From your description, I'd say your fuse clips have surface oxidation. If your fuse was melting near the end caps, then it is a certainty.
You must use the correct length 1 inch fuses. Longer ones will not have enough surface contact and your problem will recur.
Clean and polish you fuse clips. Also test for clip spring tension on the fuse. It should be difficult to remove. You should not be able to remove it with your bare fingers. A tool should be required.
Fuses melt from the heat generated by passing too much current. But, it is the heat that melts them. When operating, they heat to some degree, but stay below the melting point, and only overheat when too much current is passed through them. From an overcurrent, they almost always melt in the middle of their span. External applied heat will also melt the fuse element. If the heat is being conducted in from the end clips they will overheat and melt near the end caps.
Clean fuse clips. Check clip spring tension. Use correct length fuses.
A headlight upgrade mod is also a common issue. A higher wattage headlight draws extra power through the main fuse, which raises its operating temp and makes it more susceptible to melting from external heat injection. Does your headlight have the stock wattage?
FYI. The stock bike draws about 10 amps through the main fuse, or about 2/3 of its rating. It will be warm when you put your thumb on it, but, it won't hurt. It will be warmer when you rev the bike after the battery is fully charged. Why?
The resistive elements in your lighting and bike electrics all draw more power as the system voltage rises.
After you start your bike, the battery voltage is low, perhaps 12.4V. But, when the bike is running, it is trying to get the battery back to 14.5 V fully charged state. It will do this when the engine has been run above 2000 RPM long enough.
If the bike draws 9 Amps @ 12.4 V, it will draw nearly 11 A when the battery reaches its 14.5 V. If the headlight has been upgraded to a 55/65 for example, the current peak through the main fuse can increase by 2 amps for a total of 13A through a 15A fuse. It won't take much external heating to melt the fuse element with this kind of load placed upon it. The problem will get worse when other electrical changes are made to turn signal bulbs, lower ohms coils, etc. Lastly, their are a lot of connectors in the bikes wiring, that become more resistive with age. If your fuse clip terminals have the surface corrosion, it is highly likely that the rest of the connectors are also oxidized. Oxidation = Resistance = power wasted as heat= extra current drawn through the main fuse = poof.
One more thing. If the Vreg contacts are stuck, or the Vreg adjustment has been tampered with, long drives can make the battery voltage go well above 14.5 V. 17V is not unheard of, and quite bad for the battery. It is not inconceivable that such a voltage could draw nearly 14 Amps through the main fuse with with the stock electrical loads. Add any fuse clip heating, and bike vibration, and bye-bye fuse.
So, how's your charging system?
Cheers,
