Motorcycle Repair/honda cb160 smoke
hi bill, i am in the uk, i purchased a honda cb160 from usa ,the motor had under 5000 miles but was tight ,i freed it off but then the left cylinder started smoking very badly, a ring had broken so i had it 1st over rebored and honed ,new pistons and rings , put back together , alas after a while it started smoking on the left again, so i have stripped it again and had someone check the valves and guides, they seem to think everything is okay, so i am lost ,can you help, thanks keith
Keith, If all the parts check out, then the smoke you saw was probably leftover residues in the exhaust system. Often, when bikes are revived after long storage, there are leftover fuel/oil deposits in the exhaust header and muffler that need to burn off. Additionally, the rebored cylinder will need some break-in miles before the rings seat into the cylinder liner. There will be some oil usage until the wearing in process is completed. I have seen bikes smoke for 15-20 minutes while this process occurs. If the piston ring was broken, before you pulled it apart, there will be a lot of oil in the exhaust pipe.
Be sure that the ring gaps are staggered when the engine is reassembled and that the gaps are down in the specified range. Also be sure that the rings are in the right order and are not in upside down. The second ring is a scraper to help reduce the oil in the combustion chamber. If the top two rings appear to be the same, look carefully at the cross section. The top ring will have a square cross section. If they appear identical, the shiny one is the top ring and will have a smaller end gap in the cylinder.
These engines don't have valve stem seals, so guide/stem clearances must be nice and tight on both valves. Make sure that the guides are not loose in the head, too.
Honda specified a 600 mile break-in period for new engines, so you will have to nurse it awhile until all the parts are seated in, as designed. So no full-throttle, high-speed running for awhile.