Motorcycle Repair: 74 Honda CB360-E, honda cb360, plume of smoke

Question
Thanks for taking my question Rich!
The bike seems to run rather well for its age. It appears to have been taken pretty good care of these past 34 years. I do use this bike to commute approx 55 miles per day and I couldn’t be more smitten with the money I have saved in gas since purchasing this bike a few months ago. Problem…white smoke comes out of the L exhaust (nothing out of the R) as far as I can tell. I did clean the carbs w/ basic carb cleaner and I have changed the oil. It did have some charging issues when first purchased…but I have corrected them w/ a “trial and error” sort of approach (long story). It does idol rather high…and the idol screw on the carb is basically (bottomed out) or not even a factor. It idols around 2k…but it will surge if the throttle is left untouched (clearly a throttle cable issue which will eventually need to be addressed) I do have a “parts” bike in the garage from which salvage parts can be accessed if needed. I cleaned, and resealed the fuel tank w/ “Red Coat”. I have new fuel in the tank. I did notice the smoke prior to making all these changes and minor modifications…however I did modify the stock air filters (which looked like the original from 1974). I basically removed the old paper filter and affixed a “pre filter” type material (polystyrene?)…This was in an effort to save the $80 Honda wants to replace. I did consider an aftermarket filter…but it just will not fit. The bike did smoke w/ or w/o the filters in place though. The smoke has progressively gotten worse after approx 1000 miles of day-to-day operation. It is not a huge problem yet…but I am concerned it may be something that may need to be addressed sooner vs. later. It seems to smoke more at idol…a large plume of smoke will exit the L side exhaust if it sits at idol…then I give it some throttle. Otherwise it will smoke on a smaller and steadier level. I have recently noticed the smell of smoke when I am riding…but I can’t really tell how much. I see smoke when I downshift and eventually come to a stop. Any thoughts?

Answer
Well John, it doesn't sound good.  I think the smoke you describe is probably oil burning which could be from worn out rings, scored cylinder or piston.  If it only smoked on deceleration, it would be valve guides or seals and I wouldn't worry about it.  The fact that it smoke continuously tells me, it's a piston or ring problem.  It will get worse and it will require a rebuild to fix. It's possible that running with out the factory air filters increased the air flow and leaned out the mixture to the point it scored a piston.   

Regards
Rich