Suzuki 2 Strokes

When I turn the ignition key over, after a longer while (i.e., not immediately) the 20A fuse blows. Usually about 5-10 seconds. This has not happened while the bike is running.

The bike is as-is from 1974, any preventative maintenance or other common flaws that may cause this?

Which position is the key in when the fuse blows?
Off, On, or park?
Take the fuse out and use a multimeter to narrow down where the drain is then sort out the earth or short circuit fault. There shouldn't be anything that would draw that sort of current.
It's either a wire trapped between something and the frame or a part that is failing 'short circuit'. Without some testing, I can't be any more certain. What model is it and do you have (and understand) a wiring diagram?
This needs sorting quickly as if there is a 20 amp fuse blowing, then there is a lot of heat building up somewhere which can easily lead to a fire.

With this much current you may be able to locate the problem by looking for burn marks.
So check all of the wiring connectors you can find. Including under the gas tank & in the headlight shell.

A short can develop from abrasion of wiring insulation.
Check to see if there is any problem where the wires flex from steering the bike.

………………..
As Alan suggested you may be able to use a multimeter to find the problem.
The wiring diagram will be very useful for this task. (See the wiring diagrams topic.)
First disconnect the battery.

Check resistance at the wiring harness connectors for the battery.
For 20 amps at 12 volts you should get a reading of less than 1 ohm.

If you have less than 1 ohm you are in luck.
Then make changes & look for a big increase in resistance.
Operate the switches (lights on/off, kill switches, …).
Strategicly unplug wiring connectors. Do this one at a time so you don't get confused about what connects to what.

Let us know how this goes.

It also could be something like a diode (or diodes) in the rectifier going/gone down and back feeding through the genny.

Since the circuit blows only after running look at the socket connections such as the regulator, the rectifier, the under tank main harness connection etc.
You should also move the electrical panel and look behind there where the harness wires come close to the inner rear fender. Its a short to ground so its going to be either of the following, the red battery power wires, an orange wire(or wires), the points harness wires and or possibly the yellow phase wires to the alternator.
Dont discount the black/white earth harness wires getting shorted or pinched onto the red battery power wires either.

Thank you guys for pointing me in the right direction. The rubber buffer under the rear end of the fuel tank was gone when i got the bike (since installed) and the tank may have been rubbing excessively on the loom there. Getting into the GT tonight to also swap the crank seals, so a major project evening!

I think it'll take more than an evening to change crank seals.

when i did the crank seals, my wife actually left me. Two suitcases to her mum's. I;m not kidding you.

mj in beirut

I wish I'd known that changing crank seals would make the wife leave.
It would have saved me thousands many years ago!

Did she come back when you finished?

She did actually. But that's not because i had removed the engine from the kitchen table but more cos --- well, i can't say

Can you help me find the cause of my blowing fuse? there' a short somewhere and i dont know where to start.
thanks