ArboristSite.com Sponsors
 
 


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

Discussion in 'Chainsaw' started by sledrat, Feb 25, 2013.

  1. sledrat

    sledrat ArboristSite Operative

    Joined:
    Feb 1, 2013
    Messages:
    392
    Likes Received:
    56
    Location:
    Canada
    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..
     
  2. fearofpavement

    fearofpavement Trying them all

    Joined:
    Jul 26, 2010
    Messages:
    4,919
    Likes Received:
    2,366
    Location:
    middle Georgia
    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.
     
  3. sledrat

    sledrat ArboristSite Operative

    Joined:
    Feb 1, 2013
    Messages:
    392
    Likes Received:
    56
    Location:
    Canada
    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..
     
    Last edited: Feb 25, 2013
  4. Roanoker494

    Roanoker494 Addicted to ArboristSite

    Joined:
    Sep 9, 2010
    Messages:
    5,188
    Likes Received:
    483
    Location:
    Narrows, Va 24124
    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.
     
  5. Cantdog

    Cantdog Tree Freak

    Joined:
    Feb 17, 2009
    Messages:
    10,635
    Likes Received:
    2,600
    Location:
    Beautiful Rockbound Coast of Maine
    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...
     
  6. PaladinMan187

    PaladinMan187 ArboristSite Operative

    Joined:
    Mar 22, 2009
    Messages:
    277
    Likes Received:
    41
    Location:
    Boondocks MD
    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.
     
  7. sledrat

    sledrat ArboristSite Operative

    Joined:
    Feb 1, 2013
    Messages:
    392
    Likes Received:
    56
    Location:
    Canada
    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:
     
  8. kurt

    kurt ArboristSite Lurker

    Joined:
    Mar 22, 2014
    Messages:
    11
    Likes Received:
    1
    Location:
    Newport News, Virginia
    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.
     
  9. PaladinMan187

    PaladinMan187 ArboristSite Operative

    Joined:
    Mar 22, 2009
    Messages:
    277
    Likes Received:
    41
    Location:
    Boondocks MD
  10. Guido Salvage

    Guido Salvage Supreme Saw Whoreder

    Joined:
    Apr 27, 2010
    Messages:
    9,242
    Likes Received:
    3,379
    Location:
    Formerly Richmond, VA now Lexington, KY
    Curious as to what evidence you have that a 50:1 mix caused your problems.
     
    blsnelling likes this.
  11. blsnelling

    blsnelling Site Sponsor

    Joined:
    Nov 8, 2006
    Messages:
    44,488
    Likes Received:
    10,201
    Location:
    Franklin, OH
    50:1 didn't cause your problems.
     
  12. joe25DA

    joe25DA Addicted to ArboristSite

    Joined:
    Mar 5, 2009
    Messages:
    3,241
    Likes Received:
    685
    Location:
    Worcester Ma
    X2
     
  13. Cantdog

    Cantdog Tree Freak

    Joined:
    Feb 17, 2009
    Messages:
    10,635
    Likes Received:
    2,600
    Location:
    Beautiful Rockbound Coast of Maine
    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......
     
    joe25DA likes this.

Share This Page