Picked up a 291 for giggles from a buddy. Problem is that it’s “blown”. Barely holds 20psi on a pressure test. I’ve yet to dig into it for an in depth diagnosis but I was looking for recommendations on rebuild kits. Looking to stay at or beyond the 56cc but want an exact fit. OEM is a fairly steep option in my quick browsing. TIA!
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Hi Tcirre, I have seen a few others who get confused with compression tests and pressure tests often mixing them together like you have above.
Just to clarify - a pressure test is done to test for air leaks, you seal all ports and exits before filling a saw with air to around 7psi. Included in this test is a vacuum test which is used to test the validity of the crank shaft oil seals which often lift under vacuum.
Though each manual is slightly different, it’s generally accepted that if it holds for a minimum of 20+ seconds it’s a pass.
During the above tests it’s often recommended to spin the crank slowly too, doing so can often show leaks that aren’t present otherwise.
On the other hand a compression test is done to test piston ring wear. You open the throttle butterfly and ensure the air filter and spark arrester screen are clean / clear before attaching a compression tester to the spark plug hole. You then pull the saw over until the gauge no longer increases. This test doesn’t require the saw to hold a set reading for a set period of time. The compressed air is held by an inline shrader valve.
Hope this just clears up any possible confusion.
As for rebuild kits, often you can just pop in a new set of ORM rings which will bring the compression back up, so long as the current piston is in good condition.
I’d highly recommend investing in a pressure & vacuum tester instead of spending that money on a rebuild. You could spend 200-300 usd on a rebuild and then the saw dies after a few minutes due to an air leak. This is all you really need -
https://www.ebay.com/itm/21pcs-Hand-Held-Vacuum-Pressure-Pump-Tester-Brake-Fluid-Bleeder-Bleeding-Tool-US/274376624766?_trkparms=aid=1110009&algo=SPLICE.COMPLISTINGS&ao=1&asc=20191002091346&meid=2f0dc7df8d0640208e37d8ecd48665ef&pid=101103&rk=6&rkt=6&sd=132920168823&itm=274376624766&pmt=1&noa=0&pg=2332490&algv=default&brand=8milelake&_trksid=p2332490.c101103.m3021
Save your money, opt for just new OEM rings and invest into a pressure and vacuum tester to test the seals, carb, o-rings, gaskets, lines, atmospheric valves etc.
You may find that there are other underlying problems too, like old carb diaphragms, needle, blocked drilling’s / jet, seals leaking etc, but I wouldn’t replace parts unnecessarily.
Start with the above and then see how it runs. Fix actual problems not possible ones and if parts ain’t broke, don’t “fix” them
Sorry that was so long, I hope it helps though. Others will be in shortly with different advice and opinions. Take as much as you can get.
Warm regards, Tom
Tom, that is quite a bit to take in at once! But very insightful. I appreciate you taking the time. As far as the reading I comment, that is straight from a Stihl service dept along with the cylinder is “blown”. Again, I haven’t dove into the saw yet to see what the issues may be as the mentioned notes were also taken during a commercial repair so I’m not sure if it’s worthy or not as they don’t like to work on non homeowner marked equipment. I will continue to watch for more advice into the swap but will also take your information and use it. Thanks!Hi Tcirre, I have seen a few others who get confused with compression tests and pressure tests often mixing them together like you have above.
Just to clarify - a pressure test is done to test for air leaks, you seal all ports and exits before filling a saw with air to around 7psi. Included in this test is a vacuum test which is used to test the validity of the crank shaft oil seals which often lift under vacuum.
Though each manual is slightly different, it’s generally accepted that if it holds for a minimum of 20+ seconds it’s a pass.
During the above tests it’s often recommended to spin the crank slowly too, doing so can often show leaks that aren’t present otherwise.
On the other hand a compression test is done to test piston ring wear. You open the throttle butterfly and ensure the air filter and spark arrester screen are clean / clear before attaching a compression tester to the spark plug hole. You then pull the saw over until the gauge no longer increases. This test doesn’t require the saw to hold a set reading for a set period of time. The compressed air is held by an inline shrader valve.
Hope this just clears up any possible confusion.
As for rebuild kits, often you can just pop in a new set of ORM rings which will bring the compression back up, so long as the current piston is in good condition.
I’d highly recommend investing in a pressure & vacuum tester instead of spending that money on a rebuild. You could spend 200-300 usd on a rebuild and then the saw dies after a few minutes due to an air leak. This is all you really need -
https://www.ebay.com/itm/21pcs-Hand-Held-Vacuum-Pressure-Pump-Tester-Brake-Fluid-Bleeder-Bleeding-Tool-US/274376624766?_trkparms=aid=1110009&algo=SPLICE.COMPLISTINGS&ao=1&asc=20191002091346&meid=2f0dc7df8d0640208e37d8ecd48665ef&pid=101103&rk=6&rkt=6&sd=132920168823&itm=274376624766&pmt=1&noa=0&pg=2332490&algv=default&brand=8milelake&_trksid=p2332490.c101103.m3021
Save your money, opt for just new OEM rings and invest into a pressure and vacuum tester to test the seals, carb, o-rings, gaskets, lines, atmospheric valves etc.
You may find that there are other underlying problems too, like old carb diaphragms, needle, blocked drilling’s / jet, seals leaking etc, but I wouldn’t replace parts unnecessarily.
Start with the above and then see how it runs. Fix actual problems not possible ones and if parts ain’t broke, don’t “fix” them
Sorry that was so long, I hope it helps though. Others will be in shortly with different advice and opinions. Take as much as you can get.
Warm regards, Tom
Funny...I was given a 291 which the dealer said was not worth fixing. I am pretty sure they never even looked at the piston before making that determination.
Won't hurt anything to remove the muffler and look at the piston.
After that, I would not go any further with the teardown until you do a pressure and vac test.
Roy
Won't hurt anything to remove the muffler and look at the piston.
After that, I would not go any further with the teardown until you do a pressure and vac test.
Roy
No worries re read it a couple of times and it will sink in.
It’s quite possible it has a scored piston from an air leak.
If you pop the muffler off and have a look at the piston, you’ll probably see lots of vertical score lines.
If so, the repair is more in depth, but one step at a time
re read it a couple of times and it will sink in.It’s quite possible it has a scored piston from an air leak.
If you pop the muffler off and have a look at the piston, you’ll probably see lots of vertical score lines.
If so, the repair is more in depth, but one step at a time
No worries
It’s quite possible it has a scored piston from an air leak.
If you pop the muffler off and have a look at the piston, you’ll probably see lots of vertical score lines.
If so, the repair is more in depth, but one step at a time
I took the carb and muffler off last night actually and well, it’s toasted! I’ve never seen a piston so scored. Oddly though through the minimal view points the cylinder walls look great. Looks like a full tear down/rebuild or trash can visit is in store!
I rebuilt one that was scored and seized, and I still have it and it sees use, but it might not be worth it.
mine was less than a day old, but it still cost me close to a new one doing it myself.
There's not a whole lot of aftermarket support for these machines I could find & the parts add up fast.
mine was less than a day old, but it still cost me close to a new one doing it myself.
There's not a whole lot of aftermarket support for these machines I could find & the parts add up fast.
for wht it's worth I still don't know exactly what killed mine, I got it out of the rubbish pile behind the dealer it was bought from after I watched a tree company un box it & go to cutting a massive muddy root ball with a super dull chain.
It seized & they refunded him. I asked for it, they said sure & rebuilt it.
It was written off as straight gas at first, but I don't actually know because he used the same fuel in all the other saws on the job that day.
Either way, it practically requires tearing the entire saw down to put a cylinder & piston in & my cylinder wasn't salvageable.
I cleaned it up, it looked ok, but the exhaust side was all tore up when I felt around it with a dentists pick or fingernail. I had darn near the cost of a new saw in parts by the time it was all said and done along with a good chunk of time.
Actually more time than an entire kit saw assembly takes.
It seized & they refunded him. I asked for it, they said sure & rebuilt it.
It was written off as straight gas at first, but I don't actually know because he used the same fuel in all the other saws on the job that day.
Either way, it practically requires tearing the entire saw down to put a cylinder & piston in & my cylinder wasn't salvageable.
I cleaned it up, it looked ok, but the exhaust side was all tore up when I felt around it with a dentists pick or fingernail. I had darn near the cost of a new saw in parts by the time it was all said and done along with a good chunk of time.
Actually more time than an entire kit saw assembly takes.
I’d suggest that if you want to rebuild it for enjoyment, the challenge and reward, go for it. If you want to do it to have a working saw and aren’t bothered, I’d personally advise against it.
Here is a video of Mustie1 rebuilding a 290 (basically the same saw) to give you an idea of what’s involved, though you must pressure and vacuum test everything. I wouldn’t advise you follow his process, it’s a little haphazard and instead grab a manual from the chaps on here.
Here is a video of Mustie1 rebuilding a 290 (basically the same saw) to give you an idea of what’s involved, though you must pressure and vacuum test everything. I wouldn’t advise you follow his process, it’s a little haphazard and instead grab a manual from the chaps on here.
