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Is there any thing I can do to improve compression while i have the bottom open?

Discussion in 'XJ Technical Chat' started by Phil B, Oct 20, 2019.

  1. Phil B

    Phil B Member

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    I split the case to rebuild the transmission, replaced the the main-shaft and drive-shaft, along with the shift-shaft and change-lever. Aside from dealing with the hassle of cleaning away old gasket and cutting new gaskets (yep, Im gonna hand cut for the clutch cover and oil pan) all has gone well so far.

    I started to wonder. The compression in the top end leaves something to be desired, being a bit low. While I have the case split and exposure to the underside of the pistons is there anything i can do that might improve compression, assuming the piston rings are part of the issue???

    Im thinking about the guys who get a few extra PSI from letting some Marvel Mystery Oil soak in the cylinder for a few days or something like that. You know, a little trick to get back a few PSI?

    Anyhow....here is a link to the thread that helped me decide to split the case and do the tranny work my self...

    Ill also attach a few pics of the work, and project in general. Shes an 85 fj600 and I'm taking her Cafe!!! As you can see I striped the tank and found a bondo masterpiece!
     

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  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    pull head and cylinders measure cylinders and hone them fit new rings.
     
  3. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    What were the compression readings?
     
  4. Phil B

    Phil B Member

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    So, compression numbers are as follows but bear in mind that live at a very high altitude over 1000M. Atmospheric pressure here is is 12.8 PSI on average. Given the compression ratio on the FJ is 10:1 that equals 128 psi (max?) at spec. I get the numbers below.

    Hot engine---
    1 - 115 ( with oil 155)
    2 - 110
    3 - 109
    4 - 117


    However, if i squirt some oil into the cylinder it shoots up to 155 PSI. Which also makes me wonder what I'm supposed to infer from that? Will a shot of oil increase compression even if its already at spec?

    Anyhow, what I'm thinking is to flip the top of the engine on its head and shoot some (read a teaspoon or two) marvel or seafoam to rest on the underside of the piston rings, much like people do by putting it in through the top. My understanding is that this is to help breakup carbon deposits that may be compromising the rings.

    Could also potentially shoot some in through the spark plug holes to rest on the valves with the same intentions...but I have never seen anyone recommend that, just thinking out loud.

    Finally, this bike was sitting for a few years, but after a carb cleaning I got all 4 cylinders to fire....I only did some small test rides before cracking her open, as the transmission clearly needed attention and I didn't want to chance anything. If I were to ride it around and give it the old Italian tune job, perhaps she would work out some of the compression issues on her own. That plus the Marvel/Seafoam ring soak might just bring her back within reasonable distance from spec?

    Thoughts?
     
  5. XJ550H

    XJ550H Well-Known Member Premium Member

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    the idea of oil is to seal rings. so low comp. is typically due to worn rings. out of adjustment valves but oil does not fix this.
    if your bike has not run for a time and you think rings are stuck there are things you can do for that.
    first is to ride it and see if it comes up on its own. second is 50/50 mix atf and acetone to clean off the rings and the groves
     
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  6. Thrasher

    Thrasher Member

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    If you take the cylinders off you can leave the base gasket out for a little more compression. Use Yamabond instead.
     
  7. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    That will make it a bear of a job the next time the cylinder block has to be pulled.
    I wonder if any of the Japanese aftermarket sources make a thinner base gasket.
     
  8. Thrasher

    Thrasher Member

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    You can take .020 no problem but maybe play with the gaskets, head and base, and play with then to tighten up the squish area and gain more compression and squish which will add more power as well. About .035 in the squish area is a safe place for a street motor, you can leave out base gaskets for a starter and then take apart the layered head gasket to get the squish close, then I would think about cutting the head if that helps... I used a product called Gaskasinch. for the head gaskets and Yamabond on the base gasket area after I pulled that gasket...
     
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  9. Phil B

    Phil B Member

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    Well...

    Im closing the bottom back up and don't plan to pull the block. Still have to check the shims and see if I'm in spec. After that ill ride it a bit and maybe put it through a heavy seafoam/gas mix the hot soak the cylinders with seafoam or Marvel t see if that improves compression.

    Damn should have done more testing before i pulled this thing apart. Oh well.
     

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