Jonsered621 suddenly died

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While you're at it, for the price, replace the decomp. I just replaced the top end on a Stihl 660 that had similar damage to the piston. The saw wouldn't stay running. After tearing into it, I found that the decomp was shot. It had zero "pop" to its movement. I suspect that the bad decomp was leaking which not only killed compression but the leaked air was leaning out the mixture and created a hot spot on the piston.

Good advise...….except the 621 never came with a decomp...…..the only true Jonsereds that ever came with a decomp was the 87cc Jonsereds 90, the 87cc Jonsereds 910E/EV and the Jonsereds 111S.
 
Good advise...….except the 621 never came with a decomp...…..the only true Jonsereds that ever came with a decomp was the 87cc Jonsereds 90, the 87cc Jonsereds 910E/EV and the Jonsereds 111S.
Hi again. I got my parts today. They look good. I have to clean carbon off top of piston. Not sure what is best way. Maybe gasoline and scrub brush. Silly question, is it always better to get new piston rings or can I reuse the ones that came with the piston that look real good. I'm planning on getting the rings and gasket package unless it is better to use the old rings.20200526_191511.jpg
 
I already said that was probably an OK deal.....depending on the gasket thickness......but the reality is there is no other source....
Hi I got it going today. It runs and idles great. The chain doesn't move when it idles. Is that ok? Write back if you can. Thanks.20200602_152034.jpg
 
I already said that was probably an OK deal.....depending on the gasket thickness......but the reality is there is no other source....
The connecting rod moves side to side more than before. Almost an eighth of an inch. I hope it is ok. I might have made it worse when I put the piston pin in. I heated the piston with a hair dryer and pushed it in but it only went up to first keeper ring. I tapped it in backed up with wood. I used ethanol free gas and ultra oil. It really likes it. I loosened the chain and it moves a little when idling.
 

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The connecting rod moves side to side more than before. Almost an eighth of an inch. I hope it is ok. I might have made it worse when I put the piston pin in. I heated the piston with a hair dryer and pushed it in but it only went up to first keeper ring. I tapped it in backed up with wood. I used ethanol free gas and ultrther a oil. It really likes it. I loosened the chain and it moves a little when idling.
There is no way to tell the side play without the cyl bolted down and then there is no way to tell it after either......you have to rely on the thrust washers being correct to center the conrod....and they will.
 
Your chain should not be moving at idle. Might next to tweak idle speed down a little.
Thanks for writing. My chain wasn't moving when it was idling with the chain adjusted properly. When I loosened it it started moving a little so it is probably just right.
I'm glad I was able to get the parts. I haven't seen any pistons or cylinders except the ones you told me about. I bid on a saw, a 621. I bid 99 dollars and it sold for 100. 20200602_202512.jpg
 
Congrats on your repairs...….however......You still have not isolated the reason the original top end got toasted. I find this concerning.....I can tell you it was not from cutting large wood for a long time.....the 621 is about the absolute most robust 56 cc saw ever made. I can't help thinking you have not conquered the problem yet.....perhaps I'm wrong.....but I've taken a lot of these apart and put a lot of 'em together and in my experience the task is not yet completed.
 
Congrats on your repairs...….however......You still have not isolated the reason the original top end got toasted. I find this concerning.....I can tell you it was not from cutting large wood for a long time.....the 621 is about the absolute most robust 56 cc saw ever made. I can't help thinking you have not conquered the problem yet.....perhaps I'm wrong.....but I've taken a lot of these apart and put a lot of 'em together and in my experience the task is not yet completed.
Thanks for writing. I believe you. I may buy a mityvac 8500. It idles fine and the rubber hoses are still good after 40 years. Maybe I'm deluding myself. The cheap gas and bad oil could have caused it. Do you think a pressure test and vacuum test could put me in the clear or are there other things to do without taking it apart? I would like to buy another one and practice taking a 621 apart. I'm keeping my eyes open for parts. I may have mentioned it loves the ethanol free gas and ultra oil.
 
Congrats on your repairs...….however......You still have not isolated the reason the original top end got toasted. I find this concerning.....I can tell you it was not from cutting large wood for a long time.....the 621 is about the absolute most robust 56 cc saw ever made. I can't help thinking you have not conquered the problem yet.....perhaps I'm wrong.....but I've taken a lot of these apart and put a lot of 'em together and in my experience the task is not yet completed.
Hi again. I just found this. Do you think it has possibilities? I will write him. https://www.ebay.com/itm/Vintage-Jo...524194?hash=item23d80c0ea2:g:VdkAAOSwaspezZLm
 
Thanks for writing. I believe you. I may buy a mityvac 8500. It idles fine and the rubber hoses are still good after 40 years. Maybe I'm deluding myself. The cheap gas and bad oil could have caused it. Do you think a pressure test and vacuum test could put me in the clear or are there other things to do without taking it apart? I would like to buy another one and practice taking a 621 apart. I'm keeping my eyes open for parts. I may have mentioned it loves the ethanol free gas and ultra oil.

A pressure/vac test would tell the tale but a bit of work to gear up properly and the purchase of a MityVac etc. As I said earlier, on a saw of this age I would just replace the crank seals. This too is a bit of work but is actually cheap insurance.....and even after spending the money and time testing you'll likely find you will have to replace them anyway.

That old 621 is in rough shape and dirty but I don't believe it has had a lot of use. Likely a decent start for a project...the Total (Tsmura) bar is a plus too. A new top cover/switch and a good cleaning would improver the looks a great deal. Price is a bit high and the shipping too but this is only my perspective...and I'm pretty cheap!!
Just went back and looked at the pics more closely....hadn't noticed the case was missing a big chunk just in front of the recoil. This won't necessarily affect the operation of the saw but is a detriment none the less. At the very least it would be something to tinker with and a dandy parts saw for yours.
 
A pressure/vac test would tell the tale but a bit of work to gear up properly and the purchase of a MityVac etc. As I said earlier, on a saw of this age I would just replace the crank seals. This too is a bit of work but is actually cheap insurance.....and even after spending the money and time testing you'll likely find you will have to replace them anyway.

That old 621 is in rough shape and dirty but I don't believe it has had a lot of use. Likely a decent start for a project...the Total (Tsmura) bar is a plus too. A new top cover/switch and a good cleaning would improver the looks a great deal. Price is a bit high and the shipping too but this is only my perspective...and I'm pretty cheap!!
Just went back and looked at the pics more closely....hadn't noticed the case was missing a big chunk just in front of the recoil. This won't necessarily affect the operation of the saw but is a detriment none the less. At the very least it would be something to tinker with and a dandy parts saw for yours.
Wow. Was there ever a more sensible answer to a question. Thanks. I'm not sure why I'm afraid to take it apart but that is all that is stopping me. I can practice on another saw, watch how to videos or just go for it like I did with the top end. I 'll write again later. Take care. I know you would approve wearing chaps. Is there a way to save money on chaps or is that counter productive? Write back if you have time. Thanks again for your support.
 
A pressure/vac test would tell the tale but a bit of work to gear up properly and the purchase of a MityVac etc. As I said earlier, on a saw of this age I would just replace the crank seals. This too is a bit of work but is actually cheap insurance.....and even after spending the money and time testing you'll likely find you will have to replace them anyway.

That old 621 is in rough shape and dirty but I don't believe it has had a lot of use. Likely a decent start for a project...the Total (Tsmura) bar is a plus too. A new top cover/switch and a good cleaning would improver the looks a great deal. Price is a bit high and the shipping too but this is only my perspective...and I'm pretty cheap!!
Just went back and looked at the pics more closely....hadn't noticed the case was missing a big chunk just in front of the recoil. This won't necessarily affect the operation of the saw but is a detriment none the less. At the very least it would be something to tinker with and a dandy parts saw for yours.
Hi I bought the 95 dollar saw. I also bought another ring gasket set. I also bought a cover but it doesn't have a switch. Can I buy one at a store that will work? Thanks. Here it is.$_57 (1).jpeg
 
Hi I bought the 95 dollar saw. I also bought another ring gasket set. I also bought a cover but it doesn't have a switch. Can I buy one at a store that will work? Thanks. Here it is.View attachment 833413
Yes you could buy a switch at the hardware store that would work. But installing the "wrong" switch is a pet peev of mine. The toggle switches you can buy have toggles that are way to long, don't look right and are way to easy to get grabbed by brush and shut the saw off. I would go on the hunt for the OEM switch myself...…..again.....any switch will work but a saw with a non original switch is one of the first things I notice when first looking at an older Jonsereds saw.

Nice cover! The paint is even rather intact!! It's a very early cover or even from a 62 as the edges are slightly rounded like the 80 top covers....all the later 621covers have an angled chamfer on the top edges.
 
Yes you could buy a switch at the hardware store that would work. But installing the "wrong" switch is a pet peev of mine. The toggle switches you can buy have toggles that are way to long, don't look right and are way to easy to get grabbed by brush and shut the saw off. I would go on the hunt for the OEM switch myself...…..again.....any switch will work but a saw with a non original switch is one of the first things I notice when first looking at an older Jonsereds saw.

Nice cover! The paint is even rather intact!! It's a very early cover or even from a 62 as the edges are slightly rounded like the 80 top covers....all the later 621covers have an angled chamfer on the top edges.
Thanks for the info. I like the original switch too. I didn't notice the round edges. My saw has the beveled. Seems like it will fit. I hope.
I found this saw for parts today.
I thought I could get a p&c, filter, carb, maybe points and clutch. Am I nuts. https://www.ebay.com/itm/Vintage-JONSEREDS-Chainsaw-Chain-Saw-PART/143620700196
 
Thanks for the info. I like the original switch too. I didn't notice the round edges. My saw has the beveled. Seems like it will fit. I hope.
I found this saw for parts today.
I thought I could get a p&c, filter, carb, maybe points and clutch. Am I nuts. https://www.ebay.com/itm/Vintage-JONSEREDS-Chainsaw-Chain-Saw-PART/143620700196
Well......there is a lot of broken/worn out parts on that....I wouldn't place a lot of faith in the P&C.....not having the sparkplug in place and a few fins knocked off it. On the other hand the piston may well be usable and there are numerous parts that are still useful. Buying a saw carcass like this is a grab bag.....you just never know what you'll be getting until you have it in hand.
 
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