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Compression issue

Discussion in 'Norton Commando Motorcycles (Classic)' started by acadian, Apr 17, 2019.

  1. acadian

    acadian VIP MEMBER

    Mar 5, 2010
    Short story: new motor, less than 50 Km on engine so far, bike runs well, no visible smoking, plugs clean (aside from the usual carb sorting indicators)

    The issue: trying to pin down a major compression discrepancy between the cylinders, DS shows excellent comp but TS next to nothing (80 psi difference) - Oil film was observed on TS intake valve. No access to a leakdown tester.

    Pulled the head. Petrol test through intake/exhaust ports and in combustion chamber showed no leakage. I haven't removed the suspect valves yet but plan to, otherwise the valve action is normal and consistent, and there were no sudden changes in valve clearances through this preliminary break in period.

    Pulled the barrels to check for busted rings/cylinder damage, glazing etc
    • no "apparent" issues. Cylinders appear to be in great shape, good cross hatching, pistons don't show any damage/scoring, though the TS piston shows a bit more build up, assuming this relates to the oil residue observed on the TS intake valve
    • the coating I had applied has burnished on the thrust faces (expected)
    • the ceramic coating on the crowns show no indication of valve contact (good, because I triple checked this during assembly)
    • no indication of blow-by between the rings
    New Cam also appears to be in good shape (I guess I ran it in properly)

    So, I'm still left wondering what could account for the compression loss? Could the TS piston have glazed to such an extent? If both barrels showed close numbers I wouldn't have thought too much about it.

    Some pics:

    IMG_2841.JPG IMG_2845.JPG IMG_2846.JPG IMG_2841.JPG IMG_2843.JPG IMG_2844.JPG IMG_2842.JPG

    Attached Files:

    Last edited: Apr 17, 2019
  2. o0norton0o

    o0norton0o VIP MEMBER

    Apr 27, 2015
    Do you have any doubt about the compression test readings? When you kicked the bike over was it obviously an easier kick to turn over the cylinder in question ? Did you redo the compression test with a teaspoon of oil in each cylinder to see if you could determin which way to look?? (up or down)

    Beyond questioning the accuracy of the compression test, there's a few obvious potential culprits.
    -head gasket
    -valve seating
    -some pourosity in head/cylinder/piston
    -valve train hanging up on something, cam lobe misalignment, bent valve stem, follower hanging up in the tunnel.

    You wasted no time in pulling it all apart, so obviously you thought you would find a cause and did not. I'm sure that I would be in the same postion as you, wondering why I didn't find anything obvious.
  3. acadian

    acadian VIP MEMBER

    Mar 5, 2010
    Fair point, I thought the compression gauge may be suspect so I tested it on my T120 and A65 and got consistent results (consistent, not necessarily accurate LOL). The bad cylinder was obviously way off, even without the comp test I knew as I could easily push the kicker through by hand with a plug in that pot.

    • Head gasket was perfect, no signs of leakage with even sealing all around. I was half hoping I would find it blown, but nope
    • I suspected the valve seating, but the initial leak test shows a positive seal, that said I have yet to remove the suspect valves to check for straightness, though there's no indication of piston contact so I I'm not sure that's the culprit anymore
    • I thought of a casting issue, or possibly a crack in the head, not sure how to check for that though as a visual inspection shows nothing
    • cam/follower alignment spot on (see cam/follower pics), no indications of excessive wear either, stellite pads in good shape and confirm lobe alignment and good contact area
    • Interesting point about the follower, had not considered that, but if the follower was sticking (not completely falling back down), I'd think I would get inconsistent valve clearances... no?
    Last edited: Apr 17, 2019
  4. 1up3down


    Jan 12, 2011
    not meant to be a disrespectful question, but you fully open the throttle when you were kicking it through at least 3-4 times with ignition off and plugs in place?

    and you are certain both carb slides open fully together viewing from the rear?
  5. acadian

    acadian VIP MEMBER

    Mar 5, 2010
  6. Atlas Commando

    Atlas Commando VIP MEMBER

    Feb 27, 2018
    Valve clearance? Or rather lack thereof on the weak cylinder? Would explain the symptoms and nothing would look 'wrong' upon disassembly. Having gone this far, I would certainly want to remove the valves for detailed inspection but if there's nothing obvious with valves and seats, not sure what else it could be.
  7. Triton Thrasher

    Triton Thrasher

    Feb 10, 2009
    Is this the bad one?

  8. Nortoniggy


    Nov 17, 2010
    If it was running ok I would have put some more miles/kilos on it rather than tearing it apart. In the scheme of things 50kms is nothing. Particularly if they were new pistons and/or rings.

  9. acadian

    acadian VIP MEMBER

    Mar 5, 2010

    I would have been happy to leave it, but again, it's the discrepancy between the readings, not the readings themselves that concerned me
  10. concours

    concours VIP MEMBER

    Dec 29, 2011
    The piston ring tension/free size? Shape? End gap?
    When you measured your end gap, did you look for uneveness about the perimeter?
  11. acadian

    acadian VIP MEMBER

    Mar 5, 2010
    I simply gapped the hastings rings that came with the pistons and installed them, did not deliberately check for perimeter contact, but fair point, I will do that in the future. I've got a new set of rings on the way, I've been advises not to attempt honing or "deglazing" the cylinders

    Oh well, at least this gave me the opportunity to check the cam/follower condition
  12. Triton Thrasher

    Triton Thrasher

    Feb 10, 2009
    It looks like the last person who honed it didn’t make much of an attempt at washing the grit off. Not saying that’s what’s wrong, but it’s poor practice.

    I don’t see why it shouldn’t be honed now.

    You can get an improvement in compression by making the second ring gap 30% bigger than the top ring gap, not oiling the bore more than a rub with an oily rag and making the engine pull fairly hard immediately it starts.
  13. acadian

    acadian VIP MEMBER

    Mar 5, 2010
    Hmmm, still trying to decide whether to put new rings in as I can't see anything wrong with the existing ones, but I'd not heard of enlarging 2nd gap
  14. MexicoMike


    Jan 31, 2010
    You mentioned that the bike ran well. So, did it run well initially and then quit running well which was the reason for the compression check?
  15. acadian

    acadian VIP MEMBER

    Mar 5, 2010
    No, was running well all things considered, the comp check was just happenstance, I pulled the exhaust valve and cant see any issues with it, not bent or burnt, next the intake valve when I have some time
  16. kerinorton


    Feb 12, 2013
    Triton Thrasher, could you please let us know your qualifications for saying.

    "You can get an improvement in compression by making the second ring gap 30% bigger than the top ring gap, not oiling the bore more than a rub with an oily rag and making the engine pull fairly hard immediately it starts. ''

    As a fully qualified auto engineer, I have never heard of that.

    Arcadian, there isn't anything wrong with your engine that a good hard ride wont fix. Put it back together as it is [ don't touch the bloody rings, and don't oil them. As above , a wee bit of oil on the bore only] , and ride it hard then medium the hard etc etc..

    Ps, there only very few genius's on this forum. Your problem is finding who they are and and acting accordingly.
    Nater_Potater likes this.
  17. Triton Thrasher

    Triton Thrasher

    Feb 10, 2009
    Cycling Proficiency Certificate, playground of Pulteneytown Academy, 1966.

    Ignorance of practice doesn’t say much for your “full qualification” then, does it? Don’t you do any sort of continuing professional development? Like reading recent things?

    What else have you “never heard of?” Oh you don’t know, do you!
  18. Danno


    Feb 7, 2010
    I would set the idle up and then pull one plug lead at a time to see if you get a similar indication.

    But I would have done it before teardown and inspection
  19. marinatlas


    Nov 4, 2007
    Doe not do it while engine running if you have electronic ignition, , that could damage the magic box .......
  20. acadian

    acadian VIP MEMBER

    Mar 5, 2010
    It's not an ignition issue, I ran through the test mode on the tri spark.

    I'm going to leave the pistons/rings alone and simply clean the cylinder, apply a bit a quickseat powder, then reassemble.

    Neither valves on the problem pot are damaged, I will lap them and put them back in. I'm going to try fitting the head sans pushrods and perform a leak down.

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