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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?

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.

Burk62450 said:

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.

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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.

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

ILENGINE said:

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

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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.

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.

Hammermechanicman said:

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.

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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.

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.

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.

Hammermechanicman said:

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.

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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.

The difference between 150 and 105 is too much to just be wet vs dry ring. Those poulans just don't seem to be as robust as saws like stihls with nikasil plated cylinders. The 34cc poulans have thinly chrome plated pistons and bare aluminum cylinders (cheaper) that do not tolerate running lean for more than a few seconds as opposed to nikasil cylinders that can tolerate lean running longer before scoring.

This is an air head setup and some require an extra rich high side carburetor (as much as 3 turns out from seated) so yes it is possible has sever cylinder damage from running way too lean. Pull the muffler and check the exhaust side of the piston for damage. 105 psi is right at the bare minimum for any two cycle normally to even hit on fuel mix.

Luckily the PNC is only about 33-35 usd plus shc or about 22 usd plus shc if you go through a distributor. Really cheap compared to what they were in the past. Plus as said they are cheap for a reason; no plating.

I don't know if this is a strato saw or not. If it is Star is right about setting the mixture. These cockroach saws are clamshell crankcase design so swapping out a P&C kit would cost a customer more than a new saw. None of the dealers and big shops around me won't even work on them. I have a guy who flips stuff he finds on the side of the road or garage sales. He brings me about a half dozen of these saws a year. Most go in the traah. If i can fix them with just fuel lines and a carb clean he has me fix it and he then sells it for $20 more than i charged. I think he just like doing it because he isn't making much money.

If you work on these things and the customer complains about it crapping bar oil everywhere when it is stored tell them to let the saw cool and then open the oil reservoir cap then replace and it MIGHT not crap so much oil while stored and drain all the fuel out or be prepared to remove the fuel cap with pliers and then when it won't start replace the fuel lines.

Well I am 99% sure it is a Strato setup. Item 11 is the air head and item 12 is the carburetor.

Yup, strato.

Also this saw sold for $300 +- pending where you brought it. It is now a discontinued unit.

Scrubcadet10 said:

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.

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It had warmed up, because idled for a couple of minutes prior to finding the H adjustment that allowed WOT. Unless a couple of minutes isn't long enough. It took a little bit to find that spot. Many moments it would sit and idle while I was thinking.
Steve has a couple of good video's about how to tune a two cycle.

ILENGINE said:

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.

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I always pour a tiny bit of mix into the cylinder before i do a compression test. IIRC, it was something I learned on this forum, and it just stuck in my head. And check compression at least 3 times.

I would investigate the PNC for damage as compression number now is too close to failure level. It easy enough to pull the muffler. The damage may have already been there to start with.