Puzzlilng Compression
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Puzzlilng Compression

  1. #1
    Senior Member Bigblockbill's Avatar
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    Default Puzzlilng Compression

    Spring is here and put the boat in the water for the first time this year. Started off with a bad needle and seat dumping fuel. Had a spare and replaced the bad one. Boat ran better but still really rich at idle and air fuels would not adjust. Looking at carb the inner air bleeds are all plugged up so hopefully that’s the main idle issue. But While trying to trouble shoot a rich idle condition I did a compression test since seems like I lost 5 -6 mph over the last several years but otherwise the boat still runs strong.

    The compression results are puzzling though. I built the motor about the 13 years ago and it’s run great the whole time. Basic motor info: 523”, comp cams XR286R, file to fit rings on about 9:1 pistons.

    I had never done a comp test on this motor but what I saw today was 55-60 psi across all 8. Put some oil in one hole and it only came up a few psi. Friday I just replaced all the valve springs so blew air down each hole and did not seem like anything was leaking excessively. Also at 5000 ft elevation so comp will be down about 20% from that -Booo

    Maybe my gauge is just not good? But releasing the pressure it did not sound like much. Can the cam shaft have that much valve overlap breading off that much pressure. I ran the boat yesterday and it did not run all that bad so I have a hard time believing it’s as bad as the numbers suggest.

    Any thoughts as to the low comp numbers?
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  3. #2
    Some guy obnoxious001's Avatar
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    Got access to a leak down tester. That's a better way to check engine condition. Maybe some of the parts stores have them as "loaner" tools,, maybe same with compression gauge. If it's been sitting and the engine has not been run, you might expect some additional leakage, perhaps a bit of rust on exhaust seats or something.

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    Senior Member Bigblockbill's Avatar
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    I do own a leak down gauge. It just happened to have been 150 miles away from the lake at the house. When I did the leak down right after I built the motor it was around 5% or so if I remember right so I can compare to what it now when I go back down to the lake. I just freaked out this morning when I did the compression test and saw 60 psi on the gauge I know it’s not really a good test and the boat still runs strong so guess I am not overly worried.
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  6. #4
    Village Idiot fc-Pilot's Avatar
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    dumb question, did you have the throttle opened up all the way? Also, was the battery well charged? Either of those can make a big difference.

    Paul

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    Senior Member shan's Avatar
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    To add to Barry's comment, I've seen this. Hot heads/valves and moist air will rust things on the holes where the exhausts are open. Mine had been sitting awhile, but I wound up yanking the heads and lapping, nothing wrong, just light surface rust. Probably would have been fine if I would have just run it a bit.

    Paul's comments are relevant also. The cranking speed needs to be the same for every hole and the throttle needs to be wide open. I would still leak it for the whole picture.
    Last edited by shan; 04-02-2018 at 04:17 PM.
    Quote Originally Posted by Spike Morelli View Post
    Golfers and Fishermen are third rate bullshitters, compared to lots of hot boaters....

  8. #6
    Senior Member Bigblockbill's Avatar
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    So the battery is well charged and cranking at a decent speed, also all the plugs were out so that helped. I did open the throttle on one of the holes and did not really seem to make a difference. Maybe just took less revolutions to get to a full pressure.
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  9. #7
    Senior Member Bigblockbill's Avatar
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    Also I ran the boat for half a day on Saturday with the compression test happening on Sunday so would think any light rust would have been cleaned. I seemed to run just fine. Saw 5300 rpm a time or two. 🙂
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    i would retest with a different compression tester,had that happen to me once,first i freaked out,then i retested with a different tester,then i chilled because all was good.

  11. #9
    Senior Member ol guy's Avatar
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    If the leak down is still around 5% or so, No worries. Run It. If you need to see bigger numbers on the compression test, back all the rockers off 1 turn and run it again. Its known as cam over-lap. M

  12. #10
    Senior Member Bigblockbill's Avatar
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    Thanks and I am hopping and assuming it’s just cam overlap. I would think it would not even run if the true compression was 60. It will be a few weeks before I can get down there again to play with it.
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  13. #11
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    Default Gauge accuracy

    If you suspect an issue with any pressure gauge you can check by using a known good tire gauge vs the gauge you are suspicious of. A common problem are the Schroeder valves weakening, a tire Schroeder is not the same and will give you low readings.

  14. #12
    Senior Member Bigblockbill's Avatar
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    ToolShark hit the nail on the head. Finally got back to the lake and was able to check the compression with my other gauge and it was 135+. The hose I was using must have had a bad or automotive tire valve core in it. I pull the cores out all the time for the leak down gauge and valve spring replacements and no telling what valve core got put back in since I though they were all the same. (Time to go find the special valve core now).
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