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The Mikuni oil injection pump

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I was curious to know how these pumps work. Looking on the internet I found very little information, so I took one apart. Rotax cautions us to not do this, but due to a friend's generosity I ended up with an extra.  In taking it apart, I lost an important retainer clip, so I must side with Rotax here. My impression? The pump is a robust and clever design. Bathed in clean oil at all times, the pump should have a long service life.

Picture #1 shows the pump, minus the two banjo fittings on the outlet lines. The banjo fittings contain one-way ball valves that keep oil from being forced backwards thru the pump.  The oil inlet tube is part of the steel cap on top of the pump.  Both ends have steel caps held on with 4 screws.  Pic #2 shows the pump body with inlet tube off.  Oil comes in, and goes part way down the pump body, where it enters the center cavity through a hole.  One can see the bleed nut on the side of the pump body, which is used for initial priming of the pump.  Pic #3 shows the input drive shaft, driven by the plastic gear. Note the worm gear machined into its shaft. This worm gear drives a pinion, as seen in pic #4. This pinion rotates a cylindrical sleeve that serves as both the piston pump's outer cylinder and valving Pic #5.  Pic #6 shows the view from the other end of the cylinder.  The aluminum button serves as a bearing as the top of this cylinder is held under spring compression against the top of the pump body, pressing against the steel cap, where the inlet tube is.  Holes in the side of the cylinder act as valves to let oil in and out of the cylinder's interior at certain rotational positions, where they line up with holes in the pump's outer body.A simple spline transmits rotational torque from the pinion gear to the cylinder, while the spring tends to push the two pieces apart. The pinion shaft is actually the piston which pumps the oil, shown in pic #7.  Next to the gear you can see the two ears of the spline.

 O.K., so this thing turns!  How does it pump?  Pic #8 shows the bottom end of the cylinder with gear.  Note that with the spring & spline, this end can move up and down with respect to the rest of  the cylinder.  Note the cam surface around the end, and the button in the center.  Pic #9 gives a clue.  The cam slides on the concentric cylindrical surface of the shaft below with the eccentric machined in the middle. Pic #10 shows the parts assembled.  This cam drives the pinion gear & piston full stroke if the eccentric does not contact the central button.  This eccentric is controlled by the lever connecting to the Bowden cable.  At 50:1 the eccentric just kisses the button; but if less oil is required, it presses on the button, limiting the downward travel & hence stroke of the piston, returned by the spring. What's really clever about this design is that, if the Bowden cable should break, the cable-arm's return spring moves the eccentric to a position allowing maximum piston stroke, so the engine will not be starved for oil. Getting the desired oil ratios may require some adjustment of the Bowden cable, however.

 

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