Tighten it up and scribe a line on the bar against a non-moving part on the saw, least ways by doing that you can either confirm or refute the bar it's self moving...............Mike

 

Are the bar nuts knurled? There should be visible slashes that act as lock nuts against the bar. Probably something similar on the back side, under the bar. If not, maybe try lock washers?

That's if you're positive the bar guides aren't worn. Bars aren't exactly cheap, but maybe consider replacing it as a test.

 

Sounds like you are just loosening the nuts and moving the bar. You need to loosen the nuts on the bar until it is loose, then adjust the screw that tightens the chain by moving the bar, then tighten the nuts to secure the bar.

 

OP says:

i replaced the tensioner thinking that may be the problem.

 

Check the sprocket teeth, bar rail as mentioned, the roller for being pinched if it's a roller nose bar and make sure the bar itself has not bent. If the sprocket teeth are grooved from wear it will bind the chain causing it to jump off. If you put on a new chain and don't replace the sprocket make sure the sprocket is not deeply grooved from the old chain because it will ruin a new chain as it will set it's own wear pattern. Also use a premium bar/chain oil; I use Stihl because it is so tacky and does not sling off the bar tip as easily as others especially like the cheap stuff. Good luck.

 

Any chain will stretch as it gets hot, don't try adjusting it right away or it will be too tight when it cools off.

 

I'd say it's time to take it to a service center and have them look it over. Chances are another pair of trained eyes will spot the problem immediately.

 

I had the same issue with my Stihl. I noticed it just kept getting looser and looser this year. It’s a fairly new chain. I finally ran out of adjustment. Don’t know why. I ended up just having a link taken out of it.

The service center showed me a trick when tensioning it, but I don’t know if it applies on all saws - keep the bar angled upward when tightening the chain, so it won’t have the tendency to loosen right back up. I haven’t tried it yet.

 

I've always done that on every saw I've owned... I have a wood block on my bench just for this purpose.

 

Thats a definite on the bar whilst adjusting.
If you have trouble holding it up and tightening at same time,
use a block of 2x4 to help.
Also adjust the tension screw in the same manner.
If the bar drops while adjusting it will never set right.

 

Sounds like you are doing everything right.
You may need to get new jamb nuts for it.
That serration on the backside of the nuts helps hold in place.
I would not try the lock washer idea as it can damage the bar.
Some of the smaller,18" bar, Stihl have only one nut and they hold.
Also, and you probably do this.
Make sure the bar and saw mating surfaces are clean before assembly.

 

I tried marking the bar as mentioned. After cutting with it the chain loosened up, but the bar did not move. I forgot to mention it will also titghen itself back up after you shut the saw off and set it down. I lift the bar up when tightening the chain.
The bar doesnt look bent. It lays flat on a level surface and will even stand on both edges nicely.
The nuts that are on the saw do not have those groves and neather do the nuts on my poulan. (I know what you guys are talking about though)
The sproket that is on it is husqvarna p/n 501831601 which is discontinued. I think the replacment is 503650901.
I guess that may be my next step.

 

I tried marking the bar as mentioned. After cutting with it the chain loosened up, but the bar did not move. I forgot to mention it will also tighten itself back up after you shut the saw off and set it down. I lift the bar up when tightening the chain.
The bar doesnt look bent. It lays flat on a level surface and will even stand on both edges nicely.
The nuts that are on the saw do not have those groves and neather do the nuts on my poulan. (I know what you guys are talking about though)
The sproket that is on it is husqvarna p/n 501831601 which is discontinued. I think the replacment is 503650901.
I guess that may be my next step.

Loosening and tightening back up means the chain is getting hot, metals expand when getting hotter. You may have a lube issue. If there isn’t enough lube, the chain will get hot.

• Does it look like it’s getting enough oil? What oil do you use?
• What are you cutting? Hardwood makes the chain hotter.
• How long has it been since you sharpened the chain? Did you file the rakes too?
• Have you tried a new chain?

 

Not sure if anyone has asked this question...but for how long has this bar been in use? What kind of jobs has it done?

 

This saw was made from 99-01 or so. It was my grandfathers, but not sure if he bought it new. He cut about 8 cords of firewood a year with it (we have pine up here) but it hasnt seen much use in the past 10 years.

 

I have a 18 inch, stihl ms250 and have done some pretty tough jobs with it. I've already went through 2 bars and one sprocket. Usually when I'd start getting symptoms like yours, I would replace the bar and it would fix it. If you decide to replace that sprocket, i would consider replacing the bar along with the chain as well. Oregon makes some very good bars(from my experience better than stihl).

Like RT said, make sure oil is getting from the saw, to the bar, and on the chain. Take of the bar and run the saw and see if oil will come out. Use compressed air to clean out the oil channels in the saw and bar and blow out the chain channel on the bar.

Hope you get it fixed!

 

Heat from friction could make the chain lengthen and get looser. I would follow the above suggestions and add that I would want to be sure the oiler is working properly.
If you use the wrong lube, it may be being 'flung' from the chain too readily when the chain is moving. Alternatively, it may not be flowing as well as it should, starving the chain for lubrication, causing it to run hot.
I cut through an oak... with a small McCullough that was a brothers and had seen use. Sharpened the chain so often, but NEVER had it get hot to the touch. I have no idea why, but I surely don't remember it being hot.
tom

 

Oil (or lack thereof) could be the issue i’m having as well. Admittedly I’ve Used filtered drain oil for bar oil for years, but only recently started having this issue. I did get suspicious a couple months ago, and washed out the oil tank real good with gas, thinking maybe the filter was clogged, but noted no difference afterwards. Wonder if I should put an older chain on there and see if it repeats. Still seems to be using the normal amount of oil (pretty much matching tank for tank with the gas).

 

Use a piece of cardboard to test if your saw is flinging oil. Start the saw and and hold the tip about 4-6" from the card board and throttle up. If you see a lot of oil landing on the cardboard youre getting oiled fine. Just some times when youre using it hold it about a quarter throttle for several seconds just to ensure the entire bar and bearings are oiled.

Also scrape the bar grooves away from the sproket and make sure your sproket bearing is greased. Look for a pin hole next to the axel.

The best way to check bar straightness is to look down a groove holding it right under your eye. Ive had pool ques that rolled straight but when you look sown them theyre twisted every which way.

 

Is this a spur or rim sprocket? On a spur that's worn with valleys cut in.....You can tighten the chain on a ridge, then the drivers find the valley = instant looseness.

 

OP said it tightens up if he lets it set a bit after running. Someone said they use drain oil, but regular bar oil is a lot stickier and will lube a bar better, especially when it gets hotter. Some things you can do cheaper, but IMHO bar oil isn’t one of them.

Back to an earlier question I asked, have you tried a new chain? That should rule out the chain sharpness.

 

Definitely one that i agree on. Went through that experience myself. Ruined quite a few chains befor figured it out. Just don't do it. Using bar and chain oil is cheper in the end.

 

Over the last 10 years, I’ve saved more than enough by not buying bar oil to replace the bar and chain multiple times. I still haven’t worn out a bar (however I did upgrade to a 20” bar about 4 years back because it saved me from having to bend over as much), but have worn many chains due to repeated sharpening. I’ll stick with filtered drain oil until I have to replace at least one bar “prematurely.”

Update: I got lazy the other day and just swapped a sharp spare chain a couple years old with the newer dull chain I was having issues with loosening up. Ran through 3 tanks of gas without needing to tighten up the chain. I think it’s just a bad chain...