70 commando rebuild smoke after 1000 miles

scratching my head before I pull the other head. Full motor rebuild, guides, bored 20 over, new valves, bike started to smoke after 1000 miles on start up then clear after a minute, mostly smoke on right side but sometimes on both. It doesn’t smoke every start up, sometimes it sits for a week and no smoke then sometimes it smokes like a bug sprayer. If I go for a ride an it sits 2/3 hr it might smoke or might not on right side. Plugs look great after a good run. I do have an oil check valve and keep oil level on L I always park on side stand. I have looked in cylinder with bore scope and inspected valves for leakage, all look well. I rebuilt my 69 last year and it’s great. Driving me bonkers.

Just to be sure I would check oil level after sitting for some time. Only time mine smoked was when it had wet sumped. If all is well there , you have a motor problem of some kind.

Valve seals? These can be tricky to fully seat and may come loose after a bit of mileage. Symptom on my 850 was puffs of smoke out of right side when blipping throttle.
I used Ludwig's method to replace the seals (with Viton rubber type in place of brittle hardened black plastic oems) without needing to remove the head.

I think we're all a little apt to see the causes of our own problems when someone posts an issue with similar symptoms... So, in my case, I had a circlip pop loose a few weeks after a complete rebuild. It popped out at 65 mph with a loud "POP" sound and issued a puff of blue/white smoke out the pipe on that side, which I saw in the mirror. So, I pulled the clutch, shut the bike off, and coasted to a stop. I had a friend pick me up with my Van.

Inspection when I tore it down showed that the circlip lightly scored the cylinder wall. My mechanic friend said put it back together because the damage was slight and very low on the cylinder wall and he thought it wouldn't smoke at all. (which it doesn't thankfully) If it did smoke after I reassembled it, then I would have taken it back apart to fix the cylinder, and whatever other parts that entailed.

Perhaps you can stick a mini camera into the spark plug hole when the pistons are down in the cylinders and have a look to see if there's a verticle score mark inline with the wrist pins... If you find that scenario, then you know where you're at...

Mine did something similar when oil was coming around the outside of the guides which were loose in the head. If the old guides were just driven out without removing carbon around the ends of the guide it can score a nice oil passage. I agree that most people will post what they have experienced. Not a bad thing.

Mwood7800 said:

scratching my head before I pull the other head. Full motor rebuild, guides, bored 20 over, new valves, bike started to smoke after 1000 miles on start up then clear after a minute, mostly smoke on right side but sometimes on both. It doesn’t smoke every start up, sometimes it sits for a week and no smoke then sometimes it smokes like a bug sprayer. If I go for a ride an it sits 2/3 hr it might smoke or might not on right side. Plugs look great after a good run. I do have an oil check valve and keep oil level on L I always park on side stand. I have looked in cylinder with bore scope and inspected valves for leakage, all look well. I rebuilt my 69 last year and it’s great. Driving me bonkers.

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batrider said:

Mine did something similar when oil was coming around the outside of the guides which were loose in the head. If the old guides were just driven out without removing carbon around the ends of the guide it can score a nice oil passage. I agree that most people will post what they have experienced. Not a bad thing.

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i wish it was that simple, checked that but thanks

I would rule out wet sumping before resorting to head pulling.

You say you have "an oil check valve" ...... one of those automatic things? If so, I would get rid of it. Replace it with a ball valve, at least until you can rule out wet sumping as the cause of your problem. Auto check valves can be leaky, a new ball valve is certain to be tight. Of course, get into the habit of ensuring the ball valve is open before starting.

You say you "always keep oil level on L". What does that "L" mean? Low in the tank? Oil level in tank, or parking on side stand has little effect on gravity ..... If your oil plumbing is leaky, gravity wins.

In lieu of replacing your oil check valve with a ball valve, the most positive way to determine if your engine is wet sumping, is to drain the sump before start up, every time. A tedious process, but one that will show you exactly how much oil is getting into the sump.

If your smoke problem continues after draining the sump, then look elsewhere. If the smoke stops, then consider the AMR modification to reduce wet sumping.

Slick

Look at the inlet valves through the plug holes, if the seals are leaking or guides have gone the valve will be wet with oil. Remove the exhaust pipes and if the exhaust guides are leaking down the their sides you'll see the leak path.
Dave

If your plugs really are clean then it would point towards exhaust guides: the oil is being sucked down and turned to smoke but not being burned in the combustion chamber.

If it is burning oil in the combustion chamber, but you believe bores and guides to be good, don’t forget that with the pushrod tubes and oil return hole Norton carefully designed in 3 areas where oil could potentially leak past the head gasket into the combustion chamber.

Whatever the cause, I’m struggling to come up with any hypothesis about the intermittent nature of your problem...

Fast Eddie said:

If your plugs really are clean then it would point towards exhaust guides: the oil is being sucked down and turned to smoke but not being burned in the combustion chamber.

If it is burning oil in the combustion chamber, but you believe bores and guides to be good, don’t forget that with the pushrod tubes and oil return hole Norton carefully designed in 3 areas where oil could potentially leak past the head gasket into the combustion chamber.

Whatever the cause, I’m struggling to come up with any hypothesis about the intermittent nature of your problem...

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Synthetic oils once hot can burn very cleanly and smokeless, perhaps to save the exhaust cats in modern motors ?

batrider said:

I agree that most people will post what they have experienced. Not a bad thing.

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Wish I could post anonymously as i'm experiencing low miles smoke right now. In my case the guy who bored cylinders for hone didn't leave enough material. I was shooting for .0025" to hone out but it was .0015" under finish size. The hone appeared to take all the boring tool lines out just fine but as I racked up miles smoke appeared and got worse. Heads off now and im seeing bore tool lines in in the honed bore. Should clean up rehone at less than .006" piston clearance put it back together an see what happens

Check for cracks in the intake port around the guide.

Fast Eddie said:

Whatever the cause, I’m struggling to come up with any hypothesis about the intermittent nature of your problem...

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How about an issue with the breather ?

oldmikew said:

How about an issue with the breather ?

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Worth checking for sure...

Are you referring to the cam case breather?
I just ordered the nyc reed breather kit. I’m running a single Mikuni.

Bump