This problem seems to be related to early heads with HD studs and/or MLS gaskets?
The other thread with a CB750 K0.
Head with no ex guide seals + MLS with small oil return holes.
This case however, can be the way too low torque of 17 lbs, 22lbs will seal.
Maybe a re-torque only will fix it.
But when engine must be pulled better to be sure.
Dynoman sell with large oil return holes that match all cylinders.
https://www.dynoman.net/bikepages/sohc/gaskets-head.htmlIf have ordered gaskets from them. Ordered a bunch last time for various bores. Shipping cost some to me so better to order for future use.
Still special price for 970cc, thin MLS
29.95 when I ordered a few
Here a photo of MLS with large return holes on early cylinder with smaller return holes.