What did I do wrong?

[PTmowerMech]

Poulan Pro PP5020AV chainsaw.

Compression at about 150. Good spark. Prime it, choke it, and it cranks and idles fine but bogs when revved up. Over and over, (having to crank it a dozen times or more) I finally starts revving without bogging. I'm opening it up trying to find that strokin sound, but after at least 1.5 turns and still not hearing it, I start backing it back down. Simply because it sounds like the RPM's were too high. I get about 1/4 turn back from where it started revving up without bogging, and it dies. Then no more start.
I pulled the muffler and seen what looks like a pretty good gouge in the piston (facing the exhaust port) On the intake side, I can see the slightest bit of where the cylinder is just starting to score.

From the time I started getting WOT without bogging, to the time it died was maybe 1 minute. I was thinking I may have overheated it. But since this only has a small air filter cover, , that only uncovers the plug and the carb, then air should've been getting around the head pretty well.
I didn't check the cylinder first, only the compression. After the fact, I can barely get 105 to 110lbs of compression.

Did I do something wrong? Or do things like this just happen with Poulan saws?

 

[Burk62450]

poulan saws are really bad about bad fuel lines. the crack break and allow air into the system making it not want to rev up and almost impossible to adjust. first thing i do with them is replace fuel lines and primimer if the unit has one. far as the scoring on the piston you mentioned, if it still has 110psi of compression it should still fire. how old is your fuel and are you certain you have good fuel with proper mix. 50/1 works on pretty much every saw but alot of saws wont run on 40/1. have you checked internet for mix for your saw? is the air filter dirty at all because that will send dirt to the cylander and piston witch will cause damage.

 

[PTmowerMech]

poulan saws are really bad about bad fuel lines. the crack break and allow air into the system making it not want to rev up and almost impossible to adjust. first thing i do with them is replace fuel lines and primimer if the unit has one. far as the scoring on the piston you mentioned, if it still has 110psi of compression it should still fire. how old is your fuel and are you certain you have good fuel with proper mix. 50/1 works on pretty much every saw but alot of saws wont run on 40/1. have you checked internet for mix for your saw? is the air filter dirty at all because that will send dirt to the cylander and piston witch will cause damage.

Air filter and fuel lines were good. I checked those things when I checked the compression and spark. New fuel also. I guess I should've mentioned that first.

After rechecking the spark, found it to still be good. Tried some carb cleaner through the venturi, and still won't crank. Probably lost a flywheel key, or the compression reading I'm getting isn't accurate. I remember Bert saying these type of compression testers aren't accurate.

 

[ILENGINE]

The Husqvarna and Poulan saws don't take much cylinder damage to cause issues.

 

[PTmowerMech]

The Husqvarna and Poulan saws don't take much cylinder damage to cause issues.

So you don't think it was anything I did? Like that minute of WOT?
I should've checked through the exhaust port before I started on this. But I could tell that the saw was rarely used. This is one of my return customers. He only uses Stihl. But bought this PP when two of his stihls went down on a weekend. Said he only used it about 8 to 10 hours. IIRC, he said he bought it last year. And after getting his Stihls up and running, he just put it in the shed.
The plug was coated. More so than one might think with only 10 hours on it.

 

[Hammermechanicman]

Not for sure but any scoring is not good. You said it started at 150 and now 105??? If that is true you did something wrong. You may have an air leak or dirty carb causing lean running at WOT. And another one gone and another one gone another one bites the dust hey hey.

 

[PTmowerMech]

Not for sure but any scoring is not good. You said it started at 150 and now 105??? If that is true you did something wrong. You may have an air leak or dirty carb causing lean running at WOT. And another one gone and another one gone another one bites the dust hey hey.

Can carbon breaking loose cause something like that. Note, there was no gouge in the ring. Only on the top edge of the piston and right below the ring.

 

[Hammermechanicman]

I doubt it. Pull the carb and muffler and put the piston at BDC and look through the intake and see if there is any aluminum transfer on the exhaust side of the cylinder. If there is then cylinder is toast.

 

[Scrubcadet10]

I remember from a video Steve did, he said not allowing a chainsaw to warm up can cause the cylinder to score and rings can get stuck. something to do with the piston expanding faster than the cylinder.

 

[ILENGINE]

Not for sure but any scoring is not good. You said it started at 150 and now 105??? If that is true you did something wrong. You may have an air leak or dirty carb causing lean running at WOT. And another one gone and another one gone another one bites the dust hey hey.

My thought was the difference between a wet compression and a dry compression. If the first compression reading was done with fuel/oil in the rings could give a higher compression reading but after running and drying out the rings will be lower since they don't seal as well.