So my new dual carb knuck had some oil return issues from the rocker boxes. For those of you who are unfamiliar with knucklehead engines, they have a top end oil return system unlike any of the other big twins. On most big twins, oil is pumped to the rocker boxes and then allowed to gravity feed back to the flywheel or cam compartment. With the knuckle there are steel hoses that actually suck the oil out of the cups, through the “knuckles”, down the pushrod tubes, around the tappets, down tappet block passages, into the cam deck, and over into the breather gear pocket. When the pistons are on their way up a secondary breather gear window opens allowing the negative pressure to suck. Oil then gets pulled down into the breather gear, where it waits for a second until the main window opens, at which point it gets blasted into the cam compartment along with the rest of the bottom end oil. From there it is scavenged by the oil pump.
However, on my knuck the oil was getting trapped in the top end! I took the entire engine apart and checked all the routing for an obstruction that would have clogged it- nothing. I swapped out the breather gear for an original knuckle one ( I had an aftermarket knuckle breather)- still no dif. Then I compared my aftermarket cases to an original knuckle set- and found a slight difference. In the breather pocket the return hole that enters the breather gear pocket was approx 1/16″ to the left of there the stock one was. That means that the “window” for the top end sucker was essentially opening a few degrees too soon. I tried advancing the breather gear 1 tooth (1 tooth equaled the amount the case hole was “advanced”). No luck! shit!
This is my latest effort. I put the breather gear back to stock setting. I figured perhaps there is simply not enough suction being created by the pistons to pull the oil. I have no idea why but it is worth a shot. If there wasnt as much suction perhaps the oil could not overcome gravity as it is being “lifted” out of the cups on its way to the knuckles. What if I allowed it to gravity feed back? I removed the steel tubes from the cups and instead installed hoses. I plugged the tapped return holes in the knuckles. The 4 hoses from each cup collect at a junction block. From the block a large AN hose goes back to the breather gear through a tapped hole above it. In effect i have re-routed the oil return, but it still goes to the same final location, but eliminates the part where it goes uphill. So far it seems to be working! I know you might be saying “are the tappets running dry now?”. They are definitely recieving less but they are still exposed to the oil mist in the cam compartment- which should be fine. Here are some pics. You can see the new AN fitting above and left of the top end feed hose.
Here is the junction block for the 4 cup lines. I kinda love this thing.
I’ll report on how it works in the long run but it seems good so far! Knuckles are tricky little fuckers….
