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A65 Breather Prt #724134
02/03/18 3:27 pm
02/03/18 3:27 pm
Joined: Sep 2014
Posts: 40
GA
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eelmgren Offline OP
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eelmgren  Offline OP
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Posts: 40
GA
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.

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Re: A65 Breather Prt [Re: eelmgren] #724147
02/03/18 5:29 pm
02/03/18 5:29 pm
Joined: Aug 2015
Posts: 1,057
Bolton Lancs UK
A
Andy Higham Online content
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Andy Higham  Online Content
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Joined: Aug 2015
Posts: 1,057
Bolton Lancs UK
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


1955 BSA B31 500cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350
1967 Greeves 360 Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
Re: A65 Breather Prt [Re: eelmgren] #724157
02/03/18 6:16 pm
02/03/18 6:16 pm
Joined: Aug 2001
Posts: 4,279
Owego, NY, USA
Mark Z Online content
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Mark Z  Online Content
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Joined: Aug 2001
Posts: 4,279
Owego, NY, USA
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.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: A65 Breather Prt [Re: eelmgren] #724211
02/04/18 2:16 am
02/04/18 2:16 am
Joined: Aug 2001
Posts: 3,641
argyll. scotland, uk
gavin eisler Online content
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gavin eisler  Online Content
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argyll. scotland, uk
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.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: A65 Breather Prt [Re: gavin eisler] #724219
02/04/18 4:48 am
02/04/18 4:48 am
Joined: Jan 2004
Posts: 3,599
ca, us
D
DMadigan Online content
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DMadigan  Online Content
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Joined: Jan 2004
Posts: 3,599
ca, us
Originally Posted by gavin eisler
If the bike sits for a while then wet sumps, the breather discharge become an oil spill unless the sump is drained.

That would take a lot of oil in the case since it has to be filled to the level of the camshaft.

Re: A65 Breather Prt [Re: eelmgren] #724222
02/04/18 5:25 am
02/04/18 5:25 am
Joined: Jan 2014
Posts: 3,515
Aus
N
NickL Online content
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NickL  Online Content
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Joined: Jan 2014
Posts: 3,515
Aus
It's true David, the oil is slung around a fair bit and does blow out of the breather if the sump level is high.
That's one of the reasons i don't like that breather arrangement.



Re: A65 Breather Prt [Re: eelmgren] #724236
02/04/18 11:46 am
02/04/18 11:46 am
Joined: Aug 2001
Posts: 3,641
argyll. scotland, uk
gavin eisler Online content
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gavin eisler  Online Content
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Posts: 3,641
argyll. scotland, uk
Mine does it over a winter lay up, not sure of the case volume , its mostly full of flywheel and stuff , only seems to take a pint or two to create the spill over.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: A65 Breather Prt [Re: eelmgren] #725110
02/10/18 11:59 pm
02/10/18 11:59 pm
Joined: Aug 2001
Posts: 3,641
argyll. scotland, uk
gavin eisler Online content
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gavin eisler  Online Content
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Joined: Aug 2001
Posts: 3,641
argyll. scotland, uk
just drained the sump, 1 litre of oil came out, at that it starts dribbling out the timed breather , until the tank level reaches the same height as the timed breather.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod

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