Hi,
Rather than take off the auto-choke (which would give you trouble cold starting) first have a fiddle with your fast idle screw. As Kev says, there are three steps to the fats idle to assist with cold starting. The fast idle screw allows you to fine tune how fast this is.
The screw in question is number 35 on this drawing
https://club8090.co.uk/wiki/Fu ... -_Pierburg" onclick="window.open(this.href);return false; - it is fitted to the throttle arm.
Let the van warm up, then with the engine still running, using a torch and contorting yourself into a strange position, have a look to see if screw 35 is pushing against anything (might want to take the large back air inlet box off to make it easier to see, but make sure there is nothing small lurking around to get sucked in!)
If screw 35 is just pointing to air (when the engine is warm), then it is not holding the throttle open. However, if it is pushing aginst something then this is your problem! As a temporary fix whip the screw out, but be aware that whilst this will cause the idle to drop to normal, the engine will run very rich as chances are your choke flap would also be shut. Would do you for a short while though whilst you sort the choke out properly.
If number 35 is not touching anything when the engine is warm, try unhooking the throttle cable and opertaing the throttle by hand - you will see where the throttle should rest, and be able to see if it returns to this position. If it is sticky, then you can work at what point it sticks, and possibly what it sticks on!
Let us know how you get on!
Cheers,
Alex