Smokey Rebuilt Bonnie T140

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

 

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.

 

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

 

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.

 

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