CB160 Compression
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Thread: CB160 Compression

  1. #1
    Member amannl's Avatar
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    CB160 Compression

    Will someone confirm what the actual compression limits are for a CB160?

    The Honda Shop manual lists compression as 141.9 psi and service limits from 128 to 149, while the Clymer manual shows 140 to 170 psi. My bike test at 145 and 147 (with a 18” long hose) and increases to about 190 with a teaspoonful of oil through the spark plugs.

    I’ve had this bike for More than 10 years and I’ve never given this discrepancy a second thought, but it’s been running poorly since the spring. Noisy and weak under acceleration, and after it has picked up some momentum and the throttle is backed off slightly it quiets down. I’ve checked valve clearances and timing, and carb.

    Im assuming I need new rings but won’t know for sure until I tear it down, but am unsure if it is actually suffering from low compression, or from something else.

    Thanks
    Adam







  2. #2
    Senior Member mike in idaho's Avatar
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    Your compression numbers are plenty good for it to run well. Noisy under acceleration, do you mean pinging(spark knock) under load? What heat range are the spark plugs? I would check the timing at full advance, then try a tankful of premium grade fuel.
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  3. #3
    Senior Member ancientdad's Avatar
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    When you say "noisy and weak", what kind of noise are you referring to? Loudly ticking valves won't reduce compression unless they're extremely loose, and bad rings don't make noise... does it produce blue smoke? Use/burn oil excessively? Two pounds variation between cylinders is meaningless and hardly the indicator of engine wear. You're sure the timing is correct, as well as the operation of the advancer? If the timing is a bit retarded or the advancer is not working properly and advancing the timing fully at higher revs, the bike will feel weak as well. Did you drain the carbs over the winter? If the carbs got a little gummed up from fuel sitting in them, it might be running a bit lean as well which would cause it to feel weak
    Tom

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  5. #4
    Member amannl's Avatar
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    I think I just figured it out. I just came back from starting the bike to go for a ride just now and a small cloud of smoke (from the oil I added to the cylinders) came out in a cloud of smoke from the front of the engine, and from the right muffler.
    So I assume it’s a leaking head gasket.

    and AD, by noise I meant loud like a muffler with no baffle.

    but does anyone know what compression is for a CB160?

    because if I pull the engine, I should replace at least one shift fork, seals and possibly hone the cylinder and install new rings, if needed.

  6. #5
    Sensei 66Sprint's Avatar
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    Oil forced through the header gasket would be both noisy, and oil up/smoke at the front of the engine

    With 18 inches of hose and those numbers, your compression is very good......

    No need to tear down top end to fix transmission, shifter, or case seal problems........
    Last edited by 66Sprint; 07-05-2019 at 09:35 PM.
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  7. #6
    Senior Member TOOLS1's Avatar
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    Forget the compression test, it is telling you that you have good compression. I would be checking the temperature of the exhaust headers to see if the cylinders are running the same temp. This will tell you a lot more than a compression test.
    TOOLS

  8. #7
    Member amannl's Avatar
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    Thanks all.
    I’m going to go for a ride this morning and will check header temp when I get back as I bought an inexpensive infrared thermometer a few years back.
    I did look at the plugs yesterday; the right looked “good”: darkish coffee insulator and clean strap while the right is darker and sooty.

  9. #8
    Member amannl's Avatar
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    Tools,
    the temps of the exhausts, an inch or so below the clamp, were ~173-175 on the left and 181-183 on the right.
    what do, or what would temp differences indicate?
    Thanks
    Adam

  10. #9
    Senior Member TOOLS1's Avatar
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    That is not much difference between the two. It seems both are running, but the left might be having some trouble.I would suspect that the pilot jet could be dirty, the O-ring on the pilot screw could be bad, or the timing on that side could be retarded.
    TOOLS

  11. #10
    Senior Member Polish's Avatar
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    Ok... Have a few questions about compression.

    First, does the 18 inch hose make a difference in the reading?

    Second, why adding oil through spark plug holes increases the PSI reading?

    Third, what is the meaning of the PSI increase?

    Forth, how do you know if a compression gauge is accurate?

    This is all for my own knowledge. I'm using an 18 inch long Canadian Tire gauge. My readings are kind of low, 145 on both, FSM calls for 185 -+ 15. But bike runs awesome, pulls hard with no exhaust/oil leak, spark plug colours are light brown and the bike has only 14k on it. So, I wonder if my readings are accurate......

    Thx
    JL
    1979 CB 400 T (Cafe Racer)

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