Breathing difficulty
After 30 years of ownership, oil is coming from the breather of Charles Hallam’s 1967 Bonneville, especially at low or tickover revs. He’s fitted a new oil pump and an oil filter on the return line and the return to the tank seems fine. “Any ideas?” he asks.
While most Japanese engines are wet sump, with a single oil pump that collects oil from the sump and delivers it around the engine, British engines are generally dry sump, keeping their oil in a tank – or a separate chamber in the engine itself. Oil tanks allow the oil to cool, but require a ‘scavenge’ pump to return the oil – and as the lubricant is hot, thin, frothy and going uphill, it has to be beefier than the feed pump.
If the scavenging is weak, crankcase level will rise and, with more oil splashing around, some will inevitably blow from the breather. Bore/piston/ring wear encourages crankcase pressurisation, which will also cause heavy breathing, but I wonder if the oil filter may be a contributory factor here. Canister filters are designed for wet sump engines; on the Triumph, returnline fitting avoids a supply blockage, but may make life harder for the scavenge pump – especially at low speeds.
The only factory-fit filter I remember is the tiny one fitted to the WD BSA B40; could a large car filter, positioned higher than the sump, have enough weight and oil volume to overcome the pump with backwash?
Charles’ pragmatic solution was to route the breather pipe back into the tank – but breathings contain moisture as well as oil, so watch out for emulsification.
‘SCAVENGE PUMP HAS TO BE BEEFIER THAN THE FEED PUMP’