Positive Crankcase Ventilation

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Cheese
Cucciolo - the Lil Pup
Posts: 14
Joined: Mon Apr 08, 2013 9:29 am

Positive Crankcase Ventilation

Post by Cheese »

I've had small weepy leaks on my motor since new. It does not appear that the crankcase is under vacuum as (I think) the breather has no check valve. Anyone install one?

Thanks,
Peter
wdietz186
Cagiva Alazzura
Posts: 703
Joined: Sun Sep 10, 2006 5:40 pm

Re: Positive Crankcase Ventilation

Post by wdietz186 »

The breather is a labyrinth and has no check valve to prevent the engine "inhaling" when the piston are on the upstroke. The volume of the cases is sufficent to prevent pumping at normal revs if the ring seal is good. A one way valve wouldn't hurt if it is large enough to handle the volume of air moved in the crankcase.Most all of the PCV valves available are too small and will result in positive pressure at high revs. The one way valves used on air injection systems in older autos are big enough and the only hard part is finding one that opens with a low pressure. Better is a reed valve,the belt drive engines use them on their breathers but the thread diameter is larger than on the bevels so some fabrication would be in order. Many of the small two strokes use reed valves that open at low vacuum and are big enough,but you would need to make a housing to hold it.
Cheese
Cucciolo - the Lil Pup
Posts: 14
Joined: Mon Apr 08, 2013 9:29 am

Re: Positive Crankcase Ventilation

Post by Cheese »

wdietz186 wrote:The breather is a labyrinth and has no check valve to prevent the engine "inhaling" when the piston are on the upstroke. The volume of the cases is sufficent to prevent pumping at normal revs if the ring seal is good. A one way valve wouldn't hurt if it is large enough to handle the volume of air moved in the crankcase.Most all of the PCV valves available are too small and will result in positive pressure at high revs. The one way valves used on air injection systems in older autos are big enough and the only hard part is finding one that opens with a low pressure. Better is a reed valve,the belt drive engines use them on their breathers but the thread diameter is larger than on the bevels so some fabrication would be in order. Many of the small two strokes use reed valves that open at low vacuum and are big enough,but you would need to make a housing to hold it.
Thanks for the reply. I agree a reed type check would be best and as I have one from a recent 1100 twin I think I'll try that.

Thanks Again!
Peter
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