Briggs compression reading for 3.5hp?

Discussion in 'Mechanic and Repair' started by bobcat175, Mar 2, 2007.

  1. bobcat175

    bobcat175 LawnSite Senior Member
    Posts: 377

    Does anyone know what the expected compression reading should be on a 3.5 Briggs and Stratton? Also what is the service limit reading?

    Not sure on the engine type but it's on a Little Wonder edger 6032 that's 5+ years old.

    thanks
     
  2. lucforce

    lucforce LawnSite Member
    Posts: 223

    There are no psi limits for this.

    The test is LEAKDOWN.

    Another test for small engines(not good for little wonder blower) is to spin the flywheel backwards, by hand. The pressure in the cylinder should cause the piston to kickback, and rotate the flywheel in the opposite direction. Better seal= better kickback.
     
  3. bobcat175

    bobcat175 LawnSite Senior Member
    Posts: 377

    ??? I don't understand...of course there are acceptable limits for compression. I was just wondering what is normal for this engine....as well as what B&S considers low to the point that service is needed.
    Leakdown is a completely different test, I know that it is a better test but I don't have a leakdown tester....only a compression gauge. The other test you mentioned sounds like a broken finger waiting to happen.
     
  4. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,024

    That EXACT ^^^^ procedure is written right in the Briggs service manual....

    Let's just say 70 or below is hard to start and below 50 is no start.
     
  5. bobcat175

    bobcat175 LawnSite Senior Member
    Posts: 377

    Thanks - but wouldn't you say that the amount of "kickback" is subject to interpretation. Yes it would tell you that you have compression but it seems that numbers off of a gauge would be a lot better.
    So are you saying that B&S does not publish compression #'s?
     
  6. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,024

    Straight from the manual; " Briggs & Stratton does not publish any compression pressures, as it is extremely difficult to obtain an accurate reading without special equipment.

    It has been determined through testing, a simple and accurate indication of compression can be made as follows"

    Then they list the procedure lucforce described.

    How many lbs. compression do you have ?
     
  7. bobcat175

    bobcat175 LawnSite Senior Member
    Posts: 377

    Thanks - I'm going to look at a used Little Wonder edger on Sun. The engine is a B&S 3.5hp. I was going to do a compression test as part of my inspection. Anything major that I should look for besides the obvious...not sure what are the common issues. I don't even know what series/type it is...here's a pic...I have higher res ones if you're interested.

    LW.jpg
     
  8. dutch1

    dutch1 LawnSite Silver Member
    from Jayhawk
    Posts: 2,231

    Briggs doesn't publish compression limits and lucforce is right on with his recommendation. This is the procedure given by Briggs in their repair manual. When I was in a shop envornment, anything under 60 was usually subject to rejection. Customers were advised of low compression units that ran, that they may not run with power under a load and we let them make the call on additional work/engine replacement/new unit. On the high side we considered 90 and above satisfactory. Low compression will tell you that you have some leakby, either worn cylinder/rings, improperly seating valves, loose valve seats or a leaky headgasket. Keep in mind that numbers I gave above are for the single cylinder 3.5-6.5 hp engines.

    A leakdown test will tell you where the leak is. If you do much engine work and don't want to spend the money for a commercially made tester, you can put one together with available fittings, guage and hose at much less expense.

    We didn't do much leakdown testing. On low compression units our procedure was to pull the head and inspect gasket, head and block for leakby, check valves for proper seating and guide wear, then check for cylinder wear/galling.

    Our most common found problem was a low oil condition, leading to overheating which further leads to blown head gaskets, galled cylinder walls, stretched valve stems and loose valve seats. Of course we didn't necessarily find all those conditions at the same time.

    Other techs will have additional thoughts. Your mileage will vary.

    Dutch
     
  9. bobcat175

    bobcat175 LawnSite Senior Member
    Posts: 377

    Thanks - Excellent info and all makes sense....

    They did say that it ran but when the blade was engaged the engine stopped...this is what sparked my original post...so it is either similar to what I quoted you on or a simple issue with an operator presence control....I wanted to do the compression test to make sure that it wasn't what you said. If the compression is low I will not have the luxury of dismantling their engine (nor would I want to)....and will walk from the sale.
     
  10. dutch1

    dutch1 LawnSite Silver Member
    from Jayhawk
    Posts: 2,231

    Make sure the blade spindle is free and not locking up. Its been a while since I worked on a Little Wonder so I don't recall if the spindle is greasable or not. Spin the blade to check for noisy bearings. If the spindle locks up, this could cause the engine to die, particularly at lower speeds.

    Dutch
     

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