Positive Crankcase Ventilation

On Gen I bikes there is a breather off the rear cam cap. On later models it is off the front. When those enormous pistons come down together, they displace a lot of air. It has to go some place. Hence the vent. The vent has a line that ultimately feeds into the air box. In its line is an oil trap. It breathes both ways: in and out. Is it up to the job? Do your cam tensioner caps leak?

BMW used to race its flat twin. The last iteration had a large (2"?) pipe ducted up under the seat into a forest of reed valves. The interior of the crankcase was a near perfect vacuum when the pistons were at Top Dead Center. If an engine leaked, it leaked IN.

At idle, take out your oil fill plug. Blip the throttle. Does it seem crisper? To preclude oil leaks, and gain some free horsepower, put a one-way valve where the oil trap goes. Use an automotive PCV valve if you can find one the right size -- that's exactly what they are for: maintaining a vacuum beneath the pistons. Change the other cam cap to a vented one and one-way it, too. If you are really ambitious, put a really large (1.5" or bigger) spigot on your rear exhaust rocker cap. Run a hose off it to a reed block encased in some sort of housing.

I was going to offer a flat plate, grooved for (and including) an o-ring to install where the front-side tensioner caps go. I may still do this, but now I'm thinking of a (perhaps 15mm OD) fitting, elbowing ninety degrees and running a stainless hose up and back, through at least one check valve to a filter back at the taillight. Maybe one off each cylinder to a catch bottle under the seat? There isn't a lot of oil on that side of the slipper/tensioner, but those caps sure seem to leak quite a bit. Thus, a catchment might be a good idea: pour it back into the filler before every ride.

More to come . . .

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