Blue Smoke

[adrian58]

My 1972 K2 is blowing smoke and fouling plugs. I know that that means burning oil but as far as I know these engines are not prone to burning oil. Before I rip it to bits I want to try everything. I've just noticed that the previous owner has blocked off what appears to be a breather pipe on the oil tank under the seat. I can't see why he's done this, anyone have any idea why ?  Could this be contributing to my problems by not venting the oil tank ? Also I changed the oil to Halfords Classic 20/50. Thanks in advance for any thoughts or suggestions. Adrian

[Bryanj]

20/50 is too thick when cold for the UK, you need 10/40 or 15/40. Try Tesco, the diesel mineral oil is exactly the correct spec for the old SOHC Hondas.

Blue smoke is never normal for a Honda 4 stroke and would be either valve guide wear/ stem seals or bad piston/rings but possibly excess crankcase pressure if a breather is blocked although that usually blows seals and/or gaskets first

[Waggles]

Agree with Bryan.

A way to check which it is likely to be is to see when the smoke is at its worst. If it smokes under power but less when you back off it is most likely to be the bores / rings. If its OK under power but smokes on the overrun then its more likely to be valve related.

I would seriously un bung the breather, it is there for a reason. ( by doing so you may find out why P.O. blocked it  ) I don't think it would directly cause smoking but there could be a slight risk of oil frothing if the oil tank becomes pressurised ( no idea if it would as I've never tried it!