Just a note about catch-can or catch-bottle or none.
The catch-can on the AVL works to collect and recirculate engine oil and gasses It has two inlet and two outlets. The large inlet on the bottom of the can is the breather from the oil tank, it should have a duckbill on the inside of the can and the stock tubing will have a 90 degree bend at the oil tank. The mid size oil return line on the bottom of the can returns any oil in the catch-can back to the engine through the back of the timing chest. The tubing for this, has a 90 degree bend that goes to the back of the timing chest. The small inlet on the bottom of the can is a breather from the primary case. The outlet on the side of the catch-can, near the top, is a breather to return gasses to the engine through the air box, carburetor and into the combustion chamber to be reburned.
The catch-bottle is often used for road racing. It will catch and hold oil from the breather and allow the engine gasses to release into the air without reburning them. By using a catch-bottle no oil from the breather will deposit on the track. At the end of the race the catch-bottle is emptied and oil is added to the engine to makeup for the lose.
Open breather, no catch-can or catch-bottle. An open breather with or without a duckbill, on the AVL engine, I have found, will loose some oil, no matter how I routed the breather hose. The oil tank needed to be topped up from time to time. Also oil from the breather ended up on the ground.
I'm back to running the catch-can. I'm using a pancake air filter so I just ran the breather hose that went to the air box down behind the gear box. I also use F type ATF in the primary so I run the primary breather line to the back of the gear box too, just to keep engine oil out of my ATF in the primary. The small inlet on the catch-can I use as a overflow drain and put a short tube on it to drain over the drive chain. To date the catch-can has not filled to drain out this overflow. I don't need to empty a catch-bottle and add oil back to the oil tank. I don't loose oil out of the breather hose allover the ground, having to replenish oil to the tank every day. I just ride my motorcycle and check my oil level after the last ride of the day. I never need to add oil between oil changes. One needs to route the breather hoses with no low places and no kinks and don't over fill the oil tank for the catch-can to work the way it should. Check the oil level after the engine has run. Shut the engine off, wait about a minute or so and then check the oil level. This is what I found about the AVL engine and this subject.
Royal Enfield people are good people
Cheers,
Bill