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Thread: Enough compression?....Rebuild?

  1. #1
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    Enough compression?....Rebuild?

    Good morning folks, hope everyone is doing well. So I've got this nice little 86 125m ive been cleaning up and tinkering with which is running and riding great with absolutely no issues whatsoever. I got a hair and decided to check the compression on it and only got 150 psi. I followed the manual's suggested procedure by warming up the engine and then cranking over with the throttle open until the reading stopped climbing. I did this multiple times with the same result. According to the manual, it should have 189psi +/- 14.2 psi. Now like I said, the bike is running and riding great with no issues but I wonder if the bike would benefit from a top end rebuild just to get the compression back up. Any thoughts, ideas, suggestions?

  2. #2
    Fullsizechevy9 is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    Quote Originally Posted by LittleRed86 View Post
    Good morning folks, hope everyone is doing well. So I've got this nice little 86 125m ive been cleaning up and tinkering with which is running and riding great with absolutely no issues whatsoever. I got a hair and decided to check the compression on it and only got 150 psi. I followed the manual's suggested procedure by warming up the engine and then cranking over with the throttle open until the reading stopped climbing. I did this multiple times with the same result. According to the manual, it should have 189psi +/- 14.2 psi. Now like I said, the bike is running and riding great with no issues but I wonder if the bike would benefit from a top end rebuild just to get the compression back up. Any thoughts, ideas, suggestions?
    You could rebuild it if it makes you feel better personally i would run it 150psi isnt bad i have heard of bikes with under 100psi still running decent long as it isnt smoking i say ride it and have fun

    Sent from my SAMSUNG-SM-N920A using Tapatalk

  3. #3
    Join Date
    Apr 2011
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    Mexico
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    If it’s running well just drive it. Compression readings can vary between gauges, air density, testing methods, etc.
    It sucks to get old

  4. #4
    Join Date
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    I couldn't have said it any better than these guys did.

  5. #5
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    Apr 2011
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    I probably should have added that the best way to check the health of a 4 stroke is a leak down test. It not only tells you if you have a problem, it tells you where the problem is.
    It sucks to get old

  6. #6
    Join Date
    Apr 2012
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    FYI, service manual spec is 175-203psi, it's a little low, but not exactly horrible. Engines can run down to around 100psi but generally when a 9:1 compression ratio engine gets that low on compression it will be burning oil pretty bad. If it runs well enough for your wants/needs, run it as is, if you want more power, look at replacing the rings and while it's apart atleast check the head/valves out. Compression loss is generally rings or valves, wouldn't hurt to check your valve adjustment just to be sure it's not too tight and bleeding compression off.

    There's a trick of putting a small amount of oil in the cylinder (just enough to coat the rings) and performing the compression test again, if the number jumps up it's the rings that are worn, else it's likely the valves.

  7. #7
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    Mar 2019
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    Monroe, NC
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    Good morning again folks and thanks for all of the replies. So this weekend I purchased a leakdown tester and gave it a shot. On a cold engine at TDC I input 20 psi and got a return reading of 20 psi, turned it up to 30 psi input and got a return of 30psi, turned it up to 40 psi and got a return reading of just a hair under 40, maybe 39. These seem like super good results to me so I guess I'll just leave it alone and ride it. Is there something else I should have been doing or did wrong?

  8. #8
    Join Date
    Apr 2011
    Location
    Mexico
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    Read and watch these.

    https://www.youtube.com/watch?v=CC1EZCxDnLc


    https://mobiloil.com/en/article/car-...-leakdown-test

    Using the stethoscope I assume your kit came with (a couple feet of hose works as well) you can find the source of any leaks. Exhaust pipe for the exhaust valve, carb throat for the intake valve and air breather vent, or oil fill plug for ring leaks. If you suspect a head gasket leak you can use soap and water to find it. A cracked head can be hard to find, but if you have one it's usually between the valve seats or spark plug holes.

    You will ALWAYS have a small amount of leakage past the rings, that's normal due to the ring gap, but all other areas should have zero leakage on an engine in good condition.
    Last edited by El Camexican; 09-09-2019 at 10:07 AM.
    It sucks to get old

  9. #9
    Join Date
    Oct 2002
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    I was told as a young lad, if it blows your thumb off it's enough to run...
    All our government does is distract us while they steal from us, misspend our tax $ and ruin our country

  10. #10
    Join Date
    Apr 2012
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    USA
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    I can hold back 140psi with my thumb, that's what pressure my dad runs his air compressor at, granted it takes a lot of effort to seal it well and the surface area of a spark plug hole is a fair bit bigger than an air gun. Probably not a bad test if you have an idea of how much effort is needed to be put in to holding your thumb over the spark plug hole.

    Also, some engines naturally have low compression, like 6.5:1 lawn mower engines that run at like 90psi, while our atv engines are around 8:1 and run 120-150+ psi.

  11. #11
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    if you look at a approved air nozzle, it's got holes on the sides. you dont want to be putting your finger or any other body parts over the port of the nozzle trying to hold back the air! if the air gets under your skin and into your bloodstream it can kill you. so please be careful! Sorry, dont want to sound like the old guy. jim
    81 185s
    82 185s with 85 200s motor
    suspended 185s
    85 atc70
    1984 200s
    85 350x 3rd owner
    and a 72 ct70

    projects coming along

    85 200s
    81 suspended 185s (can't believe I came across 2)
    1982? 185s

  12. #12
    Join Date
    Apr 2012
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    USA
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    Yea I learned about that long after I did that when I was a kid. The air gun wasn't approved lol, the tip was removed and a brake line installed for low volume high pressure air blowing. Now my dad uses what we call the "air cannon", it's about like an air pistol, it has a trigger that's effectively a 1/2 turn valve and is a straight through passage and the tip has the holes like a normal air gun. No clue where he got it but man does that thing blow.

    I'm not sure how strong skin is against air pressure, I think it requires a cut for air to get in your blood stream, but yea it can kill you if you get enough air in your blood stream where your heart looses it's "prime" since it is just a pump after all. Clearly my family doesn't abide by limitations placed on most things, almost all air tools recommend something like 80-100 psi, my dad runs 140psi and his brother runs 180psi. My uncle's impact is so torqey that it can very easily break lug nuts off. It's a bit overkill really.

  13. #13
    Join Date
    Apr 2011
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    Mexico
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    Quote Originally Posted by Jim mac View Post
    if you look at a approved air nozzle, it's got holes on the sides. you dont want to be putting your finger or any other body parts over the port of the nozzle trying to hold back the air! if the air gets under your skin and into your bloodstream it can kill you. so please be careful! Sorry, dont want to sound like the old guy. jim
    If “old” means wise please carry on. A grease monkey in my hometown was killed about 40 years ago when one of his workmates thought it would be funny to shoot a blast of air into the butt crack of his coveralls.
    It sucks to get old

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