low compression problem

Discussion in 'Mechanic and Repair' started by kdlklm, May 8, 2006.

  1. kdlklm

    kdlklm LawnSite Member
    from Canada
    Posts: 64

    I've got an older grasshopper mower with a 18 hp Briggs and Stratton engine. I took it into get serviced, and the guy told me it has low compression and that it's not worth fixing, and to just run it until it dies. The only problem I was having is that the idle is not very smooth and it backfires when I shut it down. At full RPM it runs really well and has lots of power. Where should a guy start. I don't want to have to pay a fortune for a service tech to screw around with it. This is the engine model.

    Model 422437 Type 0678
     
  2. oldrustycars

    oldrustycars LawnSite Senior Member
    Posts: 301

    i'd like to see a leakdown test on this, then you'll know where your losing compression. if its just a valve job or adjustment, thats relatively easy. if its getting past the rings, generally you'd go with a new engine.
     
  3. khouse

    khouse LawnSite Bronze Member
    Posts: 1,465

    If it has plenty of power then I would guess it has plenty of compression. I'm pretty sure that the engine has a compression release that makes it easier to spin for start up and can give you a low compression reading and make you think it has low compression when it really doesn't. If it won't idle then I really think you should buy a new carb. I used to rebuild all carbs but found that it saves time just to replace the carb and time is money. Also try turning off the engine at full throttle and that should take care of your backfire when turning off. Post back with your findings.. Good luck.
     
  4. kdlklm

    kdlklm LawnSite Member
    from Canada
    Posts: 64

    That was my thought as well that if it had lots of power it should have relatively good compression. But I've never seen a motor that would have an automatic decompression for starting, normally it's a manual thing, is it not. The service tech played with the carburetor, and got it to idle fairly well. Generally what I would do to stop the backfiring, is to let it idle for about a minute before shutting it down. But that doesn't seem to do it anymore.
     
  5. oldrustycars

    oldrustycars LawnSite Senior Member
    Posts: 301

    small engines have had automatic compression release for many years. briggs from at least the mid 80's, maybe older than that. so, how exactly did the tech decide this engine has low compression? he had to have used a compression tester (the common way) or a leakdown tester (the best way). in any case, since we dont know anything about this engine, you have some options. find out exactly why it has low compression...if its valve related, i'd fix it now. if its getting past the piston and rings, run it until it pukes.
     

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