Stihl 039 troubleshooting advice needed

[KMB]

A while ago I posted about a 039 that I was looking at for a friend to buy:

I just got back from checking on the saw. Cosmetically it's not pretty. Had the black screw on caps (did the older 039's have an orange fuel cap?) Part of the flat part of the handle is broken and, the top cover is cracked. I went to turn the chain around (was on backwards or "backards" as they say here in the south), and found that the chain tensioner assembly pin was broken. The bar had no paint, but had the Stihl markings, and is a 16" hard nose. The chain was sharp with some life left on it. A few pulls on choke till it coughed, one pull with choke off and it started. Sounded good, and it revved up nice. Max compression was 140. The air filter was dirty, but not real bad. The view through the muffler looked good, but there was 2 small vertical marks on the side of the piston, just before the top of the piston. I did a approximate measure of the bore through the exhaust, and it was just short of 2"...so it looks like it is an actual 039 (49mm/1.93in bore)...don't know why the 16" bar though. The chain brake worked good, but was a little loose. I probably would have offered $25 less...

Kevin

I had told him about muffler mods, and he insisted I do one on the saw. I did the mod, retuned the saw with my tach, and gave it back to him. Yesterday I asked him how it ran and he said that it had started out good, but then when it got hot (warmed up), it wouldn’t run right. He said when he did get it started, he had to keep on the throttle to keep it running, it wouldn’t idle properly.

So here’s the info on this saw as of today. I offered to look at the saw for him, instead of taking it to a shop. I want to look at the saw for him to see if I did something wrong when I modded the muffler and retuned the saw. I would say the muffler mod I did was conservative, pretty mild:

I haven’t done a pressure or vacuum test yet. I pulled the muffler and saw that the piston is scored pretty bad (I can’t remember if I looked at the piston before I did the muffler mod…I wish I would have documented it…I think it looked the way it was when he bought it where it showed 2 small vertical marks on the side of the piston, just before the top of the piston). He hadn’t used it much since he bought it, probably not even a tank of fuel through it :

The cylinder looks good. I’m not real good on reading plug color, but it looks like the plug is showing ‘rich’? He said he used premium gas mixed 50:1 :

I had the saw set at approx. 12,500 when it was warmed up (Stihl says 13,500 max. for stock setup). The H screw on the carb was turned out 1 turn (from lightly seated).

The saw will turn over. I have tried to start the saw and it won’t start. It has good spark, but the plug doesn’t seem to be getting fuel. I’m sure the carb could use a kit. Maybe the scored piston is causing it not to start? (this is my first time dealing with a scored piston).

I’m not sure about the air filter. It looks like it should have more to it than just the screen, correct? Did I miss that in the first place?:

That’s all I can think of to pass along to you fella’s in my initial troubleshooting. So let me have it. Is the scored piston (leaned out) my fault from the muffler mod and retune? Or will a press/vac test be needed?

I should add that regardless of where the fault lays, I want to do the work for him to get it back running. I have the service manual and IPL. I have read that these saws are fun to work on...

Kevin

 

[blsnelling]

There's no way that saw will start with a piston scored that bad. The rings are stuck in the grooves and cannot seat against the cylinder wall. You're not making enough compression to draw fuel from the carb. There is no other recourse than to replace at least the piston and possibly the cylinder. I'm confused that it seized considering how rich you set it. If that's truely the case, it sounds like he ran straight gas in it to me.

How did you determine that the cylinder looks good? The only way to really tell is to take it off. Looking at the intake wall through the muffler will not tell you. The scoring happened on the exhaust wall. You might be able to see a little through the spark plug hole but not much. There's no way to score a piston that bad and not transfer aluminum onto the cylinder wall.

That filter is shot. All the fleece is worn away leaving it with nothing to filter with. He's sucking all kinds of dust past that screen, which causes wear to the piston skirt.

BTW, why would you want to do a muffler mod and then tune it 1,000 RPMs below spec? The saw would run better as it came from the factory. If your going to mod a muffler, your not going to get any gains from it until the saw is tuned properly. Also, tuning it rich isn't going to make the saw last longer. It will only serve to carbon it up.

Sorry for all the bad news

 

[parrisw]

Ouch. Well something has happened. Either gone lean. Or he mistakenly put in his lawnmower gas instead of the 50:1 mix?? Did you ask what kind of oil he used??? That air filter looks like a Maple Leaf can go through it, what's up with that??

 

[parrisw]

Dam brad you beat me again!!!

 

[THALL10326]

A while ago I posted about a 039 that I was looking at for a friend to buy:



I had told him about muffler mods, and he insisted I do one on the saw. I did the mod, retuned the saw with my tach, and gave it back to him. Yesterday I asked him how it ran and he said that it had started out good, but then when it got hot (warmed up), it wouldn’t run right. He said when he did get it started, he had to keep on the throttle to keep it running, it wouldn’t idle properly.

So here’s the info on this saw as of today. I offered to look at the saw for him, instead of taking it to a shop. I want to look at the saw for him to see if I did something wrong when I modded the muffler and retuned the saw. I would say the muffler mod I did was conservative, pretty mild:

I haven’t done a pressure or vacuum test yet. I pulled the muffler and saw that the piston is scored pretty bad (I can’t remember if I looked at the piston before I did the muffler mod…I wish I would have documented it…I think it looked the way it was when he bought it where it showed 2 small vertical marks on the side of the piston, just before the top of the piston). He hadn’t used it much since he bought it, probably not even a tank of fuel through it :

The cylinder looks good. I’m not real good on reading plug color, but it looks like the plug is showing ‘rich’? He said he used premium gas mixed 50:1 :

I had the saw set at approx. 12,500 when it was warmed up (Stihl says 13,500 max. for stock setup). The H screw on the carb was turned out 1 turn (from lightly seated).

The saw will turn over. I have tried to start the saw and it won’t start. It has good spark, but the plug doesn’t seem to be getting fuel. I’m sure the carb could use a kit. Maybe the scored piston is causing it not to start? (this is my first time dealing with a scored piston).

I’m not sure about the air filter. It looks like it should have more to it than just the screen, correct? Did I miss that in the first place?:

That’s all I can think of to pass along to you fella’s in my initial troubleshooting. So let me have it. Is the scored piston (leaned out) my fault from the muffler mod and retune? Or will a press/vac test be needed?

I should add that regardless of where the fault lays, I want to do the work for him to get it back running. I have the service manual and IPL. I have read that these saws are fun to work on...

Kevin

Do yourself a quick easy favor. Remove the air filter, press the trigger and then look down the throat of the carb and get a look at the intake side of the piston. If its scored on the intake side like it is on the exhaust side chances are good thats a striaght gas issue with that saw. The intake side of the piston should not be scored for it gets cooled as the fuel enters the engine. Without oil in the fuel though both the intake and exhaust will show the same scoring, a lack of lube scoring.

 

[KMB]

Brad,

Thanks for the 'beating', correct on all points. I ain't to proud to take constructive critism...I'm still learning.

Why I set it that rich...it seemed that when I would start the saw, and make a few initial cuts, it wood rev higher, but then seemed to settle down after it warmed up. I had set the saw to approx. 13,000 after it warmed up, but then when I went to check it later, it was reving past the 13,000 initially, but would settle back down. I guess I got 'chicken' and thought the fella might not let the saw warm up properly. But I probably should have initially checked the saw properly before doing the muffler mod. I had planned on getting the saw back later (after he had put a few tanks of fuel through it) to retune it. Looks like I need to learn some more before I work on saws...

And yes, I figured that would be the case regarding having to look at the complete inside of the cylinder.

Thanks for confirming the shot air filter.

And anything else you can pass along for future knowledge, have at it.

Kevin

 

[KMB]

Ouch. Well something has happened. Either gone lean. Or he mistakenly put in his lawnmower gas instead of the 50:1 mix?? Did you ask what kind of oil he used??? That air filter looks like a Maple Leaf can go through it, what's up with that??

I'll confirm if he mistakenly used straight gas. And the air filter, it came with the saw and is something I obviusly missed from the start...my bad. He just knows running a saw...not much on maintenance.

Kevin

 

[KMB]

Do yourself a quick easy favor. Remove the air filter, press the trigger and then look down the throat of the carb and get a look at the intake side of the piston. If its scored on the intake side like it is on the exhaust side chances are good thats a striaght gas issue with that saw. The intake side of the piston should not be scored for it gets cooled as the fuel enters the engine. Without oil in the fuel though both the intake and exhaust will show the same scoring, a lack of lube scoring.

I'll go out and have a look, thanks for the tip.

Kevin

 

[timberwolf]

It will be bitterly argued against by some 50:1 die hards, but go 40:1 esp on a modified saw.

Tuning by RPM alone can be missleading, Go on sound too. If the saw tuning was jumping around there may have been a secondary problem (air leak or carb issue)?

 

[KMB]

Do yourself a quick easy favor. Remove the air filter, press the trigger and then look down the throat of the carb and get a look at the intake side of the piston. If its scored on the intake side like it is on the exhaust side chances are good thats a striaght gas issue with that saw. The intake side of the piston should not be scored for it gets cooled as the fuel enters the engine. Without oil in the fuel though both the intake and exhaust will show the same scoring, a lack of lube scoring.

I'll go out and have a look, thanks for the tip.

Kevin

I decided to pull the carb off to get a better look, and the intake side of the piston is fine, no scoring.

Kevin

 

[blsnelling]

If the saw tuning was jumping around there may have been a secondary problem (air leak or carb issue)?

+1. The RPMs should not jump around.

I'm glad you didn't take offense. It wasn't meant that way.

 

[KMB]

It will be bitterly argued against by some 50:1 die hards, but go 40:1 esp on a modified saw.

Tuning by RPM alone can be missleading, Go on sound too. If the saw tuning was jumping around there may have been a secondary problem (air leak or carb issue)?

Thanks for that. More and more I'm realizing I should have recognized a problem earlier.

Kevin

 

[THALL10326]

I decided to pull the carb off to get a better look, and the intake side of the piston is fine, no scoring.

Kevin

Ok, that solves that question. Now one more thing to look at and look at close before digging into the saw. Check his chain. If its beat to hell there lies your over reving. If its sharp then the saw itself has a issue that lead to the scoring.

 

[KMB]

+1. The RPMs should not jump around.

I'm glad you didn't take offense. It wasn't meant that way.

For sure no offense taken. You (and others here) have good saw stuff to pass along. I really like working with/on saws and I don't mind being corrected for my learning. This already has been good for me.

Kevin

 

[blsnelling]

More and more I'm realizing I should have recognized a problem earlier.

Kevin

Hey, you're learning. That's what it's all about. How many pistons did I go through before I found the crank seal problem on the 045. It had no typical symptoms, but it was there all the same. Not all lessons are easily learned. Looks like you had nothing to do with this piston failure, but you get to learn all the same.

 

[KMB]

Ok, that solves that question. Now one more thing to look at and look at close before digging into the saw. Check his chain. If its beat to hell there lies your over reving. If its sharp then the saw itself has a issue that lead to the scoring.

I had sharpened his chain real good before giving the saw back to him. The chain still looks good with a good edge. But it was very loose (sagging) when I got it back.

Kevin

 

[KMB]

Hey, you're learning. That's what it's all about. How many pistons did I go through before I found the crank seal problem on the 045. It had no typical symptoms, but it was there all the same. Not all lessons are easily learned. Looks like you had nothing to do with this piston failure, but you get to learn all the same.

I just went back a read through your 'what caused this failure' thread. Good info there. I'll definitely be pressure and vacuum testing this saw.

Kevin