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Cylinder head pressure venting...(2012)

Discussion in 'Norton Commando Motorcycles (Classic)' started by Bob Z., Aug 1, 2012.

  1. NortonMKIIA850


    Jul 20, 2017
    The first post in this thread cites a post from the NOC forum that I haven't been able to find. But I had an interesting experience yesterday that seems relevant to this topic, and have started a thread on it there. But I'm (mostly) cross-posting my own post here, in case it's of interest, or anyone has any observations to add:

    'My Commando has, unfortunately, taken up smoking. Grey-blue smoke, just from the left cylinder. I've tried replacing the oil seals on both inlet valve guides, with generous assistance from [NOC-member] Gordon Robertson, but to no avail – I did both because it seemed sensible. Yesterday I ran the engine with the left exhaust valve cover off, so that I could see if oil was pooling there – maybe the drain there was blocked. No sign of oil pooling, so I'll have to take the head and barrels off to investigate further. But, while the valve cover was off, the smoking virtually disappeared! Of course, it came back when I put the valve cover back on.

    I've seen people discussing the virtue of a breather outlet for the head alone: from this experience I'm convinced. Is that wrong? By the way I have a one-way reed valve from HNW in the existing breather hose, and yes, I have installed it the right way around! [​IMG]'

    Noticing previous mention in this thread of a factory breathing set-up for the cylinder head installed until 1962, I wonder if it's possible to retro-fit this …?
    Last edited by a moderator: Dec 18, 2018
  2. comnoz

    comnoz VIP MEMBER

    Dec 10, 2008
    The pressure originates from the lower end. You may cause oil return problems if you try to vent it from the head. [oil has trouble going down if there is a lot of air going up]

    Your symptoms indicate either your breather check valve is not working or you have so much blow-by that the check valve can't handle it.
  3. NortonMKIIA850


    Jul 20, 2017
    Yep, pressure originates in the air-compressor under the pistons. Understood about 'a lot of air going up'. Blow-by shall be the next thing I look for – I can see that the check valve is working, when I take the oil tank filler cap off and watch and feel for it – thanks!
    Last edited: Sep 14, 2018
  4. JimC


    Oct 12, 2007
    Those vacuum check valves cannot keep up with engine pulses at speed. The heat quickly renders the rubber flapper useless. They do provide an excellent placebo effect, though.
  5. Nater_Potater

    Nater_Potater VIP MEMBER

    Apr 7, 2013
    If a placebo effect, then that means my girl is self-aware and conscious of what's been done to her. She doesn't leak oil, and I can tell when the booster valve goes bad, because I start to get some seepage from under the barrel. Granted, it's not a lot (just enough to create a dusty ring along the base), but it completely goes away once the valve's replaced. The last valve has lasted over 5,000 miles, and is still going strong.

    xbacksideslider likes this.
  6. robs ss

    robs ss VIP MEMBER

    Aug 16, 2016
    In addition to comments above I believe there are at least two good reasons not to vent to atmosphere through the rocker cover (read head for Norton heavy twins)
    1. The source of the pressure fluctuations is in the crankcase, caused by piston movement - best to treat (vent) it there.
    2. If you vent through the head you are relying on upward air movement through the rear oil gallery (only path) potentially interfering with oil drainage.

    Bob - your link doesn't go anywhere useful - at least not for me

  7. NortonMKIIA850


    Jul 20, 2017
    Me neither – but I think that's down to the site re-vamp a while back, and (yet another) job for the long-suffering Jerry Doe – I've a feeling there's a thread for internal links that don't work, where they can be posted for fixing …? Something like it anyway.
  8. Reggie


    Jul 24, 2006
    NortonMK11A850 wrote;
    When I had one of the breathers as sold by HNMW I found that I used to get emulsified oil in the breather which I don't think helped it breath at all. I got sick of cleaning it out. I changed mine to a Jim Comstock sump plug breather and have not had any more "emulsion issues."
    Just a thought?
  9. NortonMKIIA850


    Jul 20, 2017
    Yep, I'm sure you're right – that is, from memory there was emulsion in the standard breather hose before I fitted a one-way valve; certainly it's still there. It won't help breathing, but periodic cleaning does. Anyway Jim's solution does seem best all-round. It's just a bit too dear for me, unfortunately. Maybe one day!
    Last edited: Sep 17, 2018
  10. pierodn


    Nov 10, 2012
    Please, could you let me understand what does it means “blow - by”.
    Thank you.
  11. NortonMKIIA850


    Jul 20, 2017
    In this instance, Piero, it means gas escaping from the compression chamber by passing between the piston, piston rings and the cylinder wall. By 'gas' I would assume not fuel alone (varieties of English can be confusing!) but instead hot, burnt gases from the ignited air-fuel mix. HTH
    Last edited: Sep 17, 2018

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