starting issues

1985 xj700n--I haven't ridden for a while, however I have started the bike every week or so for the past month or so. When I go to start it I set the petcock at prime for a minute or so, set the choke, crank it over a few times, close the choke and usually starts with a little more cranking. I then idle it at about 1500 for a couple of minutes. If I let it go to normal idle it will probably cut of after 20-30 seconds. Then it is almost impossible to get it to start again. If I go ahead and ride it a mile or so, it restarts instantly. Any idea what might be going on here?

 

Plugs fouling.
I have the same issue if I don't actually ride my spare bike after starting it with choke.
Not so much if I can start without choke though.

I move the spare about 15metres up a steepish driveway once a week just to get it out of the way.
The daily is fine but when I switch the bikes, the same thing happens to the bike I was previously riding.

Get it off choke as fast as possible and you will have less of an issue if you don't ride it after starting.