How much Compression is needed for a old Jonsered to run?

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sledrat

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Howdy how much compression is need for a old chainsaw to start and run?

why im wondering this is i do have a older Jonsered 49sp
poor lil saw didnt have spark when i got her
yep she was used and abused..
so cleaned it up and got it sparking again a real nice clean blue spark..
Also cleaned the carb up and then i pulled it over and over and over!!
I poured a lil gas down the plug and still no go..
Seems really low in comp. when pulling it comparied to my other oldies.
Any ideas or help on how to up the compression a lil with out putting new rings in her??
I think according to my wonky old compression gauge shes running at about high 80s - 90 mark..
Most of my old outboards from the 1930s era have no trouble running some on only 50- 80 pounds!

strange...
oh would the gas mix have anything to do with this also? all i had at the time was 50:1 which im sure is not right but should still run for a few seconds on it??
Any ideas?
and thanks..
 
50:1 mix isn't the problem. Sounds like you're not sure if your compression gauge is accurate or not. If it is, that isn't enough. Double check your spark. Sometimes a plug will fire in the open air but not under load.

As far as being "easy to pull" do you have another version of that same saw you could compare it to? Or take compression on a good running saw to confirm your tester.
 
ya not to sure on the old compression gauge to be perfectly accurate
but it gives a general reading,,probly accurate to 5 to ten pounds..
but then again so does my thumb lol
No was hoping some one here might say oh theres a decompression system on this saw or something that could be stuck open lol

might have to pop off the muffler and look at the rings see if i got a sticky one or two?

never heard of plug not firing under load before.. interesting ..

As for having one to compare to no i do not..
 
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Generally speaking you need at least 100psi for a saw to run, though I have seen a few run with as little 85-90psi. I would consider 100psi as time for a rebuild and 150psi+ as healthy. There are some exceptions to that, like the Poulan 3400 would be considered healthy at 125psi.

50:1 with a good modern 2 stroke oil should run just about anything, though everyone has their personal opinions on that matter. I personally run my 2 strokes on 40:1 thinking the extra oil helps with todays ethanol fuel.
 
A 49sp should be atleast at 150lbs.....to 175 or so....these are fairly high compression engines as they have a domed piston. Yes pull your muffler and see what the piston looks like.....that will probably tell the tale. I have had them stick a ring from carbon buildup and lose compression or I have also seen them with massive ring wear and also massive piston wear.....any/all these things can stop it from starting...and of course there is scoreing from running a to lean mixure...
 
I was told one time by an old codger who really knew his stuff that in order for an internal combustion engine running on gasoline to fire it had to have 100 psi worth of compression. Obviously being able to fire and having decent power are two very different things.
 
im gonna pop the muff off today see whats going on in there
going to get to the bottom of this!
Really like the look and feel of that old jonsered!
nice saw deserves a lil TLC!:msp_wub:
 
yea i have a 920 good spark fuel compression at 95 psi is a no go. im waiting for my new piston to come in the mail old piston was chewed up from a 50:1 mix being run in it. the manual calls for 40:1 on older saws i usually run 35:1 in my 2 strokes with additional lucas fuel stabilizer in it so almost a 30:1 ratio i personally have never had a fouled plug or cylinder damage in any of my toys. cleaning up my cylinder was a real pain id rather run more oil than be sorry.
 
yea i have a 920 good spark fuel compression at 95 psi is a no go. im waiting for my new piston to come in the mail old piston was chewed up from a 50:1 mix being run in it. the manual calls for 40:1 on older saws i usually run 35:1 in my 2 strokes with additional lucas fuel stabilizer in it so almost a 30:1 ratio i personally have never had a fouled plug or cylinder damage in any of my toys. cleaning up my cylinder was a real pain id rather run more oil than be sorry.

Curious as to what evidence you have that a 50:1 mix caused your problems.
 
I tend to agree......I run all my Jonsereds on 50:1 with Stihl Ultra....some tune in around 15,000 rpm....others much slower.....never seen a scratch or score and some get a real hard workout from time to time....now 50:1 with 30wt non detergent might give some problems but with modern, good quality oil 50:1 is fine. If you toasted your 920 on 50:1 there are other issues at play here......
 

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