Nobody asked me but....
I have been doing some research on the PCV system used in the TR4A which uses the Smiths "Mushroom" diaphram valve also found on other LBC cars during the early years of engine pollution control.
The function of the valve was to draw crankcase gases into the inlet manifold to be burnt instead of letting them escape onto the road using the draft tube fitted to the TR4 and earlier TRs.
How the valve functions is well explained here: http://mgaguru.com/mgtech/engine/cv103.htm
As Barney Gaylord explains it is a balancing valve that functions based on manifold vacuum. A critical component of this system is a properly vented oil cap. It has a vent hole of a precise size (about 7/64" or 0.11" ) to control the amount of fresh air drawn into the engine.The correct cap for TR4A engines is an AC part number OC-5, Triumph part# 143393, which is NLS (no longer stocked). The cap sold as a replacement is for the closed system and I do not believe it has a vent hole. This can lead to a rough idle or worse, as described in the link.
I recently bought an original cap on eBay, which I took apart to see how it functions. The pictures are posted below. It has metering hole through the cap (.11" dia.) that draws air in through a plug of steel wool. This is contained inside a cup on top of the cap, the cover of which can be easily pried off for cleaning. On this particular cap the steel wool was clean and dry, but a bit brittle. The very thin gasket on the bottom is hard and brittle and will have to be fixed with a new bit of rubber gasket material.
With the introduction of the TR250 it appears that Triumph went to the sealed cap that is now sold. Instead there was tee used on the hose connecting the valve cover to the inlet side of the valve,with another hose connected into the back of the air box. This early air box is different from those used on later TR6s in that it appears to have metering restriction inside the air box. All of which leads me to think that if one cannot find the correct cap then a similar tee and hose connected to an air filter but using with a proper restriction could substitute for the OC-5 cap.
Edited 2 time(s). Last edit at 2019-02-28 07:22 PM by andy303.
TR4A TR250 Vented Oil Cap OC-5 and PCV System
Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster