Smokey Rebuilt Bonnie T140

Discussion in 'Vintage Classics' started by Boggie, May 19, 2019.

  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  1. Boggie

    Boggie New Member

    Nov 10, 2018
    13
    3
    Buckinghamshire
    Hi all,

    I have been looking at buying a T140 for a while and just been to see a lovely restored bike. The top end has just been rebuilt with barrels rebored, oversized pistons and new rings. It is not yet run in, in fact it has not been on the road since the engine was put back together a couple of days ago.

    The bike started easily and the engine sounded great but was rather oil smokey on start up, more so on one side and this did not clear as the engine warmed up. In fact it got a bit worse. Would that be normal for a brand new rebuild or could something else, such as valve guides be an issue? If so, is this a big job?

    Many thanks,
    Ian
     
    • Agree Agree x 1
  2. Sprinter

    Sprinter Kinigit

    Aug 17, 2014
    5,971
    1,000
    uk
    It may be burning off lube from build, or not.
     
    • Like Like x 1
    • Agree Agree x 1
  3. Boggie

    Boggie New Member

    Nov 10, 2018
    13
    3
    Buckinghamshire
    Thanks Chaps,

    I don't think it was rebuild lube as this would have burned of fairly quickly. We had the engine running for 5 minutes after warm up and the smoke remained.

    Oil pump is a concern but what is the cause? Over pressure? I unscrewed the filler cap and could see that oil was being pumped back into the frame but it was spraying out so I quickly replaced it.
     
    • Agree Agree x 1
  4. Boggie

    Boggie New Member

    Nov 10, 2018
    13
    3
    Buckinghamshire
    Been researching and chatting to a mechanic friend:

    As a general rule; smoke on acceleration is usually rings, smoke on deceleration is usually guides.

    When the engine accelerates, manifold vacuum is low and so any smoke could caused by oil passing the scraper ring, into the chamber and burning. Oil consumption will be very high and plugs will be very oily. When the engine decelerates, manifold vacuum is high, and oil can be drawn via the valve guides, into the chamber and burn with copious amounts of smoke at idle and immediately after taking off from a standstill. Oil consumption will be up and plugs will have some oily black deposits.

    Typically, oil passing the rings will be continuous under all running conditions but more so under acceleration / revving up. It's quite possible that she is getting oil from both rings and guides. This bike is creating smoke under both conditions but more on acceleration (revving up) and more so on one side than the other so probably not an issue common to both cylinders.

    Note: A little smoke at start up is normal and caused by oil from the cylinder head (as a little oil drains from the guides while sitting after being out for a run). However, this is at idle and for a period after starting, usually clearing after a short period of time. As this bike did not clear then I don’t think this is the issue.

    Piston rings:
    Broken oil control rings would cause the issues but this is unlikely as they are new. As the top end had been rebuilt recently, it might be worth checking the ring gaps. Should be around .010" for the compression and scraper rings. Too big gaps and oil can get through. This could be the problem here but probably not.

    Valve Guides:
    Guides can crack and this could be the problem but probably not. Worn guides are becoming more common. They used to last forever but modern unleaded fuel needs additives to help lubricate the guides. This could be the problem.

    Oil where it shouldn’t be:
    Leaks around the head gasket - have the head bolts been re-torqued after heat-cycling the engine? This is where I would check first as the engine has just been rebuilt so could be the issue.

    Check to make sure the oil in the rocker boxes is draining properly.

    Blocked engine breather – excessive pressure in the engine could be forcing oil up the bores.

    Wet sumping – too much oil in the sump being splashed up by the crank and con rods. However, this should be about the same on both cylinders if you are on the main stand so probably not the problem.

    Oil pressure relief valve spring within spec? A short spring will allow too much oil in the sump but again, I don’t think this is the problem as the smoke was more on one side.

    Bottom line:
    The issue is probably easy to fix, however the costs will be in identifying what needs to be done. I could be looking at quite a large labour bill to find the fix. If the valve guides are cracked or worn, there will be a cost for either a replacement head or some machine shop work to replace them. Best case scenario is that a head bold re-torque fixes it at 1h labour. Worst case scenario is that after a lot of mechanic’s time the problem of valve guides is identified. The labour plus parts could be £500 - £1000.

    Any other thoughts please? I really like this bike and it is a rare model too. I have negotiated a price with the dealer of potentially taking the bike as it is and taking the risk but it is only works out that the bike is around £400 cheaper.

    Should I take the risk? Apart from the smoking and a few minor things, this is a really nice restoration of a limited edition T140.
    Cheers, Ian
     
    • Agree Agree x 1
  5. Rocker

    Rocker Elite Member

    May 1, 2016
    1,659
    800
    Suffolk
    #5 Rocker, May 19, 2019
    Last edited: May 20, 2019
    Could be rebuild oil as mentioned above or wet sumping due to oil leakage from the tank through the oil pump the amount of oil in the sump could be quite a lot and should only be about an egg cups worth, drain the sump fire her up again and smoke should clear within 5 mins hth
     
    • Agree Agree x 1
  6. Boggie

    Boggie New Member

    Nov 10, 2018
    13
    3
    Buckinghamshire
    Thanks Rocker, is the oil pump the cause of wet sumping? If so I could buy a Morgo unit for under £100. That would be a relatively cheap fix, assuming they are not hard to fit?

    I am not sure if this is the issue though, the bike was running on the main stand and I assume wet sumping would affect both cylinders the same as both pistons crank unions and big ends would be throwing up the same amount of oil. However, the RH cylinder was much worse.
     
    • Agree Agree x 1
  7. stevethegoolie

    stevethegoolie Elite Member

    Oct 16, 2014
    2,450
    800
    East Riding of Yorkshire
    Just a couple of ideas:- are the ring gaps spaced evenly? From what I understand, best practice is to have them 120 degrees apart if there are three of the little buggers, that way nowt much gets past them - or is that more to do with compression?
    Also, you could use the bike for a while to see if the rings/oil use/smoke etc settle down a bit as everything beds in.
    Just thinking out loud!
     
    • Agree Agree x 1
  8. Rocker

    Rocker Elite Member

    May 1, 2016
    1,659
    800
    Suffolk
    Morgo or new pump would help especially with scavaging from the sump but I've been told it can depend on what position the pump stops too.
    My British bikes wet sump if not used for a while sometimes I drain the sump and sometimes I just go feck it and burn it off:)
    But back to yours who rebuilt it? do you trust them? did they snap a ring on assembly? rebuild oil should clear quite quickly with usually more smoke from the outside of the engine:joy:
     
    • Agree Agree x 1
  9. Ken walburn

    Ken walburn Noble Member

    Jun 28, 2017
    887
    300
    Essex
    Personally, I would go back again, run the engine, ride the bike, see if smoking goes away. Leave standing for 1/2 hour, restart, see what happens. Is it smokes, something’s has gone wrong during rebuild, more than likely piston/bore related
     
    • Agree Agree x 2
  10. Ken walburn

    Ken walburn Noble Member

    Jun 28, 2017
    887
    300
    Essex
    Personally, I would go back again, run the engine, ride the bike, see if smoking goes away. Leave standing for 1/2 hour, restart, see what happens. Is it smokes, something’s has gone wrong during rebuild, more than likely piston/bore related
     
    • Agree Agree x 1
  11. darkman

    darkman Crème de la Crème

    Oct 26, 2015
    6,893
    1,000
    Southcoast of the UK Earth
    Sounds like a badly rebuilt top end to me, budget on doing a complete top end rebuild to be safe and then ask yourself if you are going that far what about the bottom end.
     
    • Like Like x 1
    • Useful Useful x 1
  12. Boggie

    Boggie New Member

    Nov 10, 2018
    13
    3
    Buckinghamshire
    Thanks everyone for all the info, what a great forum! I collated all the potential issues and passed them to the dealer and using them his mechanic discovered it was indeed worn valve guides, as I originally suggested to him. So he has fixed this but added £500 to the price we had agreed so I walked.

    I just bought a US spec (peanut tank and high bars) but genuine UK supplied T140E instead. It needs some TLC but is a reliable, ready to ride bike, been with the current owner 36 years and has been updated with alloy barrels, forged con con rods and pistons, "half-race" cams, updated pushes with alloy tubes, 1.5 inch headers with straight-through pea shooters, alloy rims with stainless spokes, Koni adjustable shocks and a 12 inch front disk.

    It comes with loads of spares and has no significant oil leaks either! It will need paint, new loom, handlebar switch clusters, lower bars, indicators etc and a general tie-up. Perfect winter resto but importantly; the mechanics and mods make for a solid base that has significantly more power able to mix it with modern traffic. I test-rode it 10 miles and it felt quite a bit faster than a standard Bonnie but well sorted and handled/stopped better too.

    Pictures coming soon!
    Cheers, Ian
     
    • Like Like x 6
Loading...

Share This Page