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Compression Question on Briggs 18hp.

Discussion in 'Mechanic and Repair' started by Jason Pallas, Feb 27, 2006.

  1. Jason Pallas

    Jason Pallas LawnSite Bronze Member
    Posts: 1,325

    Can anyone tell me what the correct compression range is for a Briggs Twin Cylinder 18hp? I think my buddy's got a bad cylinder and I told him I'd check it for him - Thanks.

    NIXRAY LawnSite Senior Member
    Posts: 511

    Check Briggs And Stratton's Web Site Or, Give Me The
    Model Number, Serial, Spec And Type Numbers And I'll
    Check My Software. ( I've B/s's Technician Software On My Shop Computer)
  3. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Right from the Service 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: Insert compression gauge in one cylinder and crank engine over until no further increase in pressure and record reading, Repeat on the other cylinder and record that reading. The difference between both cylinders should not exceed 25%. More than 25% indicates loss of compression in the cylinder with lower pressure.
    From my experience 50 lbs. or below is not enough to run, From 50 to 75 lbs. will run but be lower on power, 75 lbs. and up is good.
  4. Jason Pallas

    Jason Pallas LawnSite Bronze Member
    Posts: 1,325

    Thanks guys - perfect. I didn't even think of measuring what I believe to be the good cylinder - then using that as a baseline. Will get a chance to try to diagnose his problem - I might be writing back. I was working on it for him at the end of the season last year - put it away for the winter.
    Seems to run fine while not under load. However, when you engage the blades, the engine seems to loose power and develop a "miss". I isolated the problem to the left cylinder. The plug has a spark (under load and while idling not under load). Under load, if I pull the wire on the plug, the engine runs the same - like it's "missing" (however, there is still a spark - tested it with another plug). Under load, if I pull the good cylinder spark wire, the engine dies immediately.
    So, because it doesn't appear to be an ignition problem, I'm thinking that it is either a bad valve or some other low compression issue. Any other suggestions/troubleshooting tips from the pros? Thanks again for the help.
  5. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Sounds like you have it narrowed down to a dead hole I wouldn't waist any more time with testing. Go ahead and pull the head on the dead hole and check the valves for wear and the valve guides, Also check to see the piston moving up and down. I've seen rods go and still run on one, If you happen to do a valve job on one side do the other side also to even the cylinders back out.
    Valve clearance springs installed; .004/ .006 intake .007/.009 exhaust.
    Valve clearance without springs; .006/.008 intake .009/.011 exhaust.

    Good Luck
  6. jblawns123

    jblawns123 LawnSite Senior Member
    Posts: 340

    They have compession releases, with just a simple gauge you cannot get an accurate reading. As said, you can only compare the 2 cylinders
  7. lawnmaniac883

    lawnmaniac883 LawnSite Silver Member
    Posts: 2,613

    Had same problem on a 2something hp kohler...turned out to be two broken rockers ;)
  8. Jason Pallas

    Jason Pallas LawnSite Bronze Member
    Posts: 1,325

    Am going to take a look at it tomorrow - will post back.
  9. Jason Pallas

    Jason Pallas LawnSite Bronze Member
    Posts: 1,325

    OK here's what happened. Checked the compression in both cylinders - both checked out fine. So, I decided to pull off the flywheel and see if there was a problem with the shear pin (causing a slight timing/missfire). Shear pin and flywheel was fine.
    I then pulled off the OHV cover to see what I could see. There is was - the push rod on the top valve (I assume it was the exhaust) was just laying there. The stud on the rocker had come loose - allowing the pushrod to fall out of place.
    Th fix: placed the pushrod back in place. Tightened the stud on the rocker arm. Visually tested the valve rockers to see if both were working. Put it all back together - started it up and it runs like a champ. Thanks for all the help guys. Apprceciate it. Good luck all.
  10. lawnmaniac883

    lawnmaniac883 LawnSite Silver Member
    Posts: 2,613

    Hah, glad it was that easy for ya. Might want to throw some locktite on it if this happens again.

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