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MG MGB Technical - Valve Cover Vent and Idle

I have an alloy valve cover on an 18V 1950cc engine with HS6 carbs. The head is not an emmissions type head. The vent (the diameter of which is larger than the restricted original valve cover) to the valve cover is connected to the carbon filter housing and the carb side engine vent is connected to the carbs via a y-fitting and tubing.

I fitted a K&N filter to the valve cover (disconnecting it from the carbon filter housing) and noticed that the idle was slower. I then removed the K&N filter and held my thumb to the vent hold and..presto the idle was smoother and faster....I looked in the Bently, Haynes and other books, checked out the archive..Ahhhh! Someone please explain this to me

thanks.

rn
RN Lipow

It is normal for an engine with a crankcase ventilation system to run slower and rougher if the crankcase is opened to the atmosphere i.e. by removing the oil filler cap, and this seems to be what you are saying. The carbs suck on the crankcase, either via a vent pipe on the rocker cover or one on the tappet chest. Something has to let some air into the crankcase in order for the fumes to be drawn through, and this is restrictive in the form of a vented oil filler cap or a filter elsewhere. With the restriction the carb mixture is correct, without the restriction too much air is getting into the mixture so weakening it. The above is true for non-emissions systems, emissions systems are much more complex and have much more scope for blocked hoses etc. to cause problems, particularly if the original plumbing is altered.
Paul Hunt

This thread was discussed between 11/02/2002 and 12/02/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.