Rinse sump while changing Royal Enfield oil?

My wife prepares to pour clean oil into my Royal Enfield Bullet.

Getting oil on my hands (and on the garage floor) is part of owning an old-fashioned Royal Enfield Bullet motorcycle. Mine is an iron barrel Bullet, the original British design of 70 years ago, but built in India in 1999.

Modern Royal Enfields are nothing like this, I assume, as they have a single drain for motor and gearbox oil, and a spin-on oil filter cartridge.

Not so my old Bullet. Never mind the gearbox, which has its own drain plug. My motor has a drain plug for its oil tank, a drain plug for the motor's sump, and an oil soaked filter to be inserted, dripping, into its home.

Changing the oil is still a simple process but, since nearly every instance is messy enough to be memorable, I've described changing the oil before.

In the previous episode I described how only 1.75 quarts of dirty oil emerged when I drained the oil tank, the separate sump and the oil filter.

The book says that the motor holds about 2.4 quarts of oil. So either I was more than half a quart low on oil, or that much oil remains inside the motor even when drain plugs are opened.

Last time I guessed that since I only got 1.75 quarts out, I should replace only 1.75 quarts. But I found afterwards that the motor tended to show well under the halfway full mark on the dipstick after a run.

So, this time, I decided I would replace the 1.75 quarts of dirty oil I (again) recovered with slightly more than two quarts of fresh oil.

But first, I wanted to conduct an experiment.

Once, years ago, while changing the oil I accidentally added fresh oil to the oil tank before plugging up the drain holes! I watched in horror as oil shot out of the bottom of the tank onto the garage floor. But I also watched in interest and surprise, as the stream of escaping oil was at first black — not clean.

In my carelessness, had I helpfully flushed out residual dirty oil in the tank? I wanted to find out.

So, this time, I videotaped the drain holes as my wife poured a bit of clean oil into the tank. Watch the video. The stream of oil emerging from the drain hole looks pretty clean, but a photo of the recovered rinse oil shows a distinctive dirty tinge. And much less oil came out then went in!

Recaptured rinsing oil shows a distinctive dirty tinge. And there's less than went in.

Perhaps, by rocking the motorcycle from side-to-side, as I now do, I had managed to drain out more dirty oil than I used to. This is supported by the fact that less oil drained out than the sample my wife poured in; presumably this "loss" equals the amount of oil that remains in the tank but can be brought out by rocking the motorcycle.

In any case, I conclude that "rinsing" the tank is a help, but rocking the motorcycle slightly from side to side also encourages more dirty oil to evacuate.