From Rod Yeomans, RIP
Small block guys.. ignore this at your own risk.. I had a breather line come loose on the Lario during a hard run, and there was oil *everywhere*..
Engine breather system
Many of the bikes have had their original filter boxes removed and pod filters fitted to replace them. What a lot of owners don't understand is the breather system that is connected to the filter box.
The purpose of the breather system is to return oil condensate to the sump. Without this a Guzzi engine would eventually pump itself dry of oil. The engine has a shared journal crank so even though the pistons are 90° apart there is still a constantly changing volume under the pistons in the crank case as the engine turns. The volume under the pistons changes by about 70% of the engines displacement every revolution.
Oil ridge and fangs.
Oil ridge and fangs.
Photo courtesy of Rod Yeomans.
Early bikes only had a very small oil dowel (1 mm) feeding oil to the valve gear. The rest was supplied by the breather system. Oil vapour was carried up via the push rod ports to condensate on the inside of the rocker covers. This would then drain down to a ridge in the casting and there are 2 fangs that sit directly above the valves and direct the oil to fall on them. Moto Morinis of the same era used this system solely to lubricate the valve gear. On later small blocks the oil dowel was increased (3 mm) and the main oil feed is via that but the vapour system still works as long as the breather system is intact enough. Larger oil dowels were fitted from engine numbers: V35 / 24550, V50 / 25180, V65 / 1162. It is recommended that this modification is undertaken on earlier engines.
If the breather system is blocked off or restricted then the crankcase builds up pressure. This may cause the engine to leak from any of its joins and seals. At low speeds the breather will suck air in too as the pistons rise so it is a good idea to have the breather vent filtered. It originally vented into the clean, negative pressure side of the filter box. It makes me wonder if the removal or modification of the original systems and the subsequent reduction in oil condensate flow to the top end contributed to a shorter service life for the valves.
It should be noted that the breather flow is the reverse in a small block to a big block engine. The small block flows in parallel with thermal effect up the barrels and out the rocker breathers. The oil returns directly to the rear of the sump below the oil level, so it acts as a one way valve.
