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

I was given a Poulan Pro P3314 chainsaw which was sitting in a person's garage for years. It has what appears to be significant scoring on the piston as seen in this photo. I don't see scoring on the cylinder (but my only view is from the exhaust port side). Compression is 120psi. Crankcase maintains pressure/vacuum (from MityVac) just fine throughout piston travel. However it is tough to start, and near impossible to restart once warm. Carb was very clean (but sprayed passages and ran it through my ultrasonic cleaner) and diaphragms were in good shape. I know the general advice is that a machine with a scored piston will not run right. I'd like to get help from this forum to understand specifically what's going on that causes problems in a case like this, where there is a scored cylinder, yet compression and crankcase integrity look good. Does the scoring somehow affect the ingestion of fuel from the carb on the compression stroke? Or does it affect the transfer of fuel from the crankcase to the cylinder? Or the scavenging of exhaust? Or ...? Thanks for any schooling you can provide.

I don't think what you're seeing has anything to do with it not running right ,Don't get locked into this as the problem , I really doubt this is anything but normal wear / tear in a chainsaw that has seen use. I would look at your fuel system first , your compression is good so cylinder scoring is a non issue in my opinion , 2 cycle machines can be a pain for the smallest thing .
Just my 2 cts

I don't know if it's the picture resolution, but that doesn't look bad to me your compression is good, and it runs well. For starting, check the prime, and fuel lines.

I am seeing deep scratches down the left side of the piston in the picture. I will guarantee that the ring is stuck in the piston groove. Remove the spark plug and have a look at the cylinder wall through the spark plug hole with the piston in the down position. Engine is trashed.

In this case compression is lying.

Use a torch and get the muffler red hot to cook out the burnt carbon deposits.

As for the scoring, it looks minor and was likely from excess carbon that has collected on top of the piston. This problem happens quite often with people who mix their own oil/fuel concoctions.

Most people think of carbon deposits as soft and fluffy, but when cooked multiple times it becomes "coking" that is harder than the piston aluminum.

carbon deposits and getting hot.

Thanks to all those replying. Some more data. Prior to my posting, the fuel line, and both lines to the primer bulb were replaced along with the fuel filter. Also the tank was cleaned and fresh fuel used. I did just now pull the spark plug and can see significant scoring on the cylinder wall above the exhaust port. I can push on the piston ring with a screwdriver (via the exhaust port) and can see it flex and rotate ever so slightly, so I don't think the ring is stuck. I will pursue the fuel side of the issue (I have a new/clone carb on order).

This is all a learning experience for me, and my goal is to arrive at a definitive root cause so I can get better at this stuff. If the piston/cylinder scoring is the issue, how exactly is that causing a problem that doesn't reveal itself via some specific measurement or test? If it is a fuel problem, the new carb may make a difference.

Before ordering parts, confirm exactly which PP3314 that you have. There is the type 1, type 2, WS, and there may be another couple.
Scored pistons can be caused by running with a dirty air filter or improper fuel mix.

New carb may help. I believe the carb may have a diaphragm to pump fuel, and they can harden. Glad to hear you dealt with fuel lines, tank and fresh fuel. So often, that's what it needs.

ILENGINE said:

I am seeing deep scratches down the left side of the piston in the picture. I will guarantee that the ring is stuck in the piston groove. Remove the spark plug and have a look at the cylinder wall through the spark plug hole with the piston in the down position. Engine is trashed.

In this case compression is lying.

Click to expand...

The scoring on the left side of piston is significant. Granted being able to view with other pictures, or better yet, in person would be ideal. However, calling this minor scoring, or fooling yourself with a 120 psi compression reading, is not right. Erratic idle, hard starting, not staying running once it warms up, are all signs of LOW compression. What you see with your eyes in a visual inspection versus a compression reading is more important. How the saw starts and runs versus a compression reading is more important.
This is a crappy residential saw to begin with. She is toast.