[Mgs] MGA crankcase ventilation

PaulHunt73 paulhunt73 at virginmedia.com
Mon Jul 15 04:41:59 MDT 2019

If it's not compression leaking then I can't think what would cause excessive pulsing out of the breathers.  Except ...

Although two pistons are going down while the other two are going up, the movement is not symmetrical.  Don't ask me to explain the physical logic behind it, but when the crank has turned 90 degrees from TDC the piston has moved more than halfway down the stroke, and from that position to BDC the piston moves less than half the stroke.  So for that 90 degrees of crank movement the undersides of the pistons coming down the first 'half' of the stroke are displacing more air than the ones going up can 'absorb' so you get a positive pressure, but for the second half of the strokes the situation is reversed to give a slight negative pressure.  But on my MGB - different crankcase ventilation system but still two routes in and out of the crankcase - I can seal off one and I get no more than a slight fluttering in the other.

Maybe the problem is not so much excessive pulsing, with you expecting there to be no movement in the breather at all, but excessive oil in the rocker cover around the vent.  The road-draught system is pretty hit and miss as to which way air will flow through the crankcase, and I wouldn't have though that suck unfiltered air in from the road-draught tube was a Good Thing, but if the intake can suck air through the rocker cover tube at any time, then it will draw oil along with it.  The question is why there should be enough negative pressure on that tube to do it.  I don't know about the MGA but on the MGB that tube is on the 'dirty' side of the filter, which in theory shouldn't be under significant negative pressure, unlike the 'clean' side of the filter at wide throttle openings.  Unless the intake to the can is restricted somehow.  But if the tube is connected to the clean side of the filter then the intake system will be continually sucking air through the tube, and any oil droplets that are in the vicinity.


----- Original Message ----- 
  From: Stan Fickes 
  To: PaulHunt73 
  Cc: mgs at autox.team.net 
  Sent: Sunday, July 14, 2019 8:05 PM
  Subject: Re: [Mgs] MGA crankcase ventilation

  Thanks, Paul!
  I concluded when I cranked the motor with no plugs installed and still felt the pulsing air that it was not a compression issue. For completeness, I need to try the same thing with the plugs installed, to see if there is a difference. I just can't think what else would pump air into the crankcase/rocker cover! I checked the draught tube this afternoon by squirting some low-pressure air into the rocker cover tube, and felt a gentle stream of air out the bottom. I should also check that with the starter motor turning to see if it's consistent between the two tubes.
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