New to here

Recently got an 84 Crown Vic for a tow vehicle. It has 129,000 mi. with good compression. I do like the car and the ride but am not satisfied with how it starts. When hot it will restart if only off for a minute or two but if it sits 5 minutes or more it won't fire. If you hold the throttle wide open it won't fire up but it will 30 seconds later, if you leave the throttle alone. I'm not familiar with the variable venturi carburator (very recently installed). Are they a problem, as I have been told. It also idles too fast, IMO, and can't be idled down. What's up.

Also, what mileage should I expect?