One thing to keep in mind is that if they aren't wine grapes, they may be VERY acidic. The way I deal with that is to dilute with water, and add some sugar to bring up the OG.
Most local/native type grape varieties are too low in natural sugar and too high in natural acid to be fermented alone.
If they are "tart" and "sweet", it sounds like they could be very high in something like malic acid. If that is the case, a "native grape" type of recipe may suit better.
Some ideas (from Jack Keller):
http://winemaking.jackkeller.net/nativew1.asp
one more point from him as well (he's talking about mustang grapes here, but it applies to all non-wine-grape type of grapes:
If the acidity of the grapes is too high, further acid reduction may be required. Here are three methods....
Acid Reduction with Calcium Carbonate: For liquors with acid levels of 10 p.p.t. or more, calcium carbonate is traditionally used to reduce acid through precipitation. A measured 2.5 grams of calcium carbonate will reduce the acidity of one gallon of wine or liquor by one p.p.t. For best results, split the liquor into two equal portions and add the calcium carbonate to one while stirring vigorously. Carbon dioxide will be given off and cause foaming. Chill the treated liquor several days and then siphon it off the lees of calcium carbonate into the untreated portion. The addition of a teaspoon of yeast energizer may be required to reactivate fermentation after treatment.
Acid Reduction with Potassium Bicarbonate: For liquors with acid levels of 8 to 10 p.p.t., potassium bicarbonate treatment can be used to reduce acid through precipitation and neutralization. A measured 3.4 grams or 0.1 oz. of potassium bicarbonate will reduce the acidity of one gallon of wine or liquor by one p.p.t. The compound is stirred directly into the full batch, then chilled to facilitate precipitation of potassium bicarbonate lees. The addition of a teaspoon of yeast energizer may be required to reactivate fermentation after treatment.
Acid Reduction through Water Dilution: This is the least desirable method, only because the Mustang Grape flavor is diluted and the resulting wine will suffer. The acid is inversely proportional to the volume of liquor, so the steps in reducing acidity from 10 p.p.t., for example, to 7 p.p.t., are: (1) 7 / 10 = 0.70 (2) 100 / 0.70 = 1.428 (3) 1.428 x 128 (oz. per gallon) = 182.784 total oz. required (4) 182.784 (total required) - 128 (oz. per gallon) = 54.784 (oz. per gallon required to be added).
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The reason I mention this is that look very small, almost like concord size, but it's way too early for grapes in most areas of the country. I've never seen grapes that color before, except for Niagara grapes.