I have a late 70's Garanta (German made) folding bicycle with a Shimano Eagle II derailleur and thumb-shifter. I know this derailleur is out of date and was low end when new BUT - does anyone have any experience with setting this up properly. I can only get 4 gears out of 5. It will only shift 1 - 2 - 4 - 5. From the low (large) gear it takes a lot of lever movement to get it to move. Is it salvageable to keep original or time for an upgrade? If upgrade time, is there a recommended inexpensive derailleur and compatible twist grip shifter available? This is just a campground bike. Thanks.