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About a month ago I noticed white smoke coming out of my motorcycle exhaust. Not constantly, but it would maybe start smoking and then stop after riding a little. Or maybe keep smoking even after riding for a while.

At first I thought it could be bad quality gas, since it has been only about 50k kilometers since the last time I had replaced the cylinder, piston and piston rings. I also couldn't notice any black residues on the end of the exhaust, which I heard gets deposited there when the engine is burning oil (is that right by the way?).

Today however, I noticed the first residues of black carbon. Very little, but first sign that it's probably oil being burned and not bad gas. The oil level is still within range, but getting slightly closer to the lower end. I'll be keeping an eye to the oil level even more frequently now.

Now to the question title. I first noticed the motorcycle smoking through the exhaust after I push started it when I had a dead battery and had to get it running. It could be a coincidence, but part of me is asking myself if that could have been the cause. Maybe the impact of the stationary transmission on the rotating engine could have broken or displaced a ring? Since 50k kilometers usually isn't enough to make a cylinder hear kit replacement necessary, I was wondering if I prematurely caused the damage.

The motorcycle is a 150cc Honda, if that's relevant!

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    50k is probably rebuild time for a small engine... – Solar Mike Mar 2 '20 at 11:33
  • no the piston rings will not get damaged by this clutch and drivechain on the other hand might. – trond hansen Mar 2 '20 at 13:37
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    If you do it right, push starting the motorcycle should cause no damage. The drive train is designed to be spun by the engine, or in turn is fully capable of spinning the engine. I've push started my '83 Honda many times, and it's still going strong. What color is the smoke? Blue, or black? If it's blue, it's probably oil going past the rings, and you'll have to follow Solar Mike's rebuild advice. If it's black smoke, then the carburetors are probably running rich, and should be tuned, or your choke could be stuck on a little bit. – the_storyteller Mar 2 '20 at 14:27
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    @PhilG 4-stroke: "The oil level is still within range, but getting slightly closer to the lower end. I'll be keeping an eye to the oil level even more frequently now." If a 2-stroke, you would expect the oil in a separate oil tank (as a few have) to be consumed, and it would be normal to burn oil (which AFAIK is why Honda discontinued making 2-strokes a while back, but might reintroduce a modern design). – Weather Vane Mar 2 '20 at 17:46
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    Are you sure the white "smoke" it isn't steam? In cold weather the water vapour released by combustion can condense, usually until the exhaust system has heated through. If your climate is usually hot you might not have noticed. Smoke contains carbon/soot particles, which can be seen if you let the exhaust blow through a white cloth. – Weather Vane Mar 2 '20 at 18:34
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The electric starter hits the engine pretty hard. There's a clutch between the starter and the crankshaft, it varies in location between bikes, sometimes it's on the output of the starter motor, sometimes between the driven starter gear and the flywheel/alternator housing or the end of the crankshaft, but there's only a little give in that clutch, and the starter accelerates the crankshaft up to its cranking speed typically within a single turn. Push starting is likely to be less aggressive than that, there's the cush drive in the rear hub that softens the impact even if you do drop the clutch in - and the flexibility of the rear tire and suspension also limits the peak forces produced - if I try to push start my 750 in first gear, it'll just skid the rear tire. So I'm certain that this wasn't the cause, probably coincidental.

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