Post by dave1800 on Jan 31, 2023 9:44:06 GMT
I recently watched a video about a major MGB engine overhaul that was undertaken as it was smoking. The problem still existed after reboring the block and fitting new pistons. A strip down after just around 50 miles revealed a slightly odd pattern of piston ring bedding in and lots of carbon in the oil which was also very sticky and contaminated with fuel.
The cause of this disaster was the PCV valve the mushroom shaped component that sits on top of the inlet manifold. It had a broken lug that prevented the diaphragm from sealing under high vacuum. This allowed additional air and oil mist to be drawn into the carburetor at idle and the mixture had been adjusted to compensate. This resulted in the car running very rich off idle and the excess fuel had prevented the rings from bedding in properly and allowed raw fuel to enter the sump. At higher RPM and high vacuum the failure of the valve to close drew in air and oil vapour that was burned and created the smoke from the exhaust. If not addressed this could lead to carbon hot spots and pre-ignition that can destroy an engine.
It is possible that the engine may not have needed an overhaul in the first place so for the sake of checking a sometimes overlooked component considerable costs were incurred.
This valve was fitted to early MGBs and also to Mk1 and MK11 Crabs. Mk111 Crabs and later UK MGBs use a different system where the crankcase ventilation is taken to a constant depression take off on the SU carb(s).
Replacement rubber diaphragms appear to be available . They deteriorate over time from the crankcase fumes and it would be wise to also check the valve for damage. Maybe it is a component that Penguin (Chris) could add to the items he makes?
David
