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  1. #1
    kmaher's Avatar
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    Expected compression raise after ring seating? (GP1300r)

    So before my rebuild, I was at 140, 140, 128 (Front through rear cylinder)

    I measured after rebuild and before firing it up, and I was at 132, 132, 131. It's a milled head machine, obviously. Is it reasonable on these to expect an 8psi raise after seating rings and completing break-in?

    Also, somewhat unrelated, how long do the big end bearings usually last on the bottom end of these machines, and are the lives reduced when running a modified craft?

    Now that I'm rebuilt and ran it on the hose for a couple minutes, I swear I'm hearing a slight "ratcheting" type sound from the bottom end. Rods didn't have any vertical play, everything was nice and smooth when moving it by hand, and I lubricated everything nicely with 2-stroke oil before reassembly. Now that I'm rebuilt, I richened my oil delivery from 40:1 to 32:1. Bottom end has never been touched, and I've got about 165 hours on the clock.

    Maybe I'm just scrutinizing. A buddy of mine said it sounds exactly the same as it did before. It's true, I tend to be neurotic for the first 10 hours after any major mechanical "surgery", but better safe than sorry.


  2. #2
    I record and measure all of compression tests with the throttle closed and open. The reason I do this is because I forget to hold the throttle wide open, so I pretend like I do it on purpose. Are you messing up the numbers like I always do the first time around?

    I am not sure how relevant these numbers would be, for the 1200r go to the drop down menu to see compression vs. dome size and octane: https://adaracing.com/personal-water...r-billet-head/

    Is your crankshaft welded? I think that is one key detail needed for slightly modified cranks to last.

  3. #3
    kmaher's Avatar
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    I always run compression cold with WOT. I know I'll see compression increase as soon as it's broken in, just not sure if 8psi is generally what's to be expected. That would get me back where I started, with the 3rd cylinder being back to where it's supposed to be.

    I've never touched the bottom end, so it's all stock right now. Man, now I'm thinking I should have just dove in and done that while I was at it.....hindsight is always 20/20

  4. #4
    steve45's Avatar
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    I don't know if you really would see an increase. Yes, in theory you should. However, if you check it before it has been run, you likely have a lot of oil in the cylinders from assembly that would artificially increase ring sealing. I think that in order to get an accurate measure of this, you would need to run the engine for a couple of minutes, then test it. This would burn off excess oil. Of course, you need to be consistent in whether you test it hot or cold.

  5. #5
    After a rebuild on my GP800, it tested 110 and 109. After 10 hours run in. Both over 120.
    I donít think it makes a difference holding the throttle open on a 2 stroke during a test. It can scavenge itís own air anyway. Not like a 4 stroke.

  6. #6
    Myself's Avatar
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    I commonly see 10 or more psi after a bit of run in.

  7. #7
    Quote Originally Posted by QBhoy View Post
    After a rebuild on my GP800, it tested 110 and 109. After 10 hours run in. Both over 120.
    I don’t think it makes a difference holding the throttle open on a 2 stroke during a test. It can scavenge it’s own air anyway. Not like a 4 stroke.

    I just finished my gpr build in the fall and got it on the lake maybe an hr running then Winter Came. I will unintentionally put this 'wot or not' compression test to the test come spring ahah. Most of my compression testing has been on my turbo car build that I finished at the end of last summer, 'wot or not' that makes a difference there, doh!

  8. #8
    kmaher's Avatar
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    I can't remember precisely, but I think the purpose for WOT on the 1300r is that when atempting to start the machine, WOT prevents the fuel injectors from spraying. Or maybe it prevents spark? Or both?

    Anyway, if you try to start a 1300r with WOT, it won't start, so I know it does SOMETHING. Someone who knows better than me can give the official answer. I'm just an idiot that someone gave wrenches and sockets to.

  9. #9
    kmaher's Avatar
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    Oh, and thanks for the replies, everyone....glad to hear I should be back compression-wise to where I started and possibly a little more.

    I think I'll take a video of it in a couple days running so an expert on here can tell me if they hear any sign of bearing wear on the bottom end or if it's just in my mind.

  10. #10
    Yeah. Apologies. It does hold the fuel off. Thatís the reason for it. Fair play.

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