 screw driver (Stihl's mostly)
Slotted screw driver
Spark plug socket (or bar wrench)
Inline spark tester
Allen Wrenches (huskies and older stihls etc)
IF your saw does not start:
1. Check compression. Most late model saws will need a minimum of 135-140psi to run and operate; vintage and older mag saws can run at 100psi. Below this number you need new rings, new piston and rings, or new piston and cylinder. .
If you do have compression move to #2.
If you are below the above numbers above, I recommend removing the muffler and checking for vertical scoring on the piston. A small flashlight is useful so you can see to the intake side of the cylinder through the exhaust outlet. After determining the cylinder condition you can decide what parts to replace. If it's scored a new top end will be needed. If the cylinder is still in good shape I recommend a new piston and rings. I have not had good luck with aftermarket rings that come with cheap pistons... Save yourself the headache and get cabers if you buy a cheap piston, or do the best thing a buy an episan or meteor piston that comes with good rings. Keep in mind you will need to determine the cause of the scoring or low compression and fix the source of the problem before running the saw again. Putting a new top end on a saw that is broken somewhere else will just score the new cylinder. More direction on this later.
2: You have good compression but saw wont start.
A.) Check your muffler screen to make sure it's not clogged.
-if clogged throw it away or clean it. saw can't run right if exhaust can't get out
B.) Air filter clean? Saws do need an air/fuel mix to run. No air = no run.
C.) Check for spark: if none, make sure your flywheel are coil are gaped correctly (I just space them with a business card. If that doesn't work it may need new coil or if older the points cleaned. If you get to this point ask questions...
D.) Make sure you are getting gas to the cylinder, is your plug wet or dry after trying to start it?
-if plug is dry will it pop when you put a shot of fuel mix directly into carb? If the answer is yes you need to check fuel filter, fuel lines, impulse line and carb. If no, ask questions and include what you have done till this point
3.) Air leaks and vac test.
Air leaks in the fuel system cause saws to torch or score the cylinders and will cause any number of other problems so that a saw wont run right. On basic principle you need everything sealed up tight with no air leaks from the intake of the carb to the exhaust outlet. This includes the fuel line from the carb to the fuel filter. The most common air leaks are in the fuel/impulse lines (cracks and holes) or the oil seals. They can also come from a carb not seated right on the intake boot or the intake boot having a hole or tear. I could go at length about how to test for this but there are good threads on here about how to use a mityvac or similar system to do this testing.
4.) basic stuff
-Use synthetic 2stroke oil. it protects better against engine wear: don't care what kind
-Don't use ethanol fuel: Don't care what people say or read it's a saftey precaution. Also ethanol binds with water and carries that through a fuel system if for no better reason
-
