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On the R90S engine that I recently rebuilt, I'm seeing some oil seepage out of the air cleaner clamshell. That makes me suspect the crankcase ventilation valve. I have the stock tube in there and it is in a stock position. Is this indicative of a problem in the engine? I doubt that it is, since it runs so incredibly well. I have been flogging it some but it seems to like that. (1976 R90S)

Chuey

A bit of oil from the breather is normal, especially when the engine oil level is at the full mark and it's revved high.
If it's excessive, a cylinder leak test will help narrow down the cause, breather or rings.
The breather tube should end where it will drip into the right carb intake tube, not into the clamshell.

It could also be a sign of rings not seating. But, like gspd says, if you have your oil at the "Full" mark, you're going to get some oil spitting out of that breather.

hello, newburt here posting first time.

first bought my '76r75/6 in 1980, sold it in 1991. bought it back in 1999 (was never ridden), sold it in 2003 and just bought it back again. this guy never rode it either, but he had an excuse - a '72 norton, triumph bonnie and a 74 bmw r/something. anyhow am just finishing up a full mechanical refresh, and also had a seep at the airbox cover. here's what i found:

hard/leaking breather assy gasket, loose breather hose, no clamp on breather hose. all fixt now, regards.
bobp