Sublime to ridiculous

Change the plugs.  Are the old ones sooty or oily?

If it's got a magneto, clean the points, check the points gap.  Clean the HT slip ring (not the cam ring!) with petrol on a rag.

A worn needle jet causes richness at low to moderate speed.

And they do wear.

Of course, richness through maladjustment on the pilot screw and/or needle clip position can also happen.

I wire brush my plugs to clean them.

If you use a wire brush don't use a steel brush. Use a brass or copper soft bristle brush or you may damage the electrodes. I soak mine with carb cleaner electrode up in a vice for 30 minutes. Then let it dry and apply a flame from a propane torch until the electrode goes red.  You can usually see where the carbon deposits are at this point. So carefully scrape off the deposits with a needle or fine scalpel .  If you don't have a gas torch the gas cooker hob is a good substitute - but it could upset her indoors.....?   If the electrodes are scratched or burnt use a 220 grade sand paper to remove the scratching and then re-gap carefully.

Steel wire brush never hurt mine.

Some they do say don't use a brass wire brush or you may leave a conductive deposit on the insulator.  Not saying 'tis so, but there's been talk that's all...

Just cleaning the gap electrode is NOT enough. Once a plug is fouled then the plug is shorting the spark up inside from the centre insulator. About the only way to clean them is grit blasting, but beware you do not want grit up inside your engine!!  The gas method above might be ok, but a set of good clean plugs to get the engine running again, might prove the issue is/was with the fouled plugs, then of course you have to sort the carburation. Needlejets in concentrics do not last more than 5K miles, the needles are usually 1 to three needle jets.

That looks horribly rich. Has the float sprung a leak or something ? Dirt in the fuel causing intermittent flooding might be a possibility.

And you're not riding around with the air slide down...?

Hi Paul ,That plug is showing rich ( not oil)  . Set the pilot when properly warmed up . I use a colourtune plug . Needle jet is a main suspect too.  Are you running an N5 plug? .  I prefer these or a KLG equivalent (old school but with a properly glazed pink insulator) . Not at all keen on wasted spark system which has one plug at a disadvantage with polarity .  My 99 DL is very reliable on the old Dissy and points helped with an old school points assist EI unit from Boyer or Pazon.  If out riding (usually the Atlas!) a sooty plug is made working by a bit of string and a soak in the petrol tank!.

You say it ran 60 miles perfectly but then 'went off' on the last 20.  That suggests there's nothing much wrong with settings, or jet or needle sizes.  I suspect Michael has the answer.

From Robert- Not at all keen on wasted spark system which has one plug at a disadvantage with polarity . This is only true for double ended coils. A pair of traditional coils cannot do this but Magnetos can do this.