Adrian Player says: “My lad has recently completed a Honda CG125M engine rebuild, but on starting oil poured out of the crankcase breather. We tried draining and refilling slightly under level, but it happened again. It looks like the crankcase is being pressurised; when we removed the oil filler plug, the engine ran smoother and seemed quite happy.
“We didn’t find any problems in the top end but someone on an internet forum had a similar problem due to incorrect crankcase centre gasket – could that be it?”
A bit off my usual beat this one, but it is possible. In my Honda shop days I only stocked genuine gaskets, pattern items were notorious for holes not properly punched through – or even missing altogether.
It seems the CG breather comes off a chamber above the gearbox, whose only entry is a small hole above the clutch – hard to see how oil can get in there in any quantity. I’d look under the clutch cover first, block that hole and blow through the breather pipe to see if it’s venting anywhere that it shouldn’t be.
ABOVE: Gaskets may have all the right holes, but not necessarily all in the right places…