I just can't understand how these bike act so differently. I can understand the getting a bad batch of components that were sourced out, but this oil in the airbox issue varies so wildly.
The strange differences in the oil in airbox problem are bizarre, and hard to reconcile.
From what I have learned on this site, my own experience, and from listening to the Norton mechanic at my shop there are 3 areas of concern with regard to the oil in airbox issue.
1. Owner unknowingly overfilling the oil tank. We’ve been through this quite bit, and Norton originally supplying the wrong length (too short) dipstick which exacerbated this.
2. Oil migrating to the engine case when the bike has sat unused for a significant period of time. On startup, the crankshaft pushes excess oil into the breather and up to the airbox. This issue can be made even worse by the owner finding the oil level down on the dipstick because some oil is in the engine sump, filling the tank, then starting the engine and driving away, only to hit the brake hard and have the engine stall/backfire or find plumes of smoke jetting from the exhaust.
3. Richard Coote has noted that the oil drain holes in the cylinder head are undersized and do not allow proper oil drainage from the head at higher RPMs. This causes oil to build up in the head and eventually be passed through the rocker cover breather and into the airbox.
Norton has pretty much solved concern #1 by offering the new longer and knurled dipstick.
Issue #2 appears to be a bigger concern for some owners than others. I know that it was my issue since I don’t get a chance to ride my 961 that much. The bike could sit for 4 or 5 weeks before I had a chance to ride it again. I also could see the oil passing from my airbox drain hose on startup sometimes. Proof enough for me to go ahead and get the oil in airbox kit installed. In 300+ miles since the installation there is still no oil in the catch bottle.
Resolving issue #3 would require removal of the cylinder head and drilling out the oil drain holes. This will most likely be a fairly costly expense. This is something that the factory should be doing on all new 961’s.