Right, two pipes it is. From about engine YE6500 and onwards they used a different pump and had two pipes into the timing chest. The oil pump is a duplex plunger, where as the original was a gear type. So probably a 1927 EW has the duplex type.
In this duplex system the oil circuit is as follows: The oil flows from the oil tank through the oil tap, and on down the fitting on the timing chest. This is the inlet for the pump. The pump returns this up to the sight glass via the other pipe. On its way it is joined (via a t-junction) by the hand pump output, which can be used to add additional oil or if the mechanical pump fails. The oil travels up and through the sight glass on the top of the petrol tank. The oil then drains down to the engine by gravity, entering via a one-way check valve screwed into the base of the front cylinder. Crankcase depression when the pistons ascend draw in the oil through the check. Note then all the oil enters the engine through the front cylinder and has to pass through the sight glass. After the oil has splashed and frolicked about the bottom end a bit, it eventually finds its way over to the timing chest cavity. There it accumulates and the level rises till the lower run of the timing gears just dips into the surface, thus lubricating the timing gear train, cams, and tappets. There is an overflow pipe that leads excess oil away to pee on the primary chain.
Also dipping into this pool of oil is a siphon screwed onto the bottom of the oil pump. This is connected to the lower portion of the duplex pump, and it is that portion that pumps oil via a quill, into the end of the crankshaft, and thence via galleries through the crankshaft to the connecting rod big end bearings. This quickly escapes the usually very loose big end bearings, joins the rest of the oil in the crankcase, and eventually finds its way back over to the timing chest. Often via the hollow cam spindles and the whole process repeats itself over and over. The oil pump does not return oil to the oil tank; it only pumps it up to the top of the tank to run through the sight glass. It is not a scavenging system; rather it is a total loss system. The only way for oil to escape this cycle of abuse is to leak out every orifice and joint, or work up past the piston rings, and out to freedom via the exhaust pipe. This is because oil has an affinity to return to the ground, whence it came.
Because oil for the crankshaft is picked up from the level in the timing chest, it is important before starting these engines after a rebuild to fill the timing chest with some oil, as it may be a while in the normal course of 'running' before enough oil accumulates to submerge the end of the oil pump siphon. Meanwhile the crank has to survive on oil mist being thrashed around the crankcase. If the mechanical pump fails and you are operating the machine solely on the hand pump (like it were a 2-3/4hp model) the crank shaft is not being feed oil via the internal galleries. Also if the engine has set for a while, some priming with the hand pump will help fill the oil lines before starting.
So if the engine is using too much oil, one is letting too much in for a start. But maybe it is just burning too much oil. Beyond the obvious worn bores and rings, there are other situations that can make smoke. If the oil level in the crank case is too high, even the best cylinder and piston will be unable to cope. Often this occurs because the oil in the lines leaks down while the bike stands unused, and ends up in the crankcase. But this should eventually burn off and equilibrium be restored. Also make sure the overflow standpipe pipe passing up through the bottom of the timing chest is not blocked or intentionally sealed off. If it continues to use too much oil, the drip rate is set too high. Otherwise it may be the plumbing is not routed as the makers originally intended. If the oil line were to run from the tap on the oil tank directly to the non-return valve, the engine is going to suck in as much oil as it can on each stroke, like a greedy puppy. It would not pass through the sight glass, and therefore there would be no metering or regulation of the amount of oil entering the engine. It should only be sucking in the tiniest amount of oil each stroke, not a whole teaspoon full.
So that is the ‘improved’ system, and you may be wondering how the earlier system worked. Well it didn’t, or not very well anyway, which is why Douglas changed it. It relied on suction to draw oil up through the sight glass and then it could flow down in to the pump via gravity. Problem was it passed through four fittings and the sight glass itself, all of which had to be vacuum tight. The system sucked alright, but not oil.
The duplex pump used on the EW was not much better. The biggest problems is the drive gear tends to strip. This is driven by a worm on the end of the crankshaft. I suppose in order to save money; Douglas did not cut the drive gear to a corresponding helix to match the worm. The gear is straight cut, which normally would bind as the tooth of the worm passed by on its oblique path. Douglas solved this by cutting the tooth space much wider than the thickness of the remaining tooth, giving the worm some extra space. But it made the tooth weaker. Also all the wear (and load) is at each end of the gear tooth on opposite flanks, as they are the only places the gear actually touches the worm. The oil pumps used on the big twins had the gears cut on an angle to match the worm helix.
By 1931, they changed it again to a much more robust multi-stage rotary-plunger pump (similar to that used on the DT/SW) that they kept right up to the end of pre-war 250/350cc sv engine production. Also gone were the oil galleries in the crankshaft. The crankshaft is rather weak as it is, having holes drilled in it just made the center crankshaft web that much more prone to break. Instead the pump had a spray bar passing over the crankshaft dripping oil into the path of the big end bearings. These had special grooves to lead the oil into the rollers where it was alleged to have done some good.
-Doug