Oil Pump

The pump is a 35mm wide cog with a sloping top. Looking at it from the side and rotating it to see maximum slope, the high side is ~6.8mm and the low side is ~4.4mm. This cog sits in an oil sump, which has an input oil feed and an output oil path.

There is a pin which changes position with throttle, travelling along the radius of the cog (from between 6mm to 14mm from the centre – judging by the marks made on a used one). The cog is sprung to push upwards against the pin so that, as it rotates, the “up down” travel changes with the slope. When the pin is at 14mm from the centre (at full throttle) the travel is ~2mm and when it is 6mm from the centre (idle) its travel is 0.8mm. Based on a full throttle "pump" is 37 microlitres of oil and a no-throttle pump is 16 microlitres. 



The oil pump, “pumps” once every 83 revs of the crank (i.e. the cog rotates once)

Let’s look at an engine that idles at 1,000 and red lines at 7,000. Note that without actual measurement of fuel flow and actual pump flow (as opposed to calculated flow), this is all theory.

Regardless of throttle position changing pump travel – the oil delivered at full revs is 7 times that delivered at idle (assuming constant pump efficiency). If fuel delivery was directly proportional to revs, then this should indicate a fairly constant ratio of fuel to oil (i.e. more fuel = more oil). Also the carb is meant to maintain a correct and contsant air/fuel ratio over all rpm ranges, so for each spark the same amount of fuel is consumed.

If we now take into account the pump stoke ratio (i.e. strokes between 2mm to 0.8mm) – at full throttle the pump is pushing ~2.5 times more oil through than at idle. So if we assume that 7,000 revs is a 2% oil mix ratio, then idle is 0.8%! 

Here is a excel spreadsheet used for some calculations: oil_pump_calculations.xls