Compression test results (how do my numbers look)

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by crofrog, Apr 11, 2009.

  1. crofrog

    crofrog Long timer

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    So new to me 950 Adventure tested compression on a cold motor last night.

    220psi front / 215 rear.

    I've got allot more wrenching time on cars than bikes and that sounds like a good number, is it?
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  2. Hammer

    Hammer Hawlin' aZZ

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    That's fine.
    Doing cars, you must know those are big #'s.
    This is a high compression race style engine.
    Spank that bad boy!
    #2
  3. NeverQuit

    NeverQuit RoutineDefiant Adventurer

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    Did anyone ever come up with the "listed" numbers? My '04 950 just compressed 236 in the front, and 240 in the rear.

    Seems to be a huge variance in what folks are reporting. I hope it's not our gauges.....
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  4. crofrog

    crofrog Long timer

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    It wouldn't surprise me though. I'm sure the tempature of the motor has something to do with it too. My valves where "loose" also.
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  5. NeverQuit

    NeverQuit RoutineDefiant Adventurer

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    Looks like both of our motors were cold when we checked. Huge variance there somehow...
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  6. NeverQuit

    NeverQuit RoutineDefiant Adventurer

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    Looks like both of our motors were cold when we checked. Huge variance there somehow...
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  7. crofrog

    crofrog Long timer

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    I think I failed at the valve check last time. Just reset the valves after a year or riding and of course did another compression test:

    Front: 235psi
    Rear: 240psi

    Checked cold with the same gauge.
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  8. charlie264

    charlie264 Long timer

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    240psi is a lot, thatÂ’s 16.5 bar. Never checked mine but the manual states 11.5/1. My compression tester was about 20 quid 20 years ago, a pro tester that reads high and low pressure is not the same as a cheap tester that has a bourdon tube and a one way valve.

    Having said that, I would say anything over 8 bar turned over by the starter motor is acceptable.
    #8
  9. fast4d

    fast4d Long timer

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    compression test can't really be used to gauge condition of the engine.

    I use comp numbers (psi) to see if all cylinders are with a percentage of each other. that's about it.

    get a leak down tester/compressed air and do a leak down test. mine tested at 5% at the last valve adjustment.
    #9
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  10. keener

    keener Speed changes you.

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    250 rear, 180psi front, warm motor
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  11. multistraddler

    multistraddler Been here awhile

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    Oh no! I just checked my compression stone cold (over 50 degrees here today) and found the following:
    Front: 150psi
    Rear: 180psi

    35k miles on the motor. I haven't noticed the bike burning oil, but this does not look good.
    I just adjusted the valves, so they should be tight.
    Am I looking at a top end rebuild? :( Grrrrr
    A friend with engine smarts and experience in the desert said that dust in the cylinders can be quite abrasive.
    I may have just proved him right...
    The condition of my intakes before and after cleaning...
    http://advrider.com/index.php?threads/intake-dirt.1053442/#post-28641712
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  12. keener

    keener Speed changes you.

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    First of all, those numbers are NOT way of the chart. They are not great though.

    You could have some crap stuck on the valve or valve seats, specially because how u cleaned it. Did you ride the bike after cleaning?? All those stuff you cleaned must come out and if u did it with valves closed and no vacuum ,chances are something is stuck under a valve.

    Take the spark plug out, rotate the engine until intakes are fully open. Send a hose down the spark plug hole ,connect to a vacuum hose ( use a plastic bag and electric tape to make a seal)...now use the smallest BRASS wire brush you can find to clean the valve lips and valve seal the vacuum running. You can pull some brass strings of a brush and fix them to tail of a fine pair brush (with strings) to make a tool.

    Take a compression test again. But a better test is a leak down test. Because you could see/hear where the leak is and how much it is. Look up on youtube if you've never done a "leak down test".

    Take that test before and after cleaning.
    #12
  13. multistraddler

    multistraddler Been here awhile

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    Keener,
    You could be right about crap on the valves/seats.

    The pistons were at TDC when I was doing the cleaning of the intakes, so nothing was falling into the cylinders. Oh wait...that's a fib. When I first tried to clean the rear intake, I was using WD40 to loosen up the crud. Some of that probably dripped down into the cylinder. But I changed my cleaning method on the front and was just using wire brush tool on my Dremel and compressed air to knock the crap off. I have not ridden the bike since doing the valve clearances and cleaning the intakes.

    I took the compression on the front cylinder three times and every time I got a higher value. First was around 125 psi (panic), next try about 140 (concern) and the final attempt measured around 150 (harummph). Maybe I should just keep going until I get to 200??? :) A similar thing happened on the rear, but it started around 140 and moved up.

    Does this logic not apply? Atmospheric pressure at my altitude of around 900 ft above sea level is 13.2 PSI at 55 degrees. So, by extension shouldn't it be a matter of multiplying the compression ratio by the atmospheric PSI? 11.5 x 13.2 = 151 psi, which seems too low. How could you blokes be measuring > 200 unless you are below sealevel?

    That's a good idea on cleaning off the valve lips and seals before jumping to any conclusions. Off with the airbox...again!!!

    Will see if I can borrow a leakdown test from one of my gearhead neighbors.

    #13
  14. keener

    keener Speed changes you.

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    Atmospheric pressure shouldn't affect anything because you have that everywhere, including both sides of the pistons.

    Edit: I stand corrected! Altitude does make a difference.

    I guess, pressure goes up as you continue testing, because you are doing this on a cold engine and the rings get warmer by friction and expand.

    Just to be safe, try the vacuum method I suggested combined with compressed air through open intakes to take out anything that might have fallen inside the combustion chamber.

    I found ,it is better to do such cleaning job absolutely dry ,no wd40 or anything, that way nothing sticks anywhere.

    Leak down test will tell you everything you need to know. You could convert a compression tester dial tool to make a leak down tester. Look it up.

    Just for shits and giggles here is a cool, not so related video. I can't stop watching it:
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  15. uk_mouse

    uk_mouse Aquatic adventurer

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    That is a cool video :D

    Conventional wisdom says that if you get a low reading on a compression test, then add a small amount of engine oil (~5ml) to the cylinder (through the spark plug hole) and do the test again. If the second reading is higher, then it's worn rings. If the second reading is no different, it's worn valves / seats.
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  16. dieselcruiserhead

    dieselcruiserhead Long timer

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    Altitude is a massive factor for compression tests. It is a 25% difference at 6k for example, so 150 instead of 200 or about 3% for every 1000 feet. So do factory that in,

    Andre at 6800 feet
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  17. keener

    keener Speed changes you.

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    You are right! I added a note to my last post. Thx!
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  18. charlie264

    charlie264 Long timer

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    Just to add a little, conventional thinking is that the ignition of fuel forces the piston down...its the change of pressure forcing the piston down also, because the pressure is higher one side the atmospheric pressure is trying to equal that pressure buy pulling in all directions. Hence if you put a vacuum pump in the spark plug hole, decreasing the pressure the atmospheric pressure pushes the piston up.
    #18
  19. Dustodust

    Dustodust Long timer

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    Mine passsed the wrectal exam last month, about 35,000miles now on a 2007 950SM motor from a crash takeout with about 9800 miles that I put in my 2004 ADV
    I got 195 on both mine, cold with the carbs off
    leakdown was about 5%, no hissing from the intake exhaust or breather.
    The leakdown test gives a more complete indication. In fact you dont even really need the leakdown guage. You could use the hose from the compression tester and add an air fitting, turn down the regualtor on the compressor to about 30 psi, Lock the crank at TDC and and fill combustion chamber. then make a stethescope with some tubing,You will hear massive hissing at the exhaust or intakes if you have a bad valve, at the breather tube (remove check valve) if the rings are shot.
    picture of inside mine rear using a borescope front looked the same, this is where I would expect to see any detonation damage start (edges near valve cutouts)
    [​IMG]
    #19
  20. multistraddler

    multistraddler Been here awhile

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    Got around to removing the carbs & airbox today to have a closer look. The valves look pretty well coated with gunk and don't see how I can do a proper job of cleaning them up through that little orifice. So, I've started down the path of pulling the motor to get the heads off to have a better look. (Our neighborhood streets are covered with a crusty layer of ice, so there won't be any motorcycling in my near future...)
    The oil tank is off, anti-freeze drained, kickstand removed, exhaust pipes undone, and miscellaneous hoses and wires have been freed. I have piece of bent sheet metal was very helpful removing my Duc engines, but it will be utterly useless on the LC8. I'll search the forum for answers on that, but any tips most welcome.
    I'm just crossing my fingers that the cylinders are OK. I've heard tell of fesh fesh getting into combustion chambers and doing a number on rings and walls...
    #20