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how much compression in lbs.

Discussion in 'Norton Commando Motorcycles (Classic)' started by alan hodge, Feb 5, 2019.

  1. alan hodge

    alan hodge VIP MEMBER

    Joined:
    Oct 10, 2017
    this is kinda like an oil thread but here goes....I played hooky from work this afternoon (78F) and rode about 30 miles got back home and while engine warm did compression test..Wide open throttle and several stout kicks..harbor freight gauge said 50lbs or 4 bars..what? bike uses no oil...plugs look good...starts first kick..runs great..no smoke of any type at any time....how can that be?
     
  2. htown16

    htown16 VIP MEMBER

    Joined:
    Apr 29, 2009
    Bad gauge. Either try another gauge or quit worrying about nothing. It probably wouldn't run if it had much less than 100 psi hot.
     
  3. lazyeye6

    lazyeye6 VIP MEMBER

    Joined:
    Feb 28, 2014
    My Harbor Freight gauge is a POS. Useless.
     
    Mike Dunlop likes this.
  4. alan hodge

    alan hodge VIP MEMBER

    Joined:
    Oct 10, 2017
    think I will quit worrying and just enjoy riding while false spring is here
     
  5. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    I could never get a reliable reading doing a cr test with a kickstart bike.
    This is with a 1960s US made gauge that works great on autos.
    A leak down test is a far better option for us, and gives a lot more info than a cr test does.

    Glen
     
  6. Onder

    Onder

    Joined:
    May 11, 2010
    Leak down the only way to go. Why do you think they use them on aircraft....
     
  7. alan hodge

    alan hodge VIP MEMBER

    Joined:
    Oct 10, 2017
    tried gauge on the T120 same results garbage can next stop
     
  8. Onder

    Onder

    Joined:
    May 11, 2010
  9. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    I got 130/127 psi with the Alton E-start spinning the motor. Best I ever got with the kick starter turning the same motor using the same (1970's SnapOn) comp gauge was around 70 (in both cases engine warmed up to operating temp prior to test). So, as noted, a kick lever isn't worth a lick for doing a compression check.
     
  10. ashman

    ashman

    Joined:
    Jul 11, 2010
    If it not broke then leave it alone, you say its running great, use no oil or don't blow smoke means its all good just do your normal maintenance when needed and give your bike a pat on the tank when it brings you home safe.
    A lot of folks worry over nothing and how many motors get pulled down to find nothing wrong with them, just take it out when you can and enjoy it, when you worry about things then thats when you will have problems.
    Now get on it and go for a good ride.

    Ashley
     
  11. kraakevik

    kraakevik

    Joined:
    Oct 12, 2009
    I'd be very skeptical of those readings if the bike runs at all. I had a trusted Sears compression tester that fooled me into removing a perfectly sound head with 30 psi readings on each side--I then tested my Combat engine that was returning 50 mpg and went like a scalded cat--the readings were even lower


    Tim Kraakevik
    kraakevik@voyager.net
     
  12. alan hodge

    alan hodge VIP MEMBER

    Joined:
    Oct 10, 2017
    aint gwine to worry no mo....gonna ride..bike gives me a big smile...it's fast! .all 130 miles so far....chain noise gone too.....
     
    ashman likes this.
  13. Mwood7800

    Mwood7800

    Joined:
    Jul 26, 2017
    I did a leak down on mine. With the cam breather the air had no where to go.
     
  14. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    You can always pull the sump plug.

    Glen
     
  15. jbruney

    jbruney

    Joined:
    Jan 5, 2019
    I see no problem. I crank up cold I puff a tad of light smoke on the right cylinder if I goose it until it warms up. So I just quit looking behind me until the motor gets warm because it has never run as well as it does now, which takes us back to....don't look for trouble, but stay up on maintenance. Those are two separate things right?

    You guys begin to make me worry with this kind of talk because in all the years I've had this 850 it has never been what I'd call difficult to kick over. Matter of fact I don't even rear up to do it...just sort of shove the old right leg down and let it go. I mean it's really tame to start, and now here I am starting to ponder an underlying issue just because it's easy...just kidding.
     
    Last edited: Feb 5, 2019
  16. htown16

    htown16 VIP MEMBER

    Joined:
    Apr 29, 2009
    Without wandering too far off track, how are you guys doing leak down tests locking the engine. I tried it on my Trident with it in gear and the rear wheel locked but there was still enough play in the drive train that the engine turned enough to open a valve.
     
  17. dynodave

    dynodave

    Joined:
    May 28, 2003
    It is ENTIRELY based upon the ACCURATE placement of the piston at crankshaft TDC. It takes practice to obtain the skill of placement. I never have to mechanically tie up the engine, place in gear or anything like that...maybe a few false starts but I always get there.

    Kick start/low pressure? At one of my NENO tech sessions a long time ago, we were trying to do a comp test. Two bigger burly guys had both tried kicking and getting piss poor pressure 60-75? (I don't exactly remember). They concluded a tear down was needed.
    I said let me give it a try... kicking technique must be the charm. 3 kicks and bingo 150?160 the crowd was speachless or laughing
     
    Kvinnhering and motorson like this.
  18. alan hodge

    alan hodge VIP MEMBER

    Joined:
    Oct 10, 2017
    tell us your technique
     
  19. Onder

    Onder

    Joined:
    May 11, 2010
    When you do an airplane you have a lot of leverage with the prop. On a bike I put a socket on a long breaker bar and hold the alternator rotor to
    crank nut a tiny bit off TDC and as you roll up the air pressure you can push the piston all the way up. You can feel it. You can hear blow by past
    the rings into the crankcase or past the valves into the ex or inlet track. Less so on a bike but on a airplane you can also hear the rings seat if
    worn.
    It is the only way to go.
     
  20. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    To do the leak down, I put the bike in fourth gear then use a short piece of wooden 2x2 to wedge the rear brake pedal down quite firmly.
    When I first got the tester, I used it per instructions, lock engine 1/2" +- btdc on compression stroke.
    After watching one of Jim Comstock's videos where he uses a bore gauge to find greater and lesser bore wear at various points along the stroke, I now test half way along the compression stroke as well.
    If the ring seal is good the leak down from rings should be very similar halfway and near top.

    Glen
     
    Last edited: Feb 6, 2019

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