Can somebody either explain crankcase breathing or give me a link where I can educate myself?
I've got a 1970 A65 project. The engine is out of the frame. With the plugs out, I turn the engine over with the kick starter and hear a heavy exhale from the breather port forward of the drive sprocket area. Is this normal?
I know engines pressurize the crankcase as the pistons go down, but I've never noticed heavy breathing on other engines. The compression is OK and I don’t hear air from the breather when the cylinder is pressurized at TDC.
As the pistons go down the air they displace has to go somewhere so It goes out of the breather. As the pistons rise the breather closes and a partial vacuum is created in the crankcase. Keeping the crankcase below atmospheric pressure lessens the chance of oil leaks but also reduces drag on the crank and rods (windage losses) Modern racing engines use an electric vaccuum pump or exhaust ejector to reduce crankcase pressure
I've never paid attention to what extent the breathing is audible on my A65s, kicking it over with the plugs out as you say, so I can't gauge if what you're hearing is normal or not. If the breathing is indeed abnormally loud, perhaps the port is partially blocked - just a thought...
For what it's worth, some other engines (like later model Triumphs) breathe into the primary chaincase, so the sound would be muffled with the primary cover on. On a Beemer twin, the breather (reed valve type) is inside the air box on top of the engine.
A bit more detail on the timed breather. The timing side crank case has a couple of ~ 1/2 " holes for crank gas to pass through , from there it goes through the end of the cam shaft , ( an 1/8" inch hole), passes through the camshaft and exits through the timed disc driven by the camshaft at the far end, the disc has two small windows to vent through , then out at the exit above and beside the gearbox sprocket. Due to the long gas passage and the small hole in the cam the breather struggles at high revs. much over 5 K and it gets out of synch with the pulses. Originally gold stars benefited from the timed breather which superseded the flapper valve on older singles, however the GS breather route is less convoluted and works very well, when the same principle was used on the twins it was initially built for the long stroke A7, it was never improved for the later higher revving 650s. It is quite normal to hear the breather wheezing when kicked over slowly, nothing to worry about. If the bike sits for a while then wet sumps, the breather discharge become an oil spill unless the sump is drained.
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