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  1. #1
    Resident "Noob" Sabu1984's Avatar
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    Compression test question...

    Ive heard two different things, when doing a compression test are you supposed to take out all spark plugs or just the cylinder your working on?! I did one today and the results where WAY lower than they were last fall. 125, 100, 90, im assuming i did it right? Last year its 125, 120, 125, would sitting over the winter cause any issues or do it need to run a bit first?


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Sabu1984 View Post
    I've heard two different things, when doing a compression test are you supposed to take out all spark plugs or just the cylinder your working on?
    Typically I take all the plugs out, so the battery+starter can crank the fastest, and most consistently.

    If you fogged the engine, then the fogging oil can affect the readings until it gets burned off.

    Also, if the battery is weak, then as you test the cylinders, it cranks more slowly, so the last reading can be lower than the first, even though the cylinders are actually in the same condition.

    Remember to hold the throttle wide open, to minimize the load on the starter+battery, AND to maximize consistent compression readings.

    If there was moisture in the engine/exhaust, or the rings are just sticky in the pistons, then you could have corrosion or other things that are affecting the readings, that wasn't a factor last Autumn.
    Last edited by K447; 05-04-2008 at 11:22 AM. Reason: My practice is plugs out, well... sometimes

  3. #3
    AWA Member 32DegH2O's Avatar
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    Quote Originally Posted by K447 View Post
    Typically you take all the plugs out, so the battery+starter can crank the fastest, and most consistently.
    I wouldn't say typically they are taken out cause that's not true...it's done both ways but I see them left in more than not. Secondly, the cranking will be more consistant by leaving the other plugs in, not out. I mean you let it crank over fast for 2/3rds and then slam it on the test cylinder. The load on the starter keeps changing/unequal...not to good on it in my opinion. If your battery and starter are in good condition then it really makes no difference if the other plugs are in. Either way will work...but I leave mine in.
    Last edited by 32DegH2O; 05-03-2008 at 02:19 PM.

  4. #4

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    i leave em in its says in all the compression tester instructions ,

    now difference between a cold and hot engine can be tricky ,

    what i would do if what happened to you happened to me i would mix fogging oil with a little bit of 2 cly oil and poor a little bit in the clys , place your thumb over the top , sheild your eyes then crank it ,

    lube the walls and rings then do another test ,

  5. #5
    Resident "Noob" Sabu1984's Avatar
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    Yeah, i fogged it last fall, and today was the first time its been started since last October.... Battery is fully charged, same battery thats been sitting on a trickle charger over the winter...Ill try it again later with plugs in and see what numbers i get...

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Make sure the engine is warmed up. Cold engine will read lower compression. Oil in the cyl will give false readings as it will fill in the voids and increase compression. Not an accurate reading IMO.

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  8. #8
    ph2ocraft's Avatar
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    For what you're doing it doesn't matter a rip which testing method you use as long as the cylinders are within 10 percent of one another.

    Since you have a differential, warm the ski, chose your compression testing method and this time check the compression in the opposite direction.
    If your within 10 percent, call it a day. If you find that one cylinder is still low, perform a leak down and chcek piston wash. If they look good and the cylinder is still low, take it for a short ride and check it again. Careful with WOT runs as it may be a lean condition that was occuring at the end of your last ride. If you STILL have a low compression number pull the domes and check for signs of detonation and/or cylinder wear.

    Keep us posted.

  9. #9
    Moderator RX951's Avatar
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    also stay consistent when testing. If you are going to test hot, always test hot. if you are going to test cold, always test cold. Numbers can vary from hot to cold.

  10. #10
    casey67's Avatar
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    I'm sorry to contradict(spelling?) anyone.
    The Polaris manual recommends removing all spark plugs,using a good battery.Hold throttle wide open when cranking.
    In my experience I like to have a battery charger attached if checking compression several times or on 8 cylinders to guarantee even battery charge/consistent cranking RPM.But that is splitting hairs.
    The reason to remove all plugs,If 1 cylinder has more compression it will slow the cranking rpm's(lower the reading on other cyl) if 1 cyl has less comp,it will raise rpm,s(raise reading on other cyl)

    Being consistant is the most important thing.
    The amount and type of oil in cyl will affect readings.

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