Hi Bill,
Either the cable needs lubrication (remove the cable from the sleeve, clean and lube with silicone spray) or the cable is slipping at the shifter lever (tighten) or the cable broke (replace). Or the derailer itself could have a problem.
Since these bikes are older, I really suggest you take them to a reputable bike tech and let them do proper maintenance. A non-working shifter is one thing, but you sure don't want the brakes to give out while riding! And have them check the tires as well. A tire going flat while zipping down the street or trail is something better never experienced.
Also, you might look into replacing the drop style handle bars for uprights and put a comfortable seat on it (there are even seat tubes with built-in shock absorbers these days that help smooth out the ride). And for sure have new shifters installed that are located at the hand grips. There are two styles; thumb levers and grip shifters. I prefer the grip shifters because I feel using thumb levers makes you give up a good firm grip on the hand grips. Anyway, as long as the bikes you have are sized right for your height, changing the handle bars, seat and shifter will cost far less than a pair of new quality bikes. The only disadvantage is the weight of the bike itself. The newer models use more advanced materials making them far lighter than the ones you currently have. But unless you are doing some serious uphill riding or toting the bike over your shoulder to the next trail, that shouldn't make a difference. In fact I think the older, heavier bikes are more stable because of the extra weight.
Best Regards,
Dave
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Wm. David McMahan
LearnMyKeyboard
JazzItUp Band
The Modulators [This message has been edited by WDMcM (edited 09-19-2009).]