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Advice on cylinder compression test results

Discussion in 'XJ Technical Chat' started by HeckticHaze, Apr 9, 2012.

  1. HeckticHaze

    HeckticHaze Member

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    I finished the valve shim changouts and they are in spec now. I did a compression test on each cylinder. Results below.

    CYL# DRY WET
    1 150 205
    2 130 182
    3 147 207
    4 125 186

    From the Yamaha manual it was 156psi normal and 128psi was minimum. Is this a piston ring issue or valves? So do I need to perform piston/ring work or valve work? Can I live with it for a season? Any comments/suggestions would be appreciated.
     
    HeckticHaze, Apr 9, 2012
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  2. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Those numbers aren't all that scary, and the "rise" when you added oil was nearly uniform as well (and you can't apply the "difference" spec to a "wet" test.) You only have one below 128, and it's not very far below. It's not like you have three pushing 126 and one making 60.

    Did you just get your hands on this bike, are just now resurrecting it, and almost no miles have been put on the motor yet since it "woke up?

    Was the motor warmed up slightly for the "dry" test?

    If this is a fresh recommissioning, and the motor hasn't seen many hours since you woke it up, I would say without a doubt get it properly sync'ed and tuned the rest of the way and RIDE IT. I'm thinking a couple of thousand fresh miles will bring those numbers more in line.

    Do another compression test before you put it away in the fall. I'll bet it makes much happier numbers.
     
    bigfitz52, Apr 9, 2012
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  3. HeckticHaze

    HeckticHaze Member

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    Bigfitz, thanks for the reply. The engine was cold when I did the tests. I purchased the bike last July. I did some initial brake work, oil, filter, tires, electrical, and some new plugs. I put about 400 miles on it and it started having power issues. The bike started backfiring when trying to start and the fuel economy was real poor. I put it away for the season and started working on it again a few weeks ago. I still need to dive into the carbs. I was going to see if I could synch it before I pull the carbs. It may be a lost cause, but I'll give it a shot.
     
    HeckticHaze, Apr 9, 2012
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  4. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    It does sound like you have a carb issue first and foremost. Getting them cleaned and the float levels set properly will be key; before you can worry about the sync.

    My point was that you can proceed with your resurrection and not worry about pulling the top end apart just yet.
     
    bigfitz52, Apr 10, 2012
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