Poulan Pro P3314 scored piston: help me understand why this causes problems

The thing that is fooling people is the 120 psi. That may not be enough for this engine depending on compression ratio. Also that 120 is most likely a wet compression reading. Like the different between running a compression test and then adding oil to the cylinder to get a second reading. And wet test will be higher in most cases because the oil seals the rings. If you want to compare compression do a cold reading. Run the saw and then recheck when hot.

That score mark is not only bleeding off compression it is being fed directly into the crankcase which also effects the vacuum up cycle of the piston to draw in the next load of fuel/air.
 
Agree with IL about the false compression reading.

Is the cylinder becoming wet with fuel mix? If so there is one of two things normally. Poor compression or lack of spark under compression. With the scoring it is likely low compression under 100 psi until the cylinder is wet but by then the cylinder is usually flooded so it won't ignite the fuel load at that time any way.

There is a couple other possibles but they are rare and that is a clogged spark arrestor or a flywheel that has slipped its timing.

If things are right with the cylinder it should fire up a second on induced fuel mix. This would required emptying the carb of fuel and adding a little fuel via the spark plug hole.
 
OK, you guys were right about the compression. This morning I measured compression before attempting to fire up the engine and it was 70PSI. Big difference from the 120PSI that I initially measured when I started working on the chainsaw. In the future I'll know to repeat my compression measurements, especially if things start to go south. Below are better shots of the piston and cylinder on the exhaust side (intake side is clean) after disassembly. Thanks again to the gurus for all the help!
 

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scoring is much common on air cooled engines than water cooled. The depth of the scores will depend upon what scored it.
 
Not Necessarily IL. I accidentally toasted two leaf blowers that looked the same. I was in a hurry and pickup the wrong fuel can. I hated it as one was only a month old.

The initial damage is usually in the hottest area which is the exhaust port.
 
ttimtucker said:
OK, you guys were right about the compression. This morning I measured compression before attempting to fire up the engine and it was 70PSI. Big difference from the 120PSI that I initially measured when I started working on the chainsaw. In the future I'll know to repeat my compression measurements, especially if things start to go south. Below are better shots of the piston and cylinder on the exhaust side (intake side is clean) after disassembly. Thanks again to the gurus for all the help!
Click to expand...
Doneski. Toast. Cooked.
 
💀 Just to remind those who own any 2 stroke equipment. Don't loan out your equipment.
Follow the fuel mix recommendation.
That particular saw used 40:1 or 32:1 mix. Keep your mix separate from your non-mix gas.
 
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